Sound Absorbers (Part 1)
The function of 'sound absorbers' and how to build a sound absorbent wall box.
This is part one in a short series of articles which will deal with the acoustic treatment of a room from floor to ceiling. It is envisaged that a limited budget is available and whereas this may mean the cosmetics of the construction aren't up to the standard of commercially available product, it does not imply a drop in the performance quality of the design.
Studio acoustics can often present a major problem to those who endeavour to build their own studios. With a little forethought and planning there is no reason why this should be so. Whether you're just throwing a few pieces of equipment together in a spare room or seriously constructing a studio which you eventually hope to run for financial gain, it's all too easy to fall into the trap of being so concerned with the equipment you're buying, as well as trying to stop the sound getting out and annoying the neighbours, that the very important factor of the studio's own internal 'sound' is often overlooked. The result of this type of oversight is often all too evident on the recordings made in that space and can, for example, lead to the notorious and classic 'bedroom drum sound'.
The acoustic treatment of a studio, no matter what amount, should hopefully be of some benefit to the sound of your recordings. Even if you don't actually record any acoustic instruments or vocals you shouldn't be fooled into thinking that the problem doesn't affect you. The choice of monitoring loudspeakers and the acoustics of your studio directly influence your perception of the sounds you hear in that room. This particular point was discussed in greater depth by Stuart Arrowsmith in his article on Monitoring in the August issue of HSR.
What to do is the obvious question; a few calculations and some careful listening is the answer. Assuming that you can't borrow or hire a sound spectrum analyser, there are some simple yet effective tests you can try out to determine where the major problems in the room's frequency response lie.
Small studios with parallel walls tend to suffer from two main problems: 'flutter echo' is caused by the sound bouncing back and forth between two parallel surfaces and is easily recognised by clapping your hands and listening for a series of very fast repeats of the original sound. The problems created by flutter echoes and their effect on your perception of the sound can, for example, be compared to the visual world. A studio plagued by flutter echoes is like looking at the world through shattered glasses and seeing multiple images. If you could convert your studio's acoustics into a visual image you wouldn't be happy with the picture for very long, the human ear, unfortunately, is far more tolerant of distorted sound than the eyes are of bad pictures. Like a television with perfect reception, a studio should add little colouration to the sound you hear at the monitoring position.
Another problem encountered in small spaces is a pronounced boost in level around the room's natural fundamental resonance frequency; this is usually around the low or lower midrange frequencies. Sounds that contain frequencies sympathetic to the room's fundamental resonance mode will appear to be louder than other frequencies because the room will act as an acoustic amplifier at those frequencies. When you combine resonant mode problems with those of flutter echo, you have yourself a pretty horrible sounding room which results in heavy colouration of any sounds recorded or played back in that space.
The amount of acoustic treatment applied to a room depends on many factors, the text book or BBC ideal would be a room which possessed a constant reverb time across the whole audio range. This is, however, a difficult and expensive ideal to achieve in a room where you don't have the opportunity to change its physical shape too drastically. In this situation a compromise has to be reached between the job cost and what you feel you can do as a permanent installation. If you are constructing a proper studio rather than just converting a spare room you can obviously tackle the job more thoroughly. With a bedroom you will probably have to take into consideration the cosmetics of the room and the potential sale of the house when you decide to move. These considerations can often restrict the amount of acoustic treatment you are able to install and in these circumstances the use of box absorbers rather than panelling the whole wall, would be the ideal solution. This article will cover both design approaches.
Before discussing the design of the acoustic treatment it's worthwhile taking a few metric measurements of the room that you propose to turn into a studio. If you measure the length, width and height of the room, you can use these figures in a simple calculation that will give the fundamental axial resonance modes of the room. Rooms possess simple natural resonant modes between the various parallel surfaces and as we are dealing with an acoustic phenomenon there are also harmonics related to each of these fundamental modes. Fortunately, nature has been kind to us and provided a simple relationship between the fundamental and its harmonics so that all you have to do is keep on adding the fundamental number together in order to work out the harmonics ie. if the fundamental = 50, then the related harmonic series would be 100, 150, 200, 250 etc., simple isn't it?
To calculate the fundamental frequency you should divide 340 (speed of sound in metres per second) by two times the particular room measurement in metres and centimetres eg. for a room size of (L)3.50x(W)2.75x(H)2.50 metres, the fundamental frequency between the opposing walls of the room's length would be: 340/2x3.50=48.57 Hz. The harmonic series related to this would therefore be: 97.14Hz, 145.71Hz, 194.28Hz, 242.85Hz etc.
Once you have calculated the fundamental frequencies and its first four or so related harmonics, you can listen to sounds in the room to see if your ears can detect any of the resonant modes. We can then apply this data to help design the right type of acoustic treatment. Don't be put off by the maths, a calculator and confidence is all you need, it's not at all difficult to work out resonance frequencies.
Armed with the results for all three pairs of surfaces you can now start to tackle the real work of building the absorbers. The basic idea is to remove the resonance frequencies that cause the room sound to be heavily coloured. The control of walls doesn't provide much difficulty, however, the removal of troublesome modes between the floor and ceiling are a little more involved for the majority of people and therefore we shall concentrate on the walls and leave ceilings until another time.
After much careful listening and a bit of maths you will have hopefully arrived at some sort of decision regarding the acoustic corrections that are required. As we have already stated, most small rooms will show a pronounced boost in the low and mid frequency range and a fast flutter echo between the parallel walls. The solution is to break up parallel surfaces with sound absorbers to stop the build-up of flutter echoes and natural room resonance modes. Without resorting to constructing angled (non-parallel) walls, both of these problems can be remedied to some extent by the use of acoustic absorbers.
It's not necessary to cover the whole room in acoustic absorbers for if you do, your music will drop dead as soon as it escapes from the loudspeakers. A balance has to be drawn between controlling the major acoustic problems of your room and achieving the desired room sound quality. In a professional studio, the control room's acoustics are normally designed to simulate the average domestic living room, though it does have a fairly flat frequency response ie. it lacks any resonance peaks (Figure 1).
The main recording area of a studio may have several different acoustic properties all within one room. This means that it's possible to record vocals and acoustic instruments in a fairly live area as well as having at the opposite end of the studio, say, a dead area for the recording of guitars. However, this approach to studio design is often only practical in a large space and in general if you are limited to one room for all your recording work it's better to go for the control room type acoustics.
The use of acoustic absorbers provides a good, economical way of controlling room modes and flutter echo problems. In order that you may understand the design principals of the acoustic absorbers, a little acoustic theory won't do any harm.
A given body of air will vibrate at a given frequency, this being its fundamental resonance mode; harmonic frequencies other than the fundamental will also be present but usually at a lower level. If you excite a body of air with a note that is sympathetic to its natural resonant mode the loudness of that note will be greater than at any other frequency; this is in principal an acoustic amplifier. The task in hand, therefore, is to design a unit that will attenuate the resonant frequencies rather than amplify them.
The design of sound absorbers falls into two categories: Panel absorbers and Resonators. Each design has its particular uses, with the panel type being used to provide a fairly broadband sound absorber, and the resonator type used to carefully tune out narrow frequency bands. Here, we shall deal with the panel absorber and continue next month with the resonator type design.
The construction of absorbers is quite straightforward and requires only the minimum of basic woodwork skills and all the materials required being readily available from most good DIY shops. Figure 2a shows a simple design for a panel absorber box which will effectively remove large amounts of bass/mid frequencies. This unit is easily constructed from any composite wood such as ply, block-board or chipboard of between 20 and 25mm thickness.
A box is first constructed and then half filled with fibreglass loft insulation which is stuck to the back panel, a cover of normal thickness hardboard is then fixed to the front. Boxes can be around 60cms square and between 3 and 20cms deep. The depth of the box and the mass of the front cover material are directly related to the lowest frequency that the absorber will have an effect upon; and the design illustrated will absorb bass and mid frequencies, though it will have little effect upon high frequencies as the front panel is constructed of hardboard which reflects rather than absorbs high frequency energy.
This type of absorber is ideally suited to removing low frequency problems without affecting the live quality of a room. Figure 2b illustrates a similar design except that the absorption characteristic has been broadened by adding a further layer of fibreglass covered by an open weave material such as hessian, and results in a box that absorbs sound across a large part of the audio range and will simultaneously control low resonant frequency problems and high frequency flutter echoes.
An important factor in the design of all the units mentioned here is the amount of air space behind the front panel. To absorb acoustic energy the front panel must be allowed to 'give', it is therefore necessary to leave an air-space behind the front panel so that when an air pressure wave hits the panel the air behind it is compressed like a spring into the fibreglass filling and is dispersed.
It is evident from this explanation that the mass and rigidity of the panel combined with the air-space behind it are directly related to the amount of absorption the boxes offer. The frequency of maximum absorption is dependent on the resonant frequency of the panel, therefore, a simple formula which enables you to work out the fundamental resonance mode of a given combination of air-space and construction material can help you design panel absorbers that are best suited to your problem; the main problem frequency being identified earlier with the room mode calculations.
The panel resonance frequency is calculated by multiplying the mass of the panel material (in kilograms per square metre) by the depth of the air-space, finding the square root of the result and dividing it all into 600.
The mass of four commonly used panel materials are:
3mm Hardboard = 3.5 Kg/m2,
2.5mm Bitumen Felt = 2.5 Kg,
2.5mm Floor Lino = 3.0 Kg,
12mm Plywood = 7.0 Kg.
For the sake of demonstration, if we have a 3mm hardboard on a 10.8cm air-space the formula would work out as follows: mass = 3.5x(air-space)10.8cm = 37.8, find the square root, which equals 6.1481, divide into 600 = 97.59 Hz.
If you refer to the earlier calculation for the room mode resonance you will see that one of the harmonics related to the room length resonance frequency 48.57Hz was in fact 97.14Hz. Therefore, we have successfully calculated that if we build a panel absorber with a 3mm hardboard front cover on a 10.8cm air-space, its maximum absorption frequency almost matches the room's resonant mode at the first harmonic. Now although the absorber doesn't match the room's fundamental frequency mode we don't need to worry because few sounds of musical importance lie this low down.
The other main point to consider is that to build an absorber that will reach down to the room's fundamental with 3mm hardboard, you would have to multiply the air-space by four and end up with a 43.2cms space. This kind of box depth is going to seriously decrease your room dimensions and can be a little impractical. If you used 12mm plywood you would require an air-space of around 15cms and probably a panel size of a metre square or so.
On the topic of very low frequency absorbers, it's worth pointing out that if your room has some large cupboards or a wardrobe in it, these make good panel absorbers if you cover the fronts with a layer of fibreglass and then some open weave material to give it a reasonable cosmetic finish. One further tip is to stick foam draught excluder around the cupboard door frames to stop bass frequencies vibrating the doors.
Another design illustrated in Figure 3 is the 'membrane absorber'. This unit incorporates the same box structure and filling as before but this time the front is covered with one or two pieces of bitumen roofing felt. The use of roof felt as a front cover lowers the resonance frequency of the absorber and makes it particularly effective at removing difficult low frequency room modes, and again their effective absorption bandwidth can be broadened by adding further fibreglass and an open weave material cover.
The use of boxes represents a good way of tackling acoustic problems because they are semi-permanent installations which are easily removed if you eventually decide to move. If you are able to install some form of permanent acoustic treatment then it's worthwhile looking to cover fairly large chunks of wall rather than working with small box structures. Figure 4 shows a radio broadcast studio that has been treated in this way and as you can see large parts of the wall have been covered with absorbers; Figure 5 illustrates a typical structure design.
The wall itself is used as the backing on to which wooden battens of a chosen dimension to give the right air-space are attached to form a skeleton framework. Fibreglass matting is then placed into the spaces and a front covering of hardboard is pinned in place. Finally another layer of fibreglass is placed over the front panel and covered with an open weave material for a good cosmetic finish. This design is, in essence, similar to Figure 2b.
To conclude, a final word on the positioning of absorbers. The distribution of the wall box design should be done in such a way as to never have boxes of the same depth on opposing studio walls. The idea is to break up the flat surface of the walls in such a way so as to remove the possibilities of direct reflection between absorbers of the same performance characteristic. You could, for example, work on a chessboard-type pattern so that you have spaces and boxes mixed together, with a similar pattern on an opposing wall but set out so that a box on one wall is opposite an untreated part of the other wall. Experimentation is the name of the game and as long as you keep the boxes evenly distributed you shouldn't cause any strange acoustic anomalies to arise.
Thanks to Rupert Neve of Broadcast Training & Services (Newmarket) for help in the research of this article.
Feature by Paul Gilby
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