A discussion on room acoustic problems and possible solutions....By Colin Whatmough
Room acoustics is in many ways a black art; while there have been many helpful hints published on getting room acoustics right, there are no guarantees. I have attended concerts in many different venues over the years. Some concert halls, such as the Robert Blackwood Hall at Monash University in Melbourne, are excellent, while others have poor acoustics. I’m quite sure that highly qualified acoustic engineers would have designed all of these halls, yet many sound very ordinary. If acoustic engineers can get it so wrong, you can imagine how difficult it is for your average hi-fi enthusiast.
Rooms that sound boomy can be very challenging. The cause of the boominess can be poor room dimensions or the type of room construction. Rooms built of solid brick are more likely to sound fat in the bass than those made from brick veneer. The plaster-board of a brick veneer home can act as a panel absorber at bass frequencies, subjectively tightening up the bass and reducing boom. As the cause of boominess is usually the physical properties of the room, 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 boomy room will have hot spots where the sound will be particularly boomy. The corners of such a room will invariably be the boomiest position. Then there will be null points where certain bass frequencies will be practically non existent. In such a room, a group of listeners will each have different impressions of the sound quality. Those sitting in null points may complain there is a lack of bass at certain frequencies while those sitting in hot spots will 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 i.e. 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 room. In rooms with severe bass problems there will be large changes in the bass quality as you move about the room.
There are two common approaches to this dilemma: electronic (using equalisation) and acoustic (using acoustic 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 (and the system) to the room.
This would seem to be the simplest most elegant way to correct room acoustic problems, but unfortunately it has some obvious flaws. Firstly such a system can only correct response anomalies for one position in the room: where the microphone was positioned (the central listening position). Since standing waves, by their very nature, affect different parts of the room in different ways, equalisation will probably be quite wrong for positions in the room other than the central listening position.
If this central listening position had a bass peak at 100Hz, equalisation would level this out and another position in the room that was down in response at 100Hz would now be even further down. Alternatively if the central listening position had a bass suck-out at 60Hz, the compensation for this would create boom in room positions that didn’t have this 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 (requiring 100 times the power) at 60Hz. Clearly no domestic system could sustain such equalisation. A 100 watt amplifier would require 10 kilowatts at 60Hz and then 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 only partially equalise them. Since equalisation addresses response anomalies, delayed resonances will still be present, even in the central listening position which is the one position that has supposedly been equalised properly.
In my opinion, the only way to address acoustic problems is with acoustic treatment. Fiddling with electronics to cure poor acoustics is a bandaid approach at best.
At our retail outlet, A1 Audio, we have two sound lounges made from solid textured sandstone bricks. Since there is a floor above these rooms, the ceiling height is too low for ideal acoustic dimensions. As a result, these rooms are quite boomy. The standing wave problems in these rooms cause severe peaks and dips in bass response and the sound quality is very dependent on your position within the room. While the textured bricks work very well at higher frequencies (by dispersing high frequency reflections), their solidness is partly responsible for the room’s bass problems. Had I considered placing framework and plaster board over these bricks, the bass problems would be reduced to a degree, but we would lose the acoustic benefits of the textured sandstone bricks at higher frequencies. I decided to develop acoustic treatments to address the issue.
Since bass is extremely prominent in the corners of these rooms, this is where the treatment needed to be. The Whatmough Basstraps were the result of many calculations and a great deal of research. These are large corner absorbers reaching from floor to ceiling and placed in the corners of the room. While they are quite large, they needed to be attractive and not destroy the decor of the room. The difference these made to the room’s acoustics was astounding. The room was transformed from acoustically bad to acoustically excellent. Furthermore, this excellent sound quality is now available from any position within the room. The bass is now clean, tight and extended throughout the room, something electronic equalisation could not hope to achieve. While I was confident these Basstraps would improve the sound in these rooms considerably, I didn’t dream the improvement would be as significant as it was. As the bass is now much cleaner, the midrange and particularly the treble sound much clearer and more detailed. Previously these frequency ranges were being overwhelmed by the fat murky bass.
One Basstrap placed in each corner behind the front speakers would be required to clean up the bass in most problematic rooms. Our rooms had severe problems, so we put a Basstrap in all four corners of the room. The Basstraps in the corners behind the front speakers achieved about 75 per cent of the final result, while the rear Basstraps accounted for the remaining 25 per cent.
There are many manufacturers making corner acoustic treatments, but these are generally made too small (for aesthetic reasons) to be effective at the low frequencies required. Rooms with poor bass are typically most problematic between 40Hz and 100Hz. The wavelengths of these frequencies are 8.575m and 3.43m respectively. To reduce standing waves considerably at frequencies with such long wavelengths requires these devices to be quite large. The Whatmough Basstraps are almost a metre wide and span from floor to ceiling.