Acoustics Fundamentals

Introduction

Every technician eventually stands in a beautiful room with an expensive sound system and hears it sound terrible. The presenter is loud and impossible to understand. The bass booms at the back and vanishes at the front. Every word comes back twice from the glass wall. Somebody asks for more level, and it gets worse.

None of that is a fault in the equipment. It is the room. Sound leaves a loudspeaker and fills the space, bouncing off every surface, and what reaches a listener is the room's version of it. Acoustics is the study of what the room does, and a technician who understands it stops fighting rooms with faders and starts working with them: choosing where loudspeakers go, what they are aimed at, where drape will help, what cannot be fixed with EQ, and how to measure whether it worked.

What this book covers

Why the room is part of the system, direct and reflected sound, early and late reflections and critical distance; reverberation time, the Sabine equation, reverberation targets and speech intelligibility; absorption coefficients, porous and resonant absorbers and why frequency matters; room modes, flutter echo, focusing and comb filtering; background noise, isolation ratings and flanking; practical treatment, permanent and temporary; measuring level, reverberation, noise and intelligibility; and how acoustics decides loudspeaker choice and placement, coverage, EQ and gain before feedback.

It was measured against the standard handbook of acoustics, the standard text on architectural acoustics, AVIXA's acoustics training and coverage uniformity standard, the international standard for speech intelligibility, and the room noise, isolation and classroom acoustics standards, so that nothing a technician is taught elsewhere about the acoustics of the rooms they work in is missing here.

What this book does not do

It does not make you an acoustical consultant. Designing treatment and isolation for construction is specialist work. This book gives you the understanding to recognise what a room is doing, suggest practical fixes, measure the results, and talk to acousticians in their own terms.

How to Use This Book

Each chapter follows the same shape, and the shape matters more than it might look. You read the ideas, then you see them play out in a real situation, then you get the procedure, then you are shown what goes wrong and how to recognise it early. Reading only the parts that look new is the fastest way to arrive on a job holding half of something.

1. Read the chapter body first. This is the understanding layer, and it is written to be read straight through.
2. Read On the job. The same material as a real situation in a real room, which is where it stops being abstract.
3. Work through How to do it. These are procedures you can follow on a job this week.
4. Study What goes wrong. Experienced technicians are not people who never make mistakes; they are people who recognise mistakes early.
5. Do the exercise before reading the answer key. Recognition feels like learning and is not.
6. Take the checklist with you. Photograph it on your phone. Use it until you no longer need it.

Keep a notebook

One habit is worth starting today, before the first chapter: keep a small notebook of rooms. For every venue you work in, write its rough dimensions, what its surfaces are, how it sounded empty and full, what went wrong, what you did about it, and any measurements you took. After a year, that notebook is a better guide to rooms than any rule of thumb.

Write it by hand. Writing "long hard room, glass on the left, boomy at the back, drape on the rear wall fixed it" is how you remember it next time you walk into a room like it.

Then use it twice. Read a few pages on the way to a venue, and carry it on the job, because the room you are standing in is almost always like one you have met before.

A note on the words

Acoustics terms are used loosely on crews. Reverberation, reverb, echo and ambience are often used as if they were the same thing, and they are not: an echo is a distinct repeat, reverberation is a smooth decay. A room mode is a standing wave. Absorption is not isolation, even though both are called soundproofing in everyday speech. Treatment and tuning are used for both acoustic changes and loudspeaker processing.

None of that is worth arguing about on a job. What matters is that you can describe what you hear — a smear, a repeat, a boom at one seat, a flutter after a clap — because a precise description points straight at the cause.

A note on safety

The hazard in acoustics work is the sound itself. Measuring and tuning a room means playing pink noise, sweeps and loud programme, often for long periods, and high sound levels damage hearing permanently without pain or warning. Wear hearing protection when you are generating test signals at level, keep measurement sessions as short as they can be, and warn anybody else in the room before you start.

The other hazards are the ordinary ones of the work: ladders and lifts to reach loudspeakers, ceilings and measurement positions, and loudspeakers or acoustic panels that have to be hung. Nothing in this book qualifies you to rig a loudspeaker or to work at height. Those tasks belong to people trained and authorised to do them.

Where this book sits

This book assumes you have read AV Fundamentals 101, or that you already know what is in it.

Experience counts instead. If you have time on audio crews where you have already heard a system sound good in one room and bad in another, and want to know why, start here.

If none of that is true yet, read AV Fundamentals 101 first.

What pairs with this

• The Physics of Sound — the physics beneath the rooms
• AV Math Fundamentals — the decibel, distance and wavelength arithmetic this book leans on

Where it leads

These are not steps in an order. Take whichever one fits.

• Live Sound Production — putting systems into rooms, show after show
• Systems Design — designing systems for the rooms they live in

Before You Begin

Why the Room Matters

Sound doesn't travel straight to the ear — it fills the room first, and the room changes it.

The room is part of the system

When a speaker produces sound, only some of it travels straight to a listener's ears — the direct sound. The rest radiates around the room, bouncing off walls, floor, ceiling, and surfaces, and arrives at the listener slightly later as reflections. What a person actually hears is the combination of the direct sound and all these reflections, which means the room itself profoundly shapes the sound. A brilliant audio system in an acoustically poor room delivers poor sound to the audience, because the room mangles it after it leaves the speakers. The room is, in effect, part of the sound system.

This is why acoustics matters to every AV professional, not just acousticians. The best microphones, console, and speakers cannot overcome a room that smears speech into unintelligibility or booms uncontrollably. Understanding that the space is an active participant in the sound — not a neutral container — changes how you think about every audio decision, from speaker choice and placement to whether a room even needs treatment to work.

Sound behaves in a space

Sound interacts with a room in three fundamental ways, and these underpin all of acoustics. It reflects — bouncing off hard surfaces (bare walls, glass, concrete) much as light bounces off a mirror. It absorbs — soft, porous surfaces (curtains, carpet, soft seating, acoustic materials, and people) soak up sound energy rather than reflecting it. And it reverberates — in a room, sound reflects off many surfaces repeatedly, and these reflections pile up and persist, gradually dying away. Reflect, absorb, reverberate: these three behaviors determine how any space sounds.

The balance of these is what makes a room sound live (lots of hard surfaces, strong reflections and reverberation — like an empty gym or a cathedral) or dead (lots of absorption, few reflections — like a heavily furnished, carpeted room). Neither extreme is universally good; the right balance depends on the use. Understanding reflection, absorption, and reverberation is the foundation for understanding and fixing any acoustic situation.

Direct sound, early reflections and late reflections

What reaches a listener arrives in three parts, and each affects what they hear differently.

The direct sound travels straight from the loudspeaker or the talker to the ear. It arrives first, and it carries the clearest version of the sound.

Early reflections arrive shortly after, having bounced once or twice off nearby surfaces — the floor, a side wall, the ceiling above the stage. When they arrive within roughly the first fifty milliseconds, hearing fuses them with the direct sound. They make speech sound louder and fuller, and in moderation they help.

Late reflections and reverberation arrive after that, having travelled further and bounced many times. They no longer fuse with the direct sound. They blur it, and for speech they are the enemy of clarity. A single strong reflection arriving late enough — more than about fifty milliseconds after the direct sound, which is a path about seventeen metres longer — is heard as a distinct echo, which is the classic slap back from the rear wall of a hall.

So a room is not simply good or bad. The same reflection that helps when it arrives early harms when it arrives late, and the art of acoustics is keeping the helpful ones and controlling the harmful ones.

Critical distance

Close to a loudspeaker, the direct sound is much louder than the reverberation in the room. Move away and the direct sound gets quieter — six decibels for every doubling of distance — while the reverberant sound stays at roughly the same level everywhere in the room. At some distance the two are equally loud. That distance is the critical distance.

Inside it, listeners mainly hear the loudspeaker, and clarity is good. Beyond it, they mainly hear the room, and clarity falls away, especially for speech in a reverberant space. Turning the system up does not help, because it raises the direct and reverberant sound equally.

Critical distance can be estimated. A common approximation is critical distance in metres = 0.057 × √(Q × V ÷ RT60), where Q is the loudspeaker's directivity factor, V is the room volume in cubic metres and RT60 is the reverberation time in seconds. In a 3,600 cubic metre ballroom with a reverberation time of 2.5 seconds, a fairly omnidirectional loudspeaker with Q of 2 has a critical distance of about 3 metres. A directional loudspeaker with Q of 10 pushes it to about 7 metres.

The practical lessons are two of the most important in live sound. Directional loudspeakers aimed at the audience extend the distance at which the loudspeaker dominates the room. And in reverberant rooms, the way to reach listeners beyond critical distance is to bring loudspeakers closer to them — delays and distributed systems — not to turn up the main system.

What arrives at a listener

Critical distance, estimated

Dc ≈ 0.057 × √(Q × V ÷ RT60), with V in cubic metres and RT60 in seconds. An estimate for thinking, not a design.

On the job

A company town hall in a large, hard-walled atrium. The production brought two small, wide-dispersion loudspeakers on stands at the front, and in rehearsal the CEO sounds clear in the front rows and like a muffled announcement at an airport from the tenth row back.

The sound technician's first instinct is to turn up. It gets louder at the back and no clearer, and the front rows start to complain.

Thinking about critical distance, the problem is plain. The atrium is huge and reverberant, and the loudspeakers have very little directivity. Beyond a few metres, listeners hear far more room than loudspeaker, and turning up raises both.

The fix is not level. The production brings in two pairs of small column loudspeakers, which are much more directional vertically and keep sound off the ceiling and upper walls, and adds a delay pair halfway back, timed to the front system. Each listener is now within critical distance of a loudspeaker. The overall level is lower than the loud rehearsal, and the CEO is clear in every row.

How to do it

1. Listen from the back: is it too quiet, or loud but unclear?
2. If it is loud but unclear, suspect listeners are beyond critical distance.
3. Do not simply turn the system up.
4. Aim loudspeakers at the audience and away from reflective walls and ceilings.
5. Consider more directional loudspeakers.
6. Add delay or distributed loudspeakers closer to the listeners, time-aligned.
7. Listen again from the back.

What goes wrong

Practice it

Answer before checking.

Reverberation & Clarity

How long sound lingers in a room makes or breaks intelligibility.

What reverberation is

Reverberation is the persistence of sound in a room after the source stops — the lingering wash of countless reflections gradually dying away. You hear it as the echoey tail in a large hard room, or its near-absence in a heavily treated one. A common measure is reverberation time (often called RT60): roughly how long it takes the reverberation to fade away. A long reverberation time means a live, echoey room; a short one means a dead, controlled room. Reverberation time is a key way acousticians describe and design rooms.

Reverberation is not inherently bad — a concert hall's lush reverberation makes orchestral music glorious, while a recording studio's very short reverberation gives clean, controlled sound. The issue is matching reverberation to use. Too much reverberation in the wrong place is one of the most common acoustic problems, especially for speech, which the next section takes up.

Reverberation and clarity

Reverberation has a direct, critical effect on clarity — especially speech intelligibility, the ability to understand spoken words. In a very reverberant room, each word's reflections linger and overlap with the next word, smearing speech into a muddy, hard-to-understand wash. This is why a cathedral or a large hard-surfaced hall can make announcements unintelligible no matter how loud — the problem is not volume, it is reverberation blurring the words together. For any space where speech must be understood (conferences, classrooms, houses of worship, announcements), controlling reverberation is essential.

Music and speech want different things: speech needs clarity (shorter reverberation so words stay distinct), while some music benefits from more reverberation for richness. A multipurpose space is a compromise or needs adjustable acoustics. The key insight for AV is that intelligibility problems are often acoustic, not electronic — adding more volume to a reverberant room makes it worse, not better, because you are just adding more reflections. Recognizing reverberation as the cause is the first step to actually fixing it.

Estimating reverberation time — the Sabine equation

Reverberation time, RT60, is the time it takes for sound in a room to decay by 60 decibels after the source stops — roughly, from a loud shout to barely audible. It can be measured, as a later chapter shows, and it can be estimated before anybody sets foot in the room.

The classic estimate is the Sabine equation: RT60 = 0.161 × V ÷ A. V is the room's volume in cubic metres. A is the room's total absorption, found by multiplying the area of each surface in square metres by that surface's absorption coefficient — the fraction of sound it absorbs, from 0 for a perfect reflector to 1 for a perfect absorber — and adding them up. In imperial units the constant is 0.049, with volume in cubic feet and areas in square feet.

Take an empty ballroom 30 metres long, 20 wide and 6 high: 3,600 cubic metres. The floor is 600 square metres of carpet with a coefficient of about 0.3, giving 180. The ceiling is 600 square metres of plaster at about 0.03, giving 18. The walls are 600 square metres of hard finish at about 0.05, giving 30. Total absorption is 228, and RT60 = 0.161 × 3,600 ÷ 228 = about 2.5 seconds.

Now hang 150 square metres of heavy drape over part of the walls, with a coefficient of about 0.55 in place of 0.05. That adds 150 × 0.5 = 75, and RT60 falls to about 1.9 seconds. Fill 400 square metres of the floor with a seated audience, with a coefficient of about 0.8 in place of the carpet's 0.3, and it adds another 200: RT60 falls to about 1.15 seconds.

The numbers are approximations, and coefficients vary with frequency, so real rooms are measured at several frequencies. But the calculation shows immediately what matters: volume raises reverberation, absorption lowers it, and the room at sound check is not the room at the show.

Reverberation targets by use

There is no single right reverberation time. The right one depends on what the room is for, and to a lesser extent on its size.

Rooms for speech want short reverberation, so each word decays before the next arrives. Conference rooms and meeting rooms work best at well under a second, often around 0.4 to 0.6 seconds. Classroom acoustics standards set limits too; the American classroom standard, for example, calls for no more than about 0.6 seconds in ordinary-sized classrooms, along with a background noise limit. Lecture halls and drama theatres work well from about 0.7 to around 1.2 seconds.

Rooms for amplified music — concert venues for bands, clubs — generally want it short too, because the sound system provides the character and too much reverberation muddies the low end.

Rooms for unamplified music want more. Concert halls for orchestral music are often around 1.8 to 2.2 seconds, and large churches and cathedrals far longer, which suits choral music and organ and makes speech very hard.

Multipurpose rooms, hotel ballrooms and convention halls are compromises, and most AV work happens in them. Knowing the target for the event tells you which way the room needs pushing, and by how much.

Speech intelligibility and the speech transmission index

Intelligibility — how well speech can be understood — can be measured, and it is increasingly specified for conference rooms, lecture halls, transport terminals and emergency voice systems.

The most widely used measure is the speech transmission index, STI, defined in the international standard IEC 60268-16. It rates how faithfully a room and a sound system carry the fluctuations in speech that make it understandable, on a scale from 0 to 1. On the standard's scale, below 0.3 is bad, 0.3 to 0.45 poor, 0.45 to 0.6 fair, 0.6 to 0.75 good, and above 0.75 excellent. A shortened version for sound systems, STIPA, is what most handheld intelligibility meters measure.

Three things lower STI, and each has its own fix. Reverberation and late reflections smear speech in time; the fixes are absorption and directional loudspeakers aimed at listeners. Background noise masks speech; the fixes are reducing noise and raising speech well above it. And distortion or poor frequency response in the system; the fix is the system itself.

For a technician, the value of STI is that it turns an argument about whether a room is clear into a number, measured before and after a change.

The Sabine equation

RT60 = 0.161 × V ÷ A (metric), or 0.049 × V ÷ A (imperial). A = the sum of each surface's area × its absorption coefficient.

Reverberation targets

Approximate mid-frequency targets. Consultants and standards give exact values for specific rooms.

Speech transmission index

On the job

A client books a hotel ballroom for a two-day conference with panel discussions and asks why last year's attendees complained they could not follow the speakers, when the AV company had a large, expensive PA.

Before the site visit, the technician estimates. The ballroom is about 3,600 cubic metres with hard walls, a plaster ceiling and carpet, and the Sabine estimate for the empty room is about 2.5 seconds, far over the half second or so that speech wants. On site, a clap confirms a long, smooth decay.

Last year's PA had been two large loudspeakers on the stage pointing down the length of the room, into a mirrored rear wall.

The proposal changes the approach rather than the budget: heavy drape along the rear wall and part of the side walls, the stage moved to the long wall so the audience sits closer to it, column loudspeakers aimed down onto the seats, and a delay pair for the rear tables. A handheld STIPA meter, brought on the first morning, reads 0.42 from the back tables with the old configuration and 0.61 with the new one, in the full room. Nobody complains about following the panels.

How to do it

1. Measure or estimate the room's length, width and height, and calculate the volume.
2. List each surface and its area.
3. Look up a typical absorption coefficient for each surface's material, around 500 Hz to 1 kHz.
4. Multiply each area by its coefficient and add them up for total absorption.
5. Calculate RT60 = 0.161 × volume ÷ total absorption.
6. Recalculate with the audience and any drape or treatment you plan to add.
7. Compare with the target for the event, and confirm with a measurement on site.

What goes wrong

Practice it

Answer before checking.

Absorption, and Why Frequency Matters

Absorption is the most common acoustic treatment and the most commonly misunderstood. A thin foam panel that makes a room sound dramatically less echoey to a clap may do almost nothing to the bass that is really muddying the mix. A room full of drape can sound dull and boomy at once.

The reason is that absorption depends on frequency, and on the size of the absorber compared with the wavelength of the sound. This chapter covers absorption coefficients and NRC ratings, how porous absorbers work and why thickness matters, how low frequencies are controlled, and the absorber in every room that nobody installs: the audience.

Absorption coefficients and NRC

A material's absorption coefficient is the fraction of sound energy striking it that it absorbs rather than reflects, from 0 to 1. Painted concrete and glass sit close to 0 at most frequencies; a thick acoustic panel sits close to 1 in the mid and high frequencies. The coefficient in sabins per square metre is what goes into the Sabine equation.

Coefficients are measured at a series of frequencies, usually in octave bands from 125 Hz to 4 kHz, and they differ a great deal across them. Carpet absorbs high frequencies well and bass hardly at all. A thin wooden panel on a frame can absorb some bass and reflect highs.

For a quick comparison, manufacturers often give a single number, the noise reduction coefficient, NRC: the average of the coefficients at 250, 500, 1,000 and 2,000 Hz, rounded to the nearest 0.05. It is useful for comparing products for speech rooms, and it says nothing at all about low frequencies, which is why two materials with the same NRC can behave very differently in a room with a bass problem.

Porous absorbers, thickness and bass

Most acoustic panels, drapes, carpets, mineral wool and foam are porous absorbers. Sound travels into the pores, and friction in the fibres turns some of its energy into heat.

A porous absorber works best where the air particles are moving fastest, and next to a hard wall they are barely moving at all: the fastest movement is about a quarter of a wavelength away from the wall. For high frequencies, a quarter wavelength is a few centimetres, so a thin panel on a wall works. For low frequencies it is large — a quarter wavelength at 100 Hz is about 86 centimetres — so a 5 centimetre panel on a wall does very little below a few hundred hertz.

This gives two practical rules. Thicker porous absorbers, and absorbers spaced away from the wall with an air gap behind them, absorb lower frequencies. And thin treatment alone changes a room's high frequencies far more than its lows, which can leave a room sounding dull and still boomy.

Bass traps are absorbers built to reach lower: very thick porous absorbers, often placed across corners, where low-frequency pressure builds up, or resonant absorbers designed for low frequencies.

Resonant absorbers

Low frequencies can also be absorbed by structures that resonate.

A membrane or panel absorber is a panel of thin wood, plasterboard or similar mounted over a sealed air space. It flexes at a resonant frequency set by its mass and the depth of the air space, and absorbs sound in a band around that frequency. Some of the bass absorption in a room with lightweight walls and ceilings comes from exactly this, unintentionally.

A Helmholtz resonator is a cavity with a narrow neck or opening — the effect you hear blowing across a bottle. Perforated or slotted panels over a cavity act as many small resonators and absorb a band set by the size of the holes, the panel thickness and the depth of the cavity.

Resonant absorbers are tuned for particular frequency bands, which makes them powerful tools for acousticians treating a specific problem, and not something a technician improvises for a show.

The audience is an absorber

The biggest absorber in most event rooms walks in through the doors. A seated audience, in clothing, in upholstered seats, absorbs a great deal of mid and high frequency sound, and empty hard seats absorb very little.

This is why a sound check in an empty room misleads. The empty room is more reverberant and brighter than the full room will be. A system tuned to sound right empty often sounds dull and quiet when full, and a room that seems hopelessly echoey at 7 a.m. can be manageable by 9 a.m. with a thousand people in it.

Upholstered seats help, because they absorb much like an audience whether they are occupied or not, which is one reason good theatres and halls change less between empty and full.

The practical habits are to estimate the room full as well as empty, to expect the system to need adjusting once the audience is in, and to check early in the event rather than trusting the sound check.

Typical absorption coefficients

Rough values for comparison. Real products and constructions vary, and coefficients are measured in octave bands.

Quarter wavelength at a hard wall

• 100 Hz — about 86 cm.
• 250 Hz — about 34 cm.
• 1 kHz — about 8.6 cm.
• 4 kHz — about 2 cm.

On the job

A podcast studio has been built in a small office. The owner has covered two walls with 25 millimetre foam tiles, and the hosts sound close and dry — and muddy, with a boom on male voices that the editor keeps fighting with EQ.

The foam has done exactly what thin porous absorption does. It has removed most of the room's high-frequency reflections, which is why the voices sound dry, and it has done almost nothing below a few hundred hertz. The small room's low-frequency build-up is untouched, and with the highs gone it is more obvious than before.

The technician recommends thick porous absorbers, 100 millimetres deep with an air gap behind, across the room's corners, and moving the hosts' table away from the centre of the room, where the bass builds up. A measurement before and after shows the decay below 200 Hz roughly halved. The editor stops cutting 150 Hz on every episode.

How to do it

1. Describe the problem by frequency — harsh, echoey highs, or muddy, boomy lows.
2. For mid and high frequency reflections, use porous absorption at the reflecting surfaces.
3. For low frequencies, use thick porous absorption with an air gap, corner bass traps, or have an acoustician specify resonant absorbers.
4. Check the absorption coefficients across frequency, not only the NRC.
5. Remember the audience will add mid and high frequency absorption.
6. Measure or listen before and after.

What goes wrong

Practice it

Answer before checking.

Common Room Problems

Rooms go wrong in recognizable ways. Naming the problem is the first step to solving it.

Too live, and uneven bass

A few problems recur across spaces. The most common is a room that is too live (too reverberant) — hard surfaces, no absorption, so sound echoes and speech blurs; common in gyms, atriums, glass-walled rooms, and empty halls. The fix is adding absorption. A second, sneakier problem is uneven bass caused by standing waves (also called room modes): at certain low frequencies, reflections between parallel surfaces reinforce or cancel, making bass boomy in some spots and weak in others. This is why bass can sound great in one seat and muddy in another — the room itself is shaping the low end unevenly.

These problems are about how the room's size, shape, and surfaces interact with sound. A room too live needs absorption; standing-wave problems relate to room dimensions and parallel surfaces and are harder to fix (requiring bass-specific treatment or placement choices). Recognizing the symptoms — echoey and unintelligible (too live), or boomy and uneven bass (standing waves) — points you toward the cause and the fix.

Noise and other issues

Beyond reflections, noise is a major acoustic problem: unwanted sound intruding into a space — HVAC rumble, traffic and street noise, sound bleeding between rooms, equipment hum. Noise raises the background level a system must overcome and harms clarity and comfort. Controlling it is about isolation (keeping unwanted sound out, or contained) and quieting noise sources. A room with a high noise floor fights every AV system in it, so noise control is a real part of making a space work acoustically.

Other issues include flutter echo (a distinct repeating echo bouncing between two hard parallel surfaces — clap in an empty hard-walled room and you may hear it), and simple uneven coverage where some areas hear well and others poorly. Each problem has acoustic causes and acoustic solutions. The skill at this level is recognizing these problems by their symptoms — too live, boomy bass, intrusive noise, flutter echo — so you understand what a room needs, which leads to treatment.

Room modes, calculated

Between two parallel surfaces, sound at certain frequencies reflects back and forth and reinforces itself, forming a standing wave: a pattern of places where that frequency is very loud and places where it almost disappears. These are room modes, and in most rooms they are the reason bass is boomy in one seat and thin in the next.

The simplest modes, axial modes, form between a single pair of opposite surfaces, and their frequencies can be calculated. The first axial mode between two surfaces is f = c ÷ (2 × L), where c is the speed of sound, about 343 metres per second, and L is the distance between them in metres. Further modes fall at two, three and four times that frequency.

Take a meeting room 10 metres long, 6 metres wide and 3 metres high. Along its length the modes fall at 17.2, 34.3, 51.5 and 68.6 Hz. Across its width, at 28.6, 57.2 and 85.8 Hz. From floor to ceiling, at 57.2 and 114 Hz. The width's second mode and the height's first mode both fall at 57.2 Hz, so that frequency is reinforced twice, and a male voice or a kick drum near 57 Hz will boom in this room.

Modes matter most in small rooms and at low frequencies, below a transition point sometimes called the Schroeder frequency; above it, so many modes and reflections overlap that the room behaves more evenly. That is why meeting rooms, studios and small venues have far worse bass problems than large halls.

Modes are controlled by room proportions, by moving loudspeakers and listeners away from the loudest and quietest points — the middle and the walls of a room are often the worst places — by bass traps, and by careful equalisation at particular positions, not by turning the bass down everywhere.

Flutter echo, echoes and focusing

Flutter echo is a rapid, rattling repeat heard between two hard, parallel surfaces — clap between two bare walls in a corridor or an empty rehearsal room and you hear it buzz. It is caused by sound bouncing back and forth between the same pair of surfaces, and is fixed by absorbing or diffusing one of them, or by angling one surface slightly.

A discrete echo is a single strong reflection arriving late enough to be heard separately, typically from a rear wall facing the stage, a balcony front or a glass wall. Loudspeakers aimed at such a surface make it worse.

Focusing comes from concave surfaces — domes, curved rear walls, barrel-vaulted ceilings — which gather reflections to a point the way a curved mirror focuses light. Listeners near that point hear unnaturally loud reflections, sometimes seconds of rolling echo, while others nearby hear little. Convention centres with curved ceilings and ballrooms with domes are common offenders. Aiming loudspeakers away from the curved surface, and treating it where possible, are the practical fixes.

Comb filtering and boundary interference

When a sound and a single strong reflection of it arrive at a microphone or a listener close together in time, they partly cancel at some frequencies and reinforce at others. Drawn on a graph, the response has a regular series of deep notches, like the teeth of a comb, which is why it is called comb filtering.

The notches fall at frequencies set by the time difference. A reflection with a path one metre longer than the direct sound arrives about 2.9 milliseconds late, and the first notch falls at about 172 Hz, with more every 343 Hz above it. The result sounds hollow and phasey, and it changes as the source or the listener moves.

Two situations cause it constantly in AV. A microphone near a hard surface — a lectern top, a table, a glass screen — picks up the direct sound and a reflection a few centimetres later, colouring the voice; boundary microphones mounted flat on the surface avoid it. And a loudspeaker or subwoofer near a wall interferes with its own reflection: a subwoofer one metre in front of a wall produces a deep cancellation around 86 Hz, where the reflected path is half a wavelength longer. Moving it right against the wall, or much further away, shifts or removes the problem.

EQ cannot fix comb filtering, because the notches move with position; the fix is geometry.

Room modes

First axial mode f = 343 ÷ (2 × L). Further modes at 2×, 3×, 4× that frequency.

Recognising the problem by ear

On the job

A company's new boardroom, 10 by 6 by 3 metres, with a video bar at one end and a large table in the middle. The chair of the board, who sits halfway down the table, says remote participants sound "boomy and hard to understand", while the people at the ends of the table find them fine.

The technician calculates the room's axial modes and finds two of them coinciding at 57 Hz, from the width and the height. The middle of a room is where even-numbered modes along its width peak — the 57 Hz width mode is the second — and the chair's seat is almost exactly there. A measurement microphone at the chair's position shows a large peak around 57 Hz that is much smaller at the ends of the table.

Turning the bass down everywhere would thin out every other seat. Instead, the integrator adds two thick corner bass traps, and applies a narrow cut at 57 Hz in the DSP that feeds the room loudspeakers, checked at several seats rather than just one. The chair hears the remote participants clearly, and the ends of the table notice no change.

How to do it

1. Measure the room's length, width and height.
2. Calculate the first few axial modes for each dimension.
3. Note frequencies where modes from different dimensions coincide.
4. Listen, or measure, at several seats with speech or music with bass.
5. Move loudspeakers or key seats away from the room's centre and its walls where possible.
6. Add bass trapping in corners.
7. Use narrow EQ cuts only after checking several positions.

What goes wrong

Practice it

Answer before checking.

Noise and Isolation

Everything so far has been about the sound a system puts into a room. Noise is the sound that is already there — air conditioning, traffic, the kitchen behind the ballroom wall, the band in the next hall — and isolation is how rooms keep sound in or out.

A room's background noise sets how loud speech must be to be understood. A room's isolation decides whether the keynote can be heard over the neighbouring trade show, and whether the neighbouring trade show can hear the keynote. This chapter covers background noise and how it is rated, signal-to-noise for speech, isolation ratings and how walls block sound, and the flanking paths that let sound round a good wall.

Background noise and NC ratings

Background noise is the sound in a room when nothing is happening in it: ventilation, lights, equipment fans, traffic, neighbouring spaces. It is measured with a sound level meter, usually in A-weighted decibels, and for rooms with steady mechanical noise it is often rated against noise criterion curves, which give a single NC number that accounts for how noise is spread across frequency.

Lower is quieter. Recording studios and concert halls aim for very low figures, around NC 15 to 20. Theatres and large meeting rooms aim for about NC 20 to 30, and classrooms and conference rooms around NC 25 to 30. Open offices and lobbies are typically NC 35 to 45.

Ventilation is the most common source in AV spaces. It produces a broad rushing noise from air moving through ducts and grilles, and a low rumble from fans and plant. It often cannot be switched off during events, and in hotels it may be controlled by someone who is not in the room.

Noise that varies — traffic, a kitchen, a neighbouring event — is harder to rate and often more disturbing, because it draws attention.

Signal-to-noise for speech

Speech needs to be clearly louder than the background noise to be understood without effort. A common working guide is that speech should be at least 10 decibels above the background noise, and around 15 decibels above it for good intelligibility, measured where the listeners sit.

In a room with a background of 45 dBA, that means speech at around 60 dBA at every seat, which is ordinary conversational level at close range and needs a sound system in any large room. With a background of 60 dBA — a busy trade show floor, a noisy restaurant — speech must reach about 75 dBA, loud enough that it becomes tiring and pushes the system towards feedback.

This is why reducing noise can do more for intelligibility than adding level. A ventilation system turned down for a keynote, a kitchen door closed, a neighbouring session asked to lower its level can transform a room that no amount of loudspeaker could fix.

Isolation, STC and how walls block sound

Isolation is keeping sound from passing between spaces. It is a different job from absorption: absorption controls sound inside a room, isolation stops it getting through the walls, floor and ceiling. An acoustic panel on a wall does almost nothing to stop sound reaching the next room.

The isolation of a wall or partition is rated by its sound transmission class, STC, in North America, and by similar ratings such as Rw elsewhere. Higher numbers isolate better. A basic single stud wall with one layer of plasterboard on each side is around STC 33 to 35: normal speech through it is audible, and loud speech audible but hard to make out. Adding insulation and a second layer of board raises it into the 40s. Double walls with separate studs, insulation and several layers can exceed STC 55, where even very loud sounds are only faintly heard.

Three things make walls isolate. Mass: heavier walls block more, and in theory doubling a wall's mass adds about 6 decibels of isolation, though real gains are usually smaller. Airtightness: sound passes through any gap, and a small crack or an unsealed cable penetration can undo a heavy wall. Decoupling: separating the two sides of a wall so vibration cannot pass straight through, as double-stud and resilient-mount constructions do.

STC is based on speech frequencies, so it underrates the problem of bass. A wall with a good STC can still pass the thump of a subwoofer from the next hall, which is why airwalls between hotel function rooms — typically far lower in performance than a real wall — are a constant source of complaints during events.

Flanking paths

Sound does not only go through a wall. It goes around it, by flanking paths: over a suspended ceiling that runs continuously above the wall, through ventilation ducts that connect both rooms, under and around doors, through back-to-back electrical boxes, along a continuous floor or structure, and through the gaps at the ends and top of a movable airwall.

Flanking is why a wall with an excellent rating on paper can perform badly in a building, and why the complaint from the room next door often comes through the ceiling or the air vents rather than the wall itself.

For a technician, flanking is mostly diagnosis: when sound is coming from a neighbouring room, listen along the ceiling, at grilles and at door edges to find where it is really coming through. Fixing it is building work for the venue and its consultants, and the practical show-day remedies are scheduling, level agreements between neighbouring rooms, closing doors and keeping loudspeakers away from the shared wall.

Background noise targets

Typical NC ratings. Standards and consultants give exact values for specific rooms.

What an STC rating sounds like

A rough guide to hearing speech through a partition.

What makes isolation

• Mass — heavier partitions.
• Airtightness — no gaps, sealed penetrations.
• Decoupling — the two sides not rigidly connected.

Common flanking paths

• Over a continuous suspended ceiling.
• Through shared ventilation ducts.
• Under and around doors.
• Through the gaps at the ends of airwalls.

On the job

A hotel has divided its grand ballroom with an airwall into two function rooms: a pharmaceutical company's quiet scientific symposium in Salon A, and an awards lunch with a live band in Salon B. Ten minutes into the lunch, the symposium's organiser comes to the AV desk: every drum hit comes through the wall.

There is no building fix available by 1 p.m. The technician walks the dividing wall with the organiser. Most of the sound is coming through the gaps at the top of the airwall, where it meets the ceiling track, and through the shared supply duct above the wall. Bass is coming through everything.

The practical fixes are about the band side. The awards lunch's front of house engineer agrees to move the band's subwoofers from beside the airwall to the far end of Salon B, aim the main loudspeakers away from the wall, and hold the level at an agreed maximum during the symposium's morning sessions, with the loud set moved to after the symposium finishes at 3 p.m. The hotel's events manager notes the problem for booking the two rooms together again.

How to do it

1. Measure the background noise with the room as it will be — ventilation running, doors as they will be.
2. Note the sources — ventilation, equipment, kitchens, traffic, neighbouring rooms.
3. Estimate the speech level needed — at least 10 dB, preferably 15 dB, above the background at listener positions.
4. Ask whether any noise sources can be reduced during sessions.
5. Find out what is happening in neighbouring rooms, and walk shared walls and airwalls.
6. Keep loudspeakers and subwoofers away from shared walls.
7. Agree levels and schedules with neighbouring rooms where needed.

What goes wrong

Practice it

Answer before checking.

Treating a Space

Three tools shape a room's sound: absorption, diffusion, and isolation.

Absorption and diffusion

Acoustic treatment improves how a room sounds, and two key tools shape reflections. Absorption uses soft, porous materials (acoustic panels, curtains, carpet, soft furnishings) to soak up sound energy and reduce reflections and reverberation — the primary fix for a too-live, echoey room. Adding absorption shortens reverberation time and improves clarity, which is why treating a reverberant conference room with acoustic panels can transform speech intelligibility. Absorption is the most common acoustic treatment.

Diffusion uses surfaces that scatter sound in many directions rather than reflecting it as a strong, distinct echo. Instead of removing sound energy (like absorption), diffusion spreads it out evenly, reducing problem reflections and flutter echo while keeping the room from going completely dead. Good acoustic design often balances absorption and diffusion — absorbing enough to control reverberation, diffusing to keep the sound even and natural. Understanding these two tools explains most acoustic treatment you will see.

Isolation and treatment in practice

The third tool, isolation, addresses noise: physically blocking sound from entering or leaving a space — through heavier construction, sealing gaps, isolating noisy equipment, and treating the paths sound travels between spaces. Isolation is different from absorption (which controls sound within a room); isolation keeps sound from getting between rooms or in from outside. A space with a noise problem needs isolation, not just absorption, to lower its background noise level.

In practice, treating a space means diagnosing its problems (too live? boomy? noisy?) and applying the right combination: absorption to tame reverberation, diffusion to even out reflections, isolation to control noise. Often modest, well-placed treatment makes a dramatic difference — a few absorptive panels on the worst reflective surfaces can rescue a room's intelligibility. You do not need to be an acoustician to grasp this: understanding which tool addresses which problem lets you recognize what a room needs and have an informed conversation about fixing it.

Where treatment goes

The same amount of absorption does far more in some places than in others. Treatment is placed where the problem reflections happen.

First reflection points are the places on side walls, the ceiling and the floor where sound from a loudspeaker reflects once on its way to the listeners. In a room with a problem of early reflections — a meeting room, a studio, a control room — these are the first places to treat. They can be found simply: sit at a listening position while someone slides a mirror along the wall; where you can see the loudspeaker in the mirror is a first reflection point.

The rear wall facing a stage or a loudspeaker returns a strong late reflection towards the stage and the front rows, and is often the cause of echoes and of microphones picking up the system. It is a prime place for absorption or diffusion.

Parallel hard surfaces that cause flutter need only one side treated, or treated in part.

Corners are where low-frequency pressure builds up, which is why bass traps go there.

And large glass walls and windows, which reflect most sound and cannot be covered with panels, can be covered with heavy curtains or drape during an event.

Temporary treatment for events

Most AV acoustics happens in rooms nobody is allowed to change: hotel ballrooms, convention halls, marquees, historic buildings. Temporary treatment is how events improve them.

Pipe and drape is the most useful tool. Heavy velour drape absorbs mid and high frequencies well, and hung with fullness — pleated rather than stretched flat — and a little way off the wall, it absorbs more and reaches lower. Drape across a reflective rear wall, a glass wall or the sides of a stage can change a room dramatically. It absorbs little bass.

Carpet and floor coverings reduce high-frequency reflections from hard floors and footstep noise. Soft stage skirting and fabric-covered set pieces help at the front. Upholstered seating, where the event can choose it, absorbs even when empty.

Layout is treatment too. Turning a long room so the stage is on a long wall brings the audience closer to the loudspeakers. Keeping the stage away from a glass wall, and keeping microphones and loudspeakers away from reflective surfaces, prevent problems that no drape could fix later.

And the audience remains the biggest absorber of all, so plan for the full room.

Where treatment works hardest

On the job

A product launch in a converted warehouse with a concrete floor, brick walls and a glass wall along one side. On the first site visit, a clap produces a long, bright decay and a flutter between the brick end walls.

The production has no permission to fix anything to the building. The plan uses everything temporary: heavy velour drape with generous fullness along the glass wall and across the rear brick wall, carpet over the concrete in the audience area, soft skirting on the stage, and the stage turned to the long brick wall, so no seat is more than 18 metres from it. Column loudspeakers aimed at the seats keep sound off the ceiling.

Measured on the day with the drape and carpet in, the empty room's reverberation has fallen from about 2.8 seconds to about 1.6, and the flutter has gone. With the audience in, the presentation is clear, and the warehouse still looks and feels like a warehouse, which is what the client wanted.

How to do it

1. Visit or study the room and note hard surfaces, glass, parallel walls and curves.
2. Estimate the reverberation time and compare it with the event's needs.
3. Plan drape with fullness on the rear wall, glass and worst reflective surfaces.
4. Add carpet or floor covering over hard audience areas.
5. Lay out the room to bring the audience closer and keep the stage off glass.
6. Aim loudspeakers at the audience and away from treated and untreated surfaces alike.
7. Measure or listen after installation, and again with the audience in.

What goes wrong

Practice it

Answer before checking.

Measuring a Room

Ears are the first measurement tool and the last judge, and they are easily fooled: by fatigue, by expectation, by the difference between a room at 7 a.m. and at 7 p.m. Measurement turns a room's behaviour into numbers that can be compared before and after a change, between seats, and between one visit and the next.

This chapter covers the sound level meter and its weightings, measuring reverberation and response with a measurement microphone and software, measuring background noise and intelligibility, and how to take measurements that mean something.

Sound level meters and weightings

A sound level meter measures sound pressure level in decibels. Phone apps can give rough readings; calibrated meters to a recognised class are used for anything that must be relied on, such as noise limits and exposure.

Weightings adjust the measurement across frequency. A-weighting, written dBA, reduces the contribution of low and very high frequencies to approximate how the ear responds at moderate levels. It is used for background noise, environmental noise limits and hearing exposure. C-weighting, dBC, is much flatter and includes low frequencies, so it is used for loud music, subwoofer-heavy shows and peak levels. Z-weighting is unweighted.

Time weighting sets how quickly the reading follows changes: fast, about an eighth of a second, and slow, about one second. For varying sound, the equivalent continuous level, Leq — often LAeq or LCeq — gives the average energy over a period, such as fifteen minutes of a show, and is how most event noise limits are written.

So a reading always needs its weighting, its time response and where it was taken: "96 dBA slow at front of house" and "106 dBC Leq over 15 minutes at the barrier" mean very different things.

Measuring reverberation and response

Reverberation time can be measured roughly with an impulse — a balloon burst, a clapperboard or a starter pistol — recorded and analysed for how long its decay takes, or by switching off loud pink noise and watching the decay. Rooms rarely allow a clean 60 decibel decay above the background, so software measures the decay over a smaller range, such as 20 or 30 decibels, and extrapolates, reported as T20 or T30.

The more accurate method uses a measurement microphone — an omnidirectional microphone with a known, flat response — and software that plays a swept sine tone or noise through a loudspeaker and records the result. From that it calculates the room's impulse response, and from the impulse response it derives reverberation time in each octave band, early decay, clarity measures, the frequency response at that position, and speech intelligibility.

Live sound engineers use related dual-channel software to compare what goes into a loudspeaker with what the measurement microphone hears, which shows the system's response in the room and is how systems are aligned and equalised.

Measurements are only as good as the method. Use several microphone positions across the audience area rather than one, keep the microphone away from surfaces, keep the room quiet and as it will be during the event, repeat measurements to check they agree, and note every setting so that a measurement taken tomorrow can be compared with today's.

Measuring noise and intelligibility

To measure the background noise, switch off the sound system and anything the event will not have running, leave running everything it will — ventilation, lights, projectors — and measure with the meter at listener positions over a minute or more, noting the LAeq and, where needed, the octave band levels for an NC rating. Measure at the time of day the event will happen, because buildings are much noisier during business hours.

Speech intelligibility is measured with an STI or STIPA meter: a test signal that imitates the fluctuations of speech is played through the sound system at speech level, and a meter at each listener position reports the STI. Measuring several positions, and measuring with and without background noise sources running, shows both the worst seats and whether the cause is reverberation or noise.

Record each measurement with its position, the date, the time, the room condition — empty or full, drape in or out, ventilation on or off — and the equipment used. A number without its conditions cannot be compared with anything.

Weightings and what they are for

What a measurement system can report

Every measurement records

• Position.
• Date and time.
• Room condition — empty or full, treatment, ventilation.
• Weighting and time response.
• Equipment and settings.

On the job

A university asks for evidence that the new treatment in its 400-seat lecture theatre has worked, because students still say some seats are hard to hear from.

The technician measures at eight seats spread across the theatre, with the ventilation running as it does during lectures. Reverberation time at mid frequencies is now about 0.9 seconds everywhere, down from about 1.5 before treatment, according to last year's report. STIPA readings are 0.62 to 0.66 at six seats and 0.48 at the two seats under the balcony overhang.

Background noise at those two seats is 8 dB higher than elsewhere: they sit beneath a ventilation outlet. The treatment worked; the complaint is noise at two seats.

The university's facilities team reduces the airflow at that outlet during lectures, and a remeasurement a week later shows 0.61 at both seats. The report goes to the university with every reading, position and condition, so next year's measurement can be compared directly.

How to do it

1. Set up the measurement microphone, interface and software, and calibrate the level if needed.
2. Choose several positions across the listening area, away from surfaces.
3. Set the room to event conditions — ventilation, lights, doors, treatment.
4. Measure background noise with the system off.
5. Run sweeps or test signals at each position for reverberation and response.
6. Measure STI where intelligibility matters.
7. Repeat a position to confirm results agree.
8. Record each result with its position, time, conditions and settings.

What goes wrong

Practice it

Answer before checking.

Acoustics & AV Decisions

Understanding the room makes you better at every audio decision in it.

Acoustics shapes AV choices

Acoustic awareness improves countless AV decisions. Speaker choice and placement depend on the room — a reverberant space may need more directional speakers aimed to minimize reflections off hard surfaces and put sound on the audience rather than the walls. The expectations for a system are set by the room — you cannot achieve studio clarity in an untreated gym, and recognizing that early sets realistic goals and prompts treatment where it is needed. Even mixing decisions account for the room, as a live engineer must work with the space's character. The AV professional who understands acoustics makes smarter choices throughout.

It also helps you communicate and solve problems. When a client complains they can't understand the announcements, you can recognize an acoustic (reverberation) problem rather than just turning up the volume uselessly, and explain that the room needs treatment. When designing or repairing a system, you account for the room as part of the solution. Acoustic understanding turns mysterious sound problems into diagnosable, addressable ones — a genuinely valuable professional capability.

Listening to rooms

The best way to build acoustic intuition is to listen critically to the rooms you are in. Notice how a space sounds: is it echoey or dead, can you understand speech easily, is there intrusive noise, does a clap produce a flutter echo. Over time you develop an ear for acoustics — walking into a room and sensing its character and likely problems, which is enormously useful in AV. This listening, combined with the concepts from this book, builds practical acoustic judgment that pure theory cannot.

This book gives you the foundation — how sound behaves in rooms, the problems, and the treatments — which is part of the broader knowledge (alongside electrical, codes, and AV math) that underpins professional AV and certifications like the CTS. Deeper acoustics is its own specialty, but every AV professional benefits from this fundamental understanding of the spaces their systems live in. Master it, and you will never again be surprised that a good system can sound bad in a bad room — you will know why, and what to do about it.

Directivity, and aiming at the audience

In a reverberant room, where sound goes matters more than how much there is. Every part of a loudspeaker's output that lands on a wall, the ceiling or an empty balcony becomes reverberation; every part that lands on the audience is direct sound, and much of it is absorbed by the audience before it can become reverberation.

A loudspeaker's directivity describes how tightly it concentrates its output, expressed as a directivity factor Q or a directivity index in decibels. A wide, omnidirectional loudspeaker has a low Q and excites the whole room; a line array or a column loudspeaker has high directivity in the vertical plane and can put sound on the seats while keeping it off the ceiling.

So, in live rooms: choose directional loudspeakers, aim them down onto the audience, keep their coverage off reflective walls and ceilings, and avoid aiming anything straight at a hard rear wall. It is the single most effective acoustic decision a sound technician makes, and it costs nothing but thought.

Distributed and delay systems

When listeners are beyond critical distance of the main loudspeakers, the answer is to bring loudspeakers closer to them. That is the idea behind delay loudspeakers in long rooms, under-balcony fills, and distributed ceiling or column systems in conference centres, airports and places of worship.

Each listener then sits close to a loudspeaker, well within its critical distance, and hears mostly direct sound at a lower overall level. The loudspeakers are time-aligned, so their sound arrives in step with the main system, and aimed or chosen so each covers its own area.

In very reverberant spaces, many small, well-aimed loudspeakers at modest level almost always outperform a few large ones at high level.

Coverage uniformity

A system is judged by its worst seat. A room where the front rows are loud and clear and the back corners thin and muddy has failed, however good it sounds at front of house.

Coverage uniformity describes how evenly a system delivers sound across the whole listening area, in level and in frequency response. ANSI/AVIXA A102.01, the audio coverage uniformity standard, gives a method for measuring it across a listener area and classes for how uniform a system is.

The practical habit is to walk the room, listening to the same programme at the front, back, sides, corners, under balconies and near walls, and to measure several positions rather than one. Gaps are filled with aiming, fills and delays, not by turning the main system up.

What EQ can and cannot fix

Equalisation adjusts a system's frequency response, and it is powerful where the problem is the loudspeaker's own response, a consistent build-up that is the same across the audience, or a feedback frequency on a microphone.

It cannot fix reverberation: turning down the frequencies that sound muddy in a reverberant room also removes them from the direct sound, and the room is still reverberant. It cannot fix comb filtering or boundary cancellations, because they change with position and a cut deep enough to fill a notch at one seat is a boost at the next. It cannot fix room modes everywhere, because modes peak and null at different places; a cut for one seat changes every other seat. And it cannot fix noise.

So acoustic problems are solved first with placement, aiming, directivity and treatment, and EQ is used afterwards, carefully, based on measurements at several positions.

Microphones, rooms and gain before feedback

Feedback happens when sound from a loudspeaker reaches a live microphone loudly enough to be amplified again, round and round, until it rings or howls. A reverberant room makes feedback come sooner, because the room returns sound to the microphone from every direction, not only from the loudspeaker in front of it.

Gain before feedback — how much level a system can give a microphone before it rings — improves with every step that increases the ratio of the voice to the room at the microphone. Keep the microphone close to the talker. Use directional microphones pointed at the talker and away from loudspeakers. Keep loudspeakers in front of microphones, aimed away from them and at the audience. Use as few open microphones as possible, because each one adds to the room's pickup. And reduce reverberation where you can.

A presenter who holds a handheld microphone at chest height, in a live room, with loudspeakers either side of the stage, will always feed back sooner than the same presenter with a headset microphone and the loudspeakers moved forward.

Solving acoustic problems in the right order

Geometry and aiming solve most problems; processing is the last step, not the first.

First

• Layout — where the stage, audience and loudspeakers go.
• Aiming — sound on the audience, off walls and ceilings.
• Directivity — the right loudspeaker for the room.
• Distance — delays and distributed systems closer to listeners.

Then

• Treatment — absorption and diffusion where reflections happen.
• Noise reduction — ventilation, doors, neighbours.

Last

• Equalisation, from measurements at several positions.

EQ, room problem by room problem

On the job

A church asks why its new sound system, installed with two large, powerful loudspeakers either side of the altar, is harder to understand than the old one. The building is stone, with a reverberation time of about 3 seconds.

The technician's walk-round finds everything this book describes. The large loudspeakers are wide and aimed down the nave at head height, lighting up the stone walls, the vaulted ceiling and the rear wall. The congregation beyond the first few pews is well beyond critical distance. The pastor's handheld microphone rings whenever he walks forward, because the loudspeakers are behind and beside him.

The recommendation, installed over the following month, replaces the pair with column loudspeakers mounted on the pillars, each aimed down onto its own section of pews and time-aligned to the pulpit. The pastor moves to a headset microphone. The EQ, measured afterwards at eight pews, needs only gentle adjustments.

STI measurements go from 0.38 to 0.58 across the congregation. The system is quieter than before, and for the first time the back pews can follow the sermon.

How to do it

1. Estimate or measure the reverberation time and background noise.
2. Choose directional loudspeakers suited to the room.
3. Place and aim them to cover the audience and avoid walls and ceilings.
4. Add delays or distributed loudspeakers for listeners beyond critical distance.
5. Place microphones close to talkers, behind loudspeakers and away from their coverage.
6. Walk the whole listening area, and measure several positions.
7. Apply EQ last, based on the measurements, never to fight reverberation.

What goes wrong

Practice it

Answer before checking.

Glossary

What the gear actually looks like

Every device named in this book, in one place. Recognition is the whole point of Chapter 4 — use this page to attach a picture to each name before you meet it on a dock at six in the morning.

Audio

Dynamic microphone

Condenser microphone

Wireless handheld

Radio rack

Mixing console

Stage box / snake

Power amplifier

Main loudspeaker

Subwoofer

Stage monitor wedge

In-ear monitors

DI box

Video

Camera

Video switcher

Scaler / processor

Projector

LED wall panel

Confidence monitor

Lighting & control

Conventional fixture

Moving-head fixture

Lighting console

Dimmer

Show controller

Media server

Networking & infrastructure

Network switch

Patch bay

Equipment rack

Power distribution

Cable tester

A properly coiled cable

Connectors, close up

XLR

TRS / TS ¼-inch

SDI on BNC

HDMI

Cat5e / Cat6 (RJ45)

Fiber optic

DMX (5-pin XLR)

AC power — not signal