Auditorium Acoustics & Sound Systems
Reverberation control, where absorption actually belongs, and a sound system sized to the room - so the back row can hear the principal without the front row being deafened.
Acoustic Design Targets
Working targets for a school hall. Sound system design follows from these, not the other way round.
| Parameter | Target for a speech-led school hall | Note |
|---|---|---|
| Mid-frequency reverberation time, occupied | 0.8 - 1.2 seconds | Shorter in smaller halls. Music-first halls are designed considerably longer |
| Room volume per seat | 4 - 6 cu m | Set at the architectural stage; determines what RT is achievable at all |
| Rear wall treatment | Effectively all of it | The surface that causes slap-back echo back to the stage and front rows |
| Side wall treatment | First reflection zone, roughly one third to half way back | Breaks up the reflections that smear consonants |
| Front ceiling | Leave reflective | Early reflections here help carry the voice to the back rows |
| Total absorptive coverage | Typically 25 - 40% of wall and ceiling area | Placed where it works, not spread evenly |
| Background noise from HVAC | As low as the plant selection allows | A noisy air handler raises the level everyone has to talk over |
Acoustic Materials and Indicative Rates
| Material | Typical NRC | Indicative rate, supplied & installed | Where it is used |
|---|---|---|---|
| 50 mm mineral or glass wool, fabric wrapped | ~0.85 - 0.95 | Rs. 600 - 1,400 / sq ft | Rear wall and first reflection points - the workhorse absorber |
| 25 mm polyester fibre panel | ~0.5 - 0.6 | Rs. 250 - 600 / sq ft | Budget treatment, upper walls, classrooms and seminar rooms |
| Perforated MDF or gypsum on a cavity | Depends on open area and backing | Rs. 500 - 1,500 / sq ft | Where a hard architectural finish is wanted with some absorption |
| Wooden slat panel on absorptive backing | Depends on slat spacing | Rs. 700 - 1,800 / sq ft | Side walls where diffusion as well as absorption is wanted |
| Heavy acoustic drape | Varies with weight and fullness | Rs. 300 - 800 / sq ft | Variable acoustics - draw it for speech, retract it for music |
| Carpet on the seating floor | Low, but useful on footfall | Rs. 90 - 350 / sq ft | Reduces footfall and chair noise more than it changes the RT |
Indicative ex-GST ranges for budgeting. NRC figures are typical laboratory values for the constructions described - the installed performance depends on thickness, density, the air gap behind the panel and the facing material, all of which should be stated in the quotation.
Specify the construction, not just the NRC. A supplier can quote an NRC of 0.9 for a panel type and then install a thinner build tight against the wall with no cavity, which will not deliver it. Write the thickness, the core density in kg per cubic metre, the air gap and the facing into the order.
Sound System by Hall Size
Indicative packages, installed and tuned. The microphone inventory is usually what schools under-specify.
| Hall | Loudspeakers | Control | Microphones | Indicative budget |
|---|---|---|---|---|
| Up to 150 seats | Pair of full-range speakers flanking the stage, 1 monitor | Small digital or analogue mixer, amplifier | 2 wired, 1 wireless handheld, 1 gooseneck | Rs. 1.5 - 4 lakh |
| 150 - 350 seats | Pair of full-range speakers plus subwoofer, 2 monitors | Digital mixer with scene recall, DSP, amplifier | 2 wired, 3 wireless handheld, 2 lapel or headworn, 1 gooseneck | Rs. 4 - 12 lakh |
| 350 - 600 seats | Larger point-source pair or compact line array, subwoofers, delay speakers, 4 monitors | Digital mixer, DSP with delay alignment, multi-channel amplification | 4 wired, 4 wireless handheld, 4 lapel or headworn, 2 gooseneck, choir mics | Rs. 10 - 28 lakh |
| 600 seats and above | Line array with distributed fill, subwoofer array, stage monitor system | Digital console, networked DSP, redundant amplification | Full inventory with antenna distribution for the wireless system | Rs. 22 lakh + |
Indicative ex-GST ranges for budgeting, including installation, cabling and commissioning. The final figure depends on brand tier, the room, and the microphone inventory you actually need.
Fix the Room Before You Buy the System
The most common complaint we are called about is a hall that is painfully loud in the first five rows and unintelligible at the back. Schools usually assume the answer is a bigger amplifier. It almost never is.
Sound level falls off with distance from the source. If a single pair of speakers at the proscenium is the only thing serving a 20-metre-deep hall, the only way to make the back row loud enough is to make the front rows far too loud - and adding power makes both worse in the same proportion. The real fixes are coverage and time. Loudspeakers whose dispersion pattern is chosen and aimed for the actual seating geometry. Where the hall is deep, a second set of speakers part-way down the house, delayed so the sound still appears to arrive from the performer rather than from overhead.
Underneath that sits the room itself. If the reverberation time is long, every reflection arriving late smears the consonants that carry meaning, and the system is fighting the building. An extra kilowatt of amplification simply makes the smearing louder. That is why the sequence matters: measure the room, treat the surfaces causing the audible problem, then size and aim the system into the room you now have. Doing it in the other order is how schools end up with an expensive system that still does not work.
It is also why the seating specification belongs in this conversation. Upholstered auditorium chairs are absorbers, so a fully upholstered house needs less added wall treatment than the same hall with hard moulded seats - and behaves much more consistently between a full house and an empty one, which matters for rehearsals. We size the treatment against the chairs actually being ordered.
What We Do
Room survey
Measure the hall, identify the surfaces causing the audible problem, and establish what reverberation time the volume and shape can actually reach.
Treatment design
Panel type, thickness, density, cavity and placement - sized against the seating being installed, and priced by the square foot so you can see the trade-offs.
System design
Loudspeaker selection, positions and aiming for your seating geometry, delay speakers where the hall is deep, DSP, amplification and microphone inventory.
Installation
Panels mounted, speakers rigged to designed fixings, cabling through the conduit provided, floor boxes and stage connections terminated and labelled.
Tuning and commissioning
Gain structure set, equalisation and delay aligned in the finished room, presets built and locked so the daily assembly is one button.
Handover and training
A one-page operating card and a session with the two or three staff who will actually run the hall, not a manual left on the rack.
Frequently Asked Questions
What does acoustic treatment for an auditorium cost?
Indicative ex-GST rates, supplied and installed: fabric-wrapped absorber panels over a mineral or glass wool core run roughly Rs. 600 to Rs. 1,400 per square foot; polyester fibre acoustic panels roughly Rs. 250 to Rs. 600; perforated MDF or wooden slat panels on a backing roughly Rs. 500 to Rs. 1,500; heavy acoustic drapes roughly Rs. 300 to Rs. 800. In a typical school hall you are treating perhaps 25 to 40 percent of the wall and ceiling area, so a 300-seat hall commonly lands between Rs. 6 lakh and Rs. 22 lakh for treatment depending on the panel type and how much of the room needs attention.
What reverberation time should a school auditorium have?
For a hall used mainly for speech, a mid-frequency reverberation time in the region of 0.8 to 1.2 seconds when occupied is the usual design target, with the shorter end preferred in smaller halls. Halls intended primarily for music are designed longer, often 1.4 to 1.8 seconds or more. Almost every school hall should be designed at the speech end - a long reverberation time makes an unamplified announcement from the stage unintelligible in the back rows, and no sound system can undo it.
Where should acoustic panels go?
Not evenly around the room, which is the usual mistake. Priority one is the rear wall facing the stage, because that is the surface that sends a delayed slap-back echo straight to the performer and the front rows. Priority two is the first reflection points on the side walls, roughly a third to halfway back along the seating, where a hard surface smears speech. Priority three is any pair of parallel hard surfaces close together, which cause flutter echo. The ceiling directly over the front of the house is usually best left reflective - those early reflections help carry the voice to the back rows, and killing them makes the hall feel dead and forces the sound system to work harder.
What is NRC and what value should we specify?
NRC, the noise reduction coefficient, is a single-number average of how much sound a material absorbs, from 0 (fully reflective) to about 1 (fully absorptive). As typical figures, a 50 mm mineral or glass wool absorber behind an acoustically transparent fabric reaches roughly 0.85 to 0.95; a 25 mm polyester fibre panel roughly 0.5 to 0.6; a perforated panel depends heavily on the open area and the backing cavity; bare plaster or concrete is around 0.05. Specify both the NRC and the construction - thickness, density and air gap - because NRC alone can be quoted from a laboratory build that does not match what is actually installed.
What sound system does a school auditorium need?
For a 300-seat speech-led hall, a working system is a pair of full-range loudspeakers flanking the proscenium sized to cover the back row, a small subwoofer if music matters, a digital mixer with enough inputs for the microphone inventory, a DSP for equalisation and delay, an amplifier with headroom, two stage monitors, and the microphones: a couple of wired handhelds, two or three wireless handhelds, two lapel or headworn wireless sets, and a gooseneck on the podium. Indicative budget for that package is roughly Rs. 4 lakh to Rs. 12 lakh installed and tuned. Larger halls need distributed speakers or a compact line array so the back rows are served without deafening the front.
Why is our hall loud at the front and inaudible at the back?
Almost always because there is nothing serving the back. A single pair of speakers at the stage has to be turned up until the back row can hear, which makes the front rows painfully loud - the level falls off with distance no matter how much power you add. The fix is coverage, not power: either loudspeakers with a pattern that throws further and is aimed properly, or delay speakers part-way down the hall time-aligned to the stage so the sound still appears to come from the performer. A long reverberation time makes it worse by smearing what does reach the back, which is why acoustics has to be sorted before the system is specified.
Do we need a sound engineer, or can staff run it?
Staff can run a well-designed system, and it should be designed so they can. That means scene recall on the mixer with a small number of clearly labelled presets, gain structure set correctly at commissioning so nobody needs to touch it, a locked configuration so settings cannot be lost, and a written one-page operating card. Handover training for the two or three people who will actually run assemblies is part of the installation. Complex productions still benefit from someone who knows the system, but the daily assembly should be a single button.
Can acoustics be fixed in a hall that is already built?
Yes, and it is usually the cheapest large improvement available. Room volume and shape are fixed, but the absorption is not, and most problem halls fail for reasons that are treatable - a bare rear wall throwing an echo, hard parallel side walls causing flutter, a fully tiled or stone floor, and glazing with nothing to break it up. We measure what the room is actually doing, identify which surfaces cause the audible problem, and treat those rather than blanketing the room. Targeted treatment of the right 25 percent of the surfaces usually beats an even coating of the lot, and costs considerably less.
Hall Sounding Wrong?
We survey the room, tell you what is actually causing it, and quote the smallest intervention that fixes it - treatment, coverage, or both.
Related Pages
Auditorium Design
Volume per seat, ceiling height and the shell decisions that set what acoustics are achievable.
Design GuideStage Curtains
Heavy velour is acoustic absorption as well as masking - drapes and treatment interact.
Stage CurtainsAuditorium Chairs
Upholstered seating absorbs sound; the chair grade changes how much wall treatment you need.
Auditorium ChairsLED Wall for Schools
Large-format display for the hall - bright enough for a lit room, no lamp to replace.
LED Wall