Acoustics can make or break a custom installation – how to tackle a ‘boomy’ room.

In many ways room acoustics is a black art; many helpful hints have been published on getting room acoustics right, but there are no guarantees.

I have attended concerts in many venues over the years. Some concert halls, such as the Robert Blackwood Hall at Monash University in Melbourne, are excellent; others have poor acoustics. Highly qualified acoustic engineers would have designed all of them, yet many sound ordinary.

If acoustic engineers can get it so wrong, you can imagine how difficult it is for the Hi-Fi enthusiast.

Fortunately, acoustically designing a lounge or home theatre is simpler than designing a concert hall. If the room is still to be built, you may have some say in the dimensions.

Try to avoid dimensions that are equal or a multiple of one another. A cube-shaped room will almost certainly cause a system to sound boomy in the bass. Such a room will guarantee standing waves at frequencies determined by the dimensions. A room that is 2.5m x 5m x 10m is also likely to cause problems; a room 3m x 5m x 8m is a much safer bet.

Furnishings also have a large influence on the sound. A room with many hard surfaces, such as bare walls and tiled floor, is likely to sound bright and reverberant.

In such a room, a well-balanced system will sound bright and hard. If you clap your hands you will hear a sharp crack followed by reverberation (flutter echoes).

Such reverberation should not be confused with reverberation (ambience) in a concert hall.

In a concert hall, reverberated sound may be heard several hundred milliseconds after the sound eminates from the instruments. In a domestic-sized room this ambience arrives only a few milliseconds after the direct sound from the speakers.

We hear this latter ambience as a smearing of the sound rather than the pleasant ambience of a concert hall.

It is relatively simple to dampen the bright sound of a room that has many hard surfaces. The first step is to put carpet or heavy rugs on the floor. Thick drapes or wall rugs will further soften the sound.

Going too far with drapes and rugs may result in the room sounding lifeless. I have worked in anechoic chambers (no reflected sound) on numerous occasions, but I would not listen to audio for pleasure in such an environment.

Rooms with bass problems are much more difficult to fix. Often rooms with large glass areas lack bass.

Rooms with many large windows can sound bright, as windows are very reflective at mid frequencies and particularly so at high frequencies. This problem is exaggerated, as bass can also be lost through the windows.

Modern systems using subwoofers are more flexible in this area, because the subwoofer level and upper frequency crossover point can be optimised to suit your room.

Rooms that sound boomy can be very challenging. The cause can be poor room dimensions or the type of construction. Rooms built of solid brick are more likely to sound fat in the bass than those in brick veneer. The plasterboard of a brick veneer home can act as a panel absorber at bass frequencies, subjectively tightening up the bass and reducing boom.

Because the room’s physical properties are usually the cause of boominess, there is no easy fix. Large pieces of furniture such as sofas help break up standing waves and can noticeably improve bass quality, but they are unlikely to eliminate the problem.

A typical room has hot spots where the sound is particularly boomy. The corners of such a room are invariably the boomiest position. Then there are null points where certain bass frequencies are practically non-existent.

In such a room, the listeners have different impressions of sound quality. Those sitting in null points may complain about a lack of bass at certain frequencies, and those in hot spots may complain that the sound is boomy.

Not only is there too much bass in the hot spots, but the bass suffers delayed resonances depending on the frequency. That is, the bass notes don’t stop quickly enough and therefore do not sound clean and tight.

When listening to an audio system in a boomy room, it is easy to hear the acoustics change as you move around the space. In rooms with severe problems there are large differences in bass sound quality.

So, what can be done to cure boomy bass?

There are two common approaches: electronic (using equalisation) and acoustic (using materials to minimise standing waves).

Many AV amplifiers, some subwoofers and some speaker systems have built-in equalisation. There are also some stand-alone equalisation boxes, usually operating in the digital domain, that can be added to an existing system. Using a supplied microphone in the central listening position, the component will calibrate itself to the room.

This seems to be the simplest and most elegant way of correcting acoustic problems, but it has some obvious flaws.

First, such a system corrects response anomalies for only one position in the room – where the microphone is placed. Standing waves, by their very nature, affect different parts of the room in different ways, so equalisation will probably be quite wrong for other positions.

If this central listening position has a bass peak at 100Hz, equalisation will level it out. Another position that is down in response at 100Hz will then be even further down.

Alternatively, if the central listening position has a bass suck-out at 60Hz, the compensation will create boom in positions that don’t have the suck-out.

It is not uncommon for suck-outs to be 20dB or more. To equalise a 20dB suck-out at 60Hz would require a 20dB boost (100 times the power) at 60Hz. Clearly, no domestic system could sustain such equalisation. A 100W amplifier would require 10kW at 60Hz, and the speakers would have to be able to handle this power.

I know of no speaker that can handle anything like 10kW at 60Hz.

For the above reason, most equalisation systems do not attempt to equalise for severe suck-outs, or just partly equalise them. Since equalisation addresses response anomalies, delayed resonances will still be present, even in the central listening position – the one that has supposedly been equalised properly.

The only way to properly deal with acoustic problems is with acoustic treatment. Fiddling with electronics to cure poor acoustics is a patch-up job at best.

Many manufacturers provide corner acoustic treatments, but because of aesthetics these are generally too small to be effective at the low frequencies required.

Colin Whatmough has over 30 years experience in the AV industry and was an early adopter of computer-aided design in speaker research and development. He founded Whatmough Monitors in 1976, where he remains as owner and designer of Whatmough speakers.