Room For Improvement

A Guide To DIY Studio Acoustics

Published in SOS December 2007
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Technique : DIY

Are the acoustics of your studio letting down your music? Improving the sound of your room needn't cost much and pays dividends in better recordings and more transportable mixes. Read on for the SOS guide.

Hugh Robjohns

Recording studios invariably require some form of acoustic treatment, both in the control room and in the live room, as the acoustic properties of an empty shell — even when furnished and kitted out with equipment — are rarely conducive to accurate listening, nor are they flattering to acoustic instruments and voices. After all, there's no point investing your hard-earned cash on esoteric gear in order to get the right sound, if what you are hearing in the control room isn't an accurate representation of what is being sent to the speakers.

Acoustic Aims

Room For ImprovementThe acoustic requirements of control rooms and live spaces are necessarily different, but in both cases the purpose of acoustic treatment is generally the same, which is to produce the most even reverb decay time across all frequencies (though the term 'reverb' doesn't strictly apply in most small rooms). The decay time shouldn't be excessively long, and the correct treatment will also help to minimise peaks and troughs in the lower part of the frequency spectrum that are caused by standing waves related to room modes (both between opposing surfaces and more complex paths). Higher-frequency flutter echoes between facing hard surfaces also need to be addressed, particularly at the engineer's listening position, as these confuse stereo imaging and can also colour the sound in an adverse way.

It is very important to understand that acoustic treatment is not the same thing as soundproofing — a common misconception amongst the uninitiated. We can look into that another time but, as a general rule, the things you do to improve the listening accuracy of a room usually have negligible effect on the amount of sound that leaks into or out of that room. Indeed, an acoustically treated room may sound 'quieter' for a given monitor speaker level than an untreated room, and this could lead you to turn up the volume, so the sound leakage problem actually gets worse!

An untreated room can make even the best monitors sound boomy, with an ill-defined mid-range, an aggressive high end and an uneven bass response — where some notes boom out and others seem almost to vanish. Speaker designers spend a huge amount of effort trying to get a frequency response that's flat to within a few decibels, but if you put their speakers in a bad room, you can end up with peaks and dips in the low bass as high as 20 or 30dB! Our standard way to demonstrate this is to program a MIDI sequence of equal-velocity-value, staccato notes in chromatic progression covering the bottom couple of audible octaves, using a sine-wave sample as the sound source. (We've placed an MP3 of this on the SOS web site at www.soundonsound.com/sos/dec07/articles/acousticsaudio.htm). As this cycles round, you'll be able to hear whether any notes are unduly loud or quiet. You may be able to get a more even sound by moving the monitors forward or back (or side to side) by a few inches, but to really fix the problem you'll probably have to install some kind of acoustic treatment. By contrast, a suitably treated room is more relaxing to work in, the bass is tight and even, the mid-range well-focused, and the top end detailed without being harsh. Stereo imaging is also precise and stable, with a wide listening 'sweet spot'.

Of course, it isn't just about how pleasant things sound. When you are mixing you need accuracy, and if you try to mix in an untreated room, you may end up with a mix that sounds acceptable in your room, but totally wrong and unbalanced when played on any other sound system. A typical scenario is that of a room with pronounced bass resonances that lead the engineer to believe he is mixing with too much bass EQ. He adjusts to compensate, and the result is a mix that sounds bass-light when played on other systems. Alternatively, if there is a 'hole' in the room's LF response at the kick-drum fundamental, he may mix the kick drum at far too high a level to compensate and, again, the mix will sound awful when played elsewhere.

Absorb Or Diffuse?

The problem, then, is clear enough, but what do we do about it? There are two main types of acoustic treatment used in studio design: absorbers and diffusers. Absorbers, as their name suggests, absorb a portion of the incident acoustic energy to reduce the amount being reflected back into the room, while diffusers scatter the sound energy over a wide angle, rather than allowing a coherent reflection to bounce back, as it would from a flat, solid surface. In the absorber category we have the relatively thin glass-fibre Rockwool or foam panels (that are effective only in the mid-range and at high frequencies) and bass traps (often mounted across corners or in ceiling voids) that work down to lower frequencies.

Bass traps may be made from large depths of purely porous absorber, such as Rockwool or dense foam, but more effective approaches incorporate damped panels or 'limp mass' membranes — the idea being that these try to move in response to the bass energy but their heavily damped structure absorbs a proportion of the energy and converts it to heat through friction.

Acoustic foam placed at the mirror points either side of the monitoring position will help to prevent mid- and high-frequency reflections from the monitors reaching your ears, and to reduce flutter echo between the two walls either side of you. For the same reason, it is a good idea to place similar material at the mirror point on the ceiling.Acoustic foam placed at the mirror points either side of the monitoring position will help to prevent mid- and high-frequency reflections from the monitors reaching your ears, and to reduce flutter echo between the two walls either side of you. For the same reason, it is a good idea to place similar material at the mirror point on the ceiling.In general, any sound that doesn't leak out of a room is trapped within it and has to be dealt with to stop it bouncing around, so a solid brick or concrete room will need more bass trapping than one with plasterboard stud walls (where much of the really low bass escapes or is converted to heat as the wall panels vibrate in response to the sound.

It is vital to understand that the low-frequency performance of foam panels and similar absorbers is proportional to their thickness: the thicker the absorber, the more effective it is at lower frequencies. A typical two-inch foam panel, for example, ceases to have much effect below 300Hz or so when fixed directly to a wall. However, its general performance, and low-frequency performance in particular, is improved if it can be spaced away from the wall by a few inches. In practice, a piece of two-inch foam mounted two inches from a wall is almost as effective as a piece of four-inch foam glued directly on to the wall.

The required thickness of foam or other absorptive panels (and the distance they are placed from the wall) relates directly to the range of frequencies that can be absorbed — due to the simple fact that the surface of the foam needs to be spaced around a quarter wavelength or more of the frequency being reflected from the wall to have the optimum effect. Lower-pitched sounds have wavelengths measured in tens of feet, so there's no wonder that a couple of inches of foam won't have much impact here!

Absorbers work efficiently where the air movement caused by sound is at its greatest velocity. For our purposes, that is at a quarter of the wavelength. Directly adjacent to the wall surface, there is no air movement, only pressure variations — so imagine, then, how futile it is to stick carpet all over a studio's walls and ceilings! The room might not ring when you clap your hands but only the highest audio frequencies are being absorbed. This leaves resonances in the mid-range and bass end to predominate, and the result is a room that sounds boxy, honky and dull. Even when using properly specified foam or Rockwool absorbers, it is unwise to cover too much of the room's total wall and ceiling area, as you'll simply mop up all the mids and highs, leaving bass-end resonance problems. What you need is just enough treatment to stop the room sounding too live, combined with a practical range of treatments to ensue that you tame the low end to balance it with the damped mids and highs.

Mids & Highs

At mid and high frequencies, flutter echoes occur between parallel walls or between a hard floor and a ceiling. If you clap your hands you can often hear a distinct fluttery ringing sound whose pitch is affected by the spacing between the surfaces. Professional studio designers often use non-parallel walls and special ceiling geometry to eliminate this problem, but most of us working at home have to deal with an approximately rectangular room with near-parallel surfaces. Fortunately, flutter echoes are easily killed off by putting acoustic foam or some other absorber on the offending surfaces, and from the mixing seat's perspective, the most important place to start is on the side walls, level with the engineer's head.

Foam placed on the side walls in this position will not only kill off the offending flutter echoes but also reduce the amount of sound energy from the monitors that is reflected back to the listening position from the walls. Such early reflections can seriously compromise the stereo imaging, so placing absorbers either side of the engineer, extending forward to cover the 'mirror' spot (the point where, if you place a mirror flat against the wall, you can see the reflection of either monitor speaker from your listening position) will bring about a significant subjective improvement in most small studios. It is important to realise though, for the reasons mentioned earlier, that such treatment on its own will have little effect on bass frequencies.

Diffusion

In larger rooms, diffusion is often used as well as absorption, to scatter the sound energy (and to stop the room sounding too dull and oppressive), whereas in smaller rooms it is arguable whether diffusers are really useful, because they are generally too close to the listener to be really effective.

A diffuser is any reflective structure that has an irregular surface capable of scattering the reflections, but to do this effectively the irregularities have to be in the order of a quarter wavelength or more, so we need humps and bumps of at least several inches. Papering the wall using textured wallpaper, for example, won't do the trick, and even that old urban myth, the egg box, only scatters at relatively high frequencies. Commercial diffusers often comprise a series of rectangular chambers of differing depths, where their depth and spacing is based on a mathematical formula that gives the most even scattering. These look very impressive and work well, but a shelving unit partly filled with randomly positioned books, CDs and DVDs also scatters remarkably effectively.

If you are making your own absorbers or traps from, for example, Rockwool, it is important that you use acoustically transparent material to cover it. Anything you're unable to blow through will be unsuitable.If you are making your own absorbers or traps from, for example, Rockwool, it is important that you use acoustically transparent material to cover it. Anything you're unable to blow through will be unsuitable.In smaller studios, a common strategy is to have a soft sofa at the rear of the room, as this acts as an absorber to some extent, and to have shelves randomly filled with items above this. However, in small rooms where the rear wall is closer than, say, six feet from the listening position, you're likely to have more success trying to absorb the sound with deep traps than you are diffusing it. Where you do have space to experiment with diffusors, you can use semi-cylinders (made from bent ply with a rockwool filling for damping), split logs, wooden blocks and even old CDs stuck onto angled wooden blocks to break up those reflections. You don't need to spend a fortune to achieve tangible results, though it isn't easy to predict in advance just how much of an improvement there will be for any given approach, so experimentation is required.

One DIY treatment that should work well in medium-sized rooms is to cover a significant proportion of the rear wall with Rockwool trapping, several inches deep, face this with cotton (or similar 'breathable') fabric, then fix up some vertical split-log style fence posts on top, with a half-inch gap between each. The curved surfaces of the posts will provide some useful HF scattering in the horizontal plane, while the Rockwool behind will absorb some of the sound energy in the mid-range and low frequencies. In fact I've recommended this type of approach to those who've built or inherited studios with carpeted walls, as it (usually in conjunction with bass trapping) helps restore some kind of spectral balance.

Diffusers are useful in live rooms, as they create a more even, less coloured sound, and they may also help reduce spill between mics slightly (because they prevent the walls acting as straight acoustic mirrors). Another benefit of using diffusers is that they create a more even, musical sound without reducing the reverb time significantly (or whatever the equivalent of 'reverb' is in very small rooms, where the decay time doesn't follow the maths!), so you can get a great-sounding live room by incorporating large areas of diffuser.

Mid & High Absorbers

Though many of us think first of acoustic foam solutions, one of the most useful low-cost acoustic absorbing materials is the type of rigid fibreglass or mineral-wool slab used for cavity-wall insulation. This comprises compressed glass or mineral fibres, and though the resulting slabs are rigid enough to be self-supporting (especially the more dense types), they can still be bent or cut. A bandsaw is great for this, though an electric carving knife also works — but please, when doing this, wear a dust mask!

The manufacturers of 'Ready Traps' recommend filling them with Owens-Corning 703 insulation (6lbs per cubic foot), which is a material that can only just be described as rigid, as it is actually still fairly bendy. A quick web search shows that many other acoustic companies also use this (as well as the more dense 6lbs per cubic foot Owens-Corning 705 variety, which has more of a slab-like character), and equivalents are available in most parts of the world. When choosing one, the weight per cubic foot or per cubic metre is what matters, and as long as the density is correct there should be no significant performance difference between glass-fibre and mineral-fibre.

These materials are rigid enough to be self-supporting once they're fitted into a simple frame, they satisfy most fire regulation requirements (which many foams don't), and they come in conveniently sized panels: 2 x 4 feet in the US or 600 x 1200mm in Europe. Because compressed Rockwool or glass fibre is denser than foam, it is more effective at low frequencies, but above a certain density is slightly less effective at high frequencies. It doesn't present such a visually attractive surface to the world as does foam, so we often face one panel of rockwool with one sheet of two-inch foam, and fix it into a frame with an air gap of a couple of inches behind it. This is an easy way to produce a cost-effective mid/high trap that looks professional and can be easily fixed to a wall or hung like a picture frame — and the foam counteracts the reduced HF absorption of the dense rockwool. However, because Rockwool and glass-fibre can shed irritating fibres, some users recommend using a garden pump-up sprayer to spray a light coat of watered-down PVA adhesive onto the faces and edges of the material, which will seal loose fibres without adversely affecting the porosity of the material.

We've had some correspondence suggesting that the 30mm cavity wall Rockwool slab we buy from Wickes DIY store when we do our Studio SOS studio make-overs is actually more dense than is optimum for mid/high traps, as it is dense enough to reflect some high-end energy. We have never yet found this to be a problem in practice, and as most thin, porous absorbers are most effective at high frequencies, it might actually help redress the spectral balance in some cases. If you can get hold of the recommended 3lbs per cubic foot version, you may achieve even better results than we have. As a rule, the very dense Rockwool and glass-fibre products are more commonly used in constructing bass traps than mid/high absorbers. Note that there is a foil-backed version of cavity-wall insulation slab which is designed to keep damp out of buildings. Some acousticians suggest using this over an air gap with the foil at the back, as it has improved low-end absorption over the regular slab. If used with the foil facing into the room, it will reflect high and upper-mid frequencies, but will still absorb in the lower mid-range. This may be a useful strategy for helping to balance up a room that has been mistakenly treated with carpet on the walls.

Acoustic foam is less dense than the mineral-wool products described so far, and consequently it is less effective at absorbing low frequencies. Most is also sculpted to improve the aesthetics, as well as to present a more absorbent surface to sound arriving at an angle. However, these sculpted patterns reduce the average thickness, which again compromises low frequency absorption. There are purpose-designed foam bass corner traps, but these tend to be very large and we find it is more practical to use foam in combination with other traps that are effective at lower frequencies.

Two-inch acoustic foam can work very well at mid and high frequencies, even when applied directly to wall and ceiling surfaces, but the dual-layer foam/Rockwool trap described earlier, with a built-in air gap behind, gives far more performance for your money, as it works down to a lower frequency — but you need to be able to handle the simple DIY task of building a wooden frame to contain it. If you want to get serious performance using foam alone, then a four-inch foam spaced from the wall on two-inch (or, better still, four-inch) foam blocks is a good option. If you want to make a trap using only Rockwool, then a cheap strategy for covering it is to use unbleached cotton dust-sheets available from most hardware stores. Whatever fabric you use should be acoustically transparent, which you can test by trying to blow through it. We attended one Studio SOS project where the home-made traps had been covered with a painted canvas, which prevented the sound getting through into the Rockwool. This looked great, but didn't work well!

Though commercial traps are more costly than making your own, they often look better, and of course they have a guaranteed performance. RealTraps (www.realtraps.com) make some very effective panels, including some that can be used across corners to provide bass trapping, while Ready Acoustics (www.readyacoustics.com) have their new Chameleon system (reviewed elsewhere in this issue of SOS), which comprises prefabricated metal frames into which you fit your own Rockwool, and then add your own choice of covering fabric. They also offer the Ready Traps bag system — a very nicely tailored zip-up bag, which comes in a choice of colours, into which you can put your own Rockwool slabs. Yet another alternative is the Ghost modular acoustics system (www.ghostacoustics.co.uk), which is reviewed in this issue, and of course you can get acoustic foam products from companies including Auralex (www.auralex.com), Advanced Acoustics (www.advancedacoustics-uk.com), Sonex (www.acousticalsolutions.com) and Primacoustic (www.primacoustic.com).

Speaker Stands

Room For Improvement Most project studio owners put their speakers on stands, or on the desk holding the mixing desk or computer. I prefer stands, where possible, as they move the speakers a little further from the reflective surface of the desk and also make it easier to position the speakers at the right height. Hollow metal stands that can be filled with dry 'playground' sand give good results, and standing the speakers on blobs of Blu Tak or non-slip kitchen matt is as good as anything. If the speakers must be placed on the desk, make sure they are angled so that the tweeters are facing your head, and consider using rigid foam support pads, as these help keep mechanical vibration from the speaker from getting into the desk structure. Avoid anything between you and the monitors (such as the corners of computer screens or gear racks), and remember not to put equipment that is likely to get hot directly below your speakers, as the convected heat can distort the audio path through the air and adversely affect the sound.

The Bass End

I'll come back to the placement of mid/high absorbers later, but for now we should look at bass trapping, as this is something that's routinely overlooked in the project studio. Low frequencies are more difficult to deal with because of their longer wavelengths, but in many cases the shape of the room doesn't do us any favours either. Gently sloping walls don't help much when it comes to evening out the bass end, but then few home studio owners have the luxury of changing their wall angles anyway!

Bass problems arise because of what are known as room modes which, in turn, result in so-called standing waves. These are created when sound energy bounces back and forth between solid surfaces. Each reflection aligns with the previous one, so that the peaks and dips combine. The frequencies affected most are where the distance between the walls is related to multiples of the quarter-wavelength of the sound. In different points in the room these reflections either add to or subtract from the sound that comes direct from your monitors, and thus cause big humps and dips in the frequency spectrum — not only at the fundamental modal frequencies, but also at all their multiples. This occurs between all opposing surfaces (front/back walls, side/side walls, floor/ceiling) and there are also weaker (but still significant) modes created when sound makes a round trip via two or more different surfaces (think about the way a snooker ball can bounce off the cushions around the table, and you'll get some idea of how this works in a room).

These frequency-response bumps and dips occur at different places within the room, and though having an acceptably even response at the listening position is primarily what we're aiming for, addressing any bass problems that exist will create a more even sound everywhere in the room.

It isn't just the thickness of foam that is important. Spacing it from the wall (or other surface), rather than mounting it flush, makes it more effective as an acoustic absorber. This principle can be seen put to good use here with a Ready Traps bag.It isn't just the thickness of foam that is important. Spacing it from the wall (or other surface), rather than mounting it flush, makes it more effective as an acoustic absorber. This principle can be seen put to good use here with a Ready Traps bag.All rooms have room modes, and part of the studio designer's job is to pick room dimensions that space the modes out as evenly as possible, so that none dominate. What we don't want is a set of room modes created by one pair of surfaces that occur at exactly the same frequencies as those from another pair of surfaces, because not only does that increase the amplitude at the humps and dips, it also leaves a bigger gap between the modes — there are fewer to fill in and level out the response. Larger rooms tend to generate more closely spaced modes and therefore tend to have fewer modal problems, whereas smaller rooms support fewer modal frequencies and consequently are more problematic. This is because the lowest modal frequency is determined by the wall spacing corresponding to the quarter wavelength of the fundamental frequency. Clearly, a square room is bad news, and a small square one is even worse, as there are relatively few modal frequencies in the crucial bass region and those from both sets of walls stack up at the same frequencies. Rooms with dimension almost exactly twice that of another dimension are almost as bad.

The worst case is a square room that is as high as it is wide. In other words, a cube — does that sound like anybody's bedroom studio? As anyone who reads our regular Studio SOS features will know, we've come across a number of these. Aside from having an uneven bass end, there's a zone in the exact centre of the room where the low end seems to vanish altogether, and unfortunately, in a typical small-room setup, that's usually where your head ends up once you're sitting in front of the desk with your gear on it! To date we've found no practical solutions that completely fix the problems in small, cuboid rooms, though it is possible to make improvements by adding trapping, providing the listener moves away from that central dead zone when making critical mixing decisions about the low end. One useful tip (that may annoy your housemates!) is that leaving the door open can help even out the low end in small rooms. In theory, you could trap a small room to work well at low frequencies, but the amount of trapping required to do a thorough job would take away a significant amount of space and may therefore not be practical in a domestic setting. Rooms with dimensions that are not multiples of each other are invariably best.

To reduce the peaks and dips at the bass end, and to reduce the duration of any bass resonances, it is necessary to reduce the amplitude of low-frequency reflections using bass traps. The most effective placement for these is in corners, as that's where you get the maximum coincidence of modes. These can be vertical corners or wall/ceiling corners, and though symmetry is desirable, this isn't as important at low frequencies as it is further up the spectrum. In the case of our small 'problem' room, traps across the wall/ceiling junction are often the most practical solution. The 'tri-corners' are ideal too — the junction between two walls and the ceiling — and Real Traps make a 2 x 2 foot panel with mounting hardware that's designed to fit into such 'three-dimensional' corners.

It is important to note that the purpose of a bass trap is not to reduce the amount of low end you hear in a room, but to reduce destructive reflections and thereby even out the level fluctuations that occur at different frequencies in untreated rooms, especially those with solid walls. Referring back to the description of room modes, what we're trying to do is attenuate those reflections that might otherwise cancel out the direct sound from the monitors, causing those unwelcome dips and peaks in the room response. If the bass trapping is done correctly, the chromatic note test should show no overly loud or quiet notes and no obvious bass ringing or resonances. Then simple fine-tuning of the monitor position may be all that's needed to achieve an acceptable low-end response. On a subjective level, the bass end will sound tighter and more predictable, and with less change when you move away from the sweet spot. If you were to take before and after measurements on a room, you'd find that after installing effective bass trapping, any resonant peaks would be of lower amplitude and wider bandwidth, so instead of having a series of pronounced resonant peaks that cause certain notes to boom out, the overall response of the room would be much flatter.

The Role Of Headphones

Room For Improvement Can you use headphones to evaluate a mix, so as to remove room acoustics from the equation? Headphones are, without doubt, very useful tools in mixing (for more on this read Martin Walker's 'Mixing On Headphones' feature in our January 2007 issue), but the listening experience is quite different from using monitor speakers and, in particular, the low end may be perceived differently, depending on the shape of your ears and the fit of the headphones. Pan placement can seem more obvious heard over headphones, and reverb character becomes better defined, but rather than regarding headphones as a replacement for monitor speakers it would be safer to consider them as a valuable 'second opinion', capable of showing up detailed faults that listening only on speakers may miss, such as noise and distortion. Given that so many people now listen to music on personal players, checking mixes on both headphones and ear-buds, as well as monitor speakers, is not a bad idea.

Practical Bass Traps

Few studios have space to accommodate several feet of dense Rockwool, which is what you'd need for a purely absorptive trap but, as touched upon earlier, placing solid wedges of dense foam or rigid rockwool panels across as many corners as possible will bring about a big improvement. This works because corner placement leaves a large air space (or thickness of foam, in the case of foam solutions) behind the traps, which in turn helps their ability to absorb low frequencies. You won't get the quarter-wavelength air gap needed for maximum effectiveness at very low frequencies (at 50Hz that would be around five feet!) but you'll be surprised at how well they can work. The reason we need the material spaced away from the wall is the same as for the basic foam or mineral-wool panel traps described earlier: out in the room, the air molecules move back and forth in response to the movement of the speaker cones, but at a room boundary such as a wall the air can't move (as it encounters a solid surface), so the sound energy converts from air movement to air pressure and it is this pressure that relaunches the reflections, just as a bouncing tennis ball stops dead when it hits a wall, then bounces back. All porous traps work by turning some sound energy from the moving air into heat via friction, but if you place them right on the wall where there's no air movement (only a pressure change), there's no frictional loss. Note, however, that having bass traps extend all the way from floor to ceiling, or along the whole length of a wall/ceiling junction, improves their ability to absorb low frequencies, because waves approaching from an angle encounter a greater effective thickness.

In a rectangular room you'll get a set of Axial modes for each single room dimension (length, width, and height), plus a further set of Tangential modes involving two surfaces (for example, across the diagonal dimension of the room) with levels 3dB lower than the Axial ones, and yet another set of Oblique modes involving three surfaces (the most obvious being between the top corner on one side of the room and the bottom corner of the other), with levels 6dB lower than the Axial ones.In a rectangular room you'll get a set of Axial modes for each single room dimension (length, width, and height), plus a further set of Tangential modes involving two surfaces (for example, across the diagonal dimension of the room) with levels 3dB lower than the Axial ones, and yet another set of Oblique modes involving three surfaces (the most obvious being between the top corner on one side of the room and the bottom corner of the other), with levels 6dB lower than the Axial ones.Thick, triangular foam corner wedges tend to be costly, and most aren't large enough to be really effective unless you use lots of them to run the full height or width of the room, but cost-wise it's perfectly practical to put two-foot wide Rockwool panels across two or more corners. A simple wooden frame can be fixed to the walls to support them. In this role the heavier grades of Rockwool (6lbs per cubic foot or greater) are most effective, and the greater the thickness, the more effective the trap will be over a range of frequencies. Indeed, you can pack the space behind porous traps with Rockwool insulation to improve their effectiveness over a wider range of frequencies; if you simply use a thin material with an air gap behind, the traps work most effectively on the frequency where the quarter wavelength corresponds to the air-gap depth, but less effectively at other frequencies.

Some acousticians suggest that using foil-backed Rockwool with no packing behind also helps low-end efficiency, and we've also had success using limp membrane material (mineral-loaded vinyl, sometimes known as barrier mat or deadsheet) as a free-hanging curtain behind thinner sheets of high-density (non-foil-backed) Rockwool. As this material is very heavy — typically 10 to 20kg per square metre — the fabric-backed variants are best, as they won't deform under their own weight. If you use limp membranes, don't use foil-backed Rockwool, as the Rockwool needs to be porous to make this work.

We've also improvised bass traps on Studio SOS visits by rolling up unused sheets of foam, or taking packs of unopened loft insulation, or even rolled up duvets, and stacking them in corners. This doesn't look great but it definitely helps, as does leaving cupboard doors open if the cupboards are stuffed full of clothes or bedding. It's also a good way of checking the effectiveness of bass trapping before spending your money or building something more permanent.

Bass traps mounted elsewhere in the room are less effective than those placed in corners, but if the corners are full, adding more to the walls may still be beneficial: it is very difficult to overdo bass trapping. One common DIY approach is to build a wooden frame to support some foil-backed Rockwool, leaving an inch or two of air gap at the rear. As mentioned earlier, if you have the foil facing the room the trap is more effective at low frequencies but reflects in the mid-range and at the high end of the spectrum, so you could once again finish off the front with acoustic foam if you need the high/mid absorption (providing it is spaced a little way from the foil, so as not to impede its vibration). In smaller rooms, where the ceiling height permits, this can be a good way to deploy additional trapping without it eating up too much space.

Modes In Differently Sized Rooms

Martin Walker The ModeCalc utility from Realtraps (www.realtraps.com/modecalc.htm) displays the first 16 axial modes for each room dimension up to 500Hz.

A small room (as shown in this screen) will have relatively few modes below a few hundred Hertz, and if some of its dimensions are close to or equal to each other, these modes will pile up at some frequencies (resulting in a huge peak), with large gaps between them (creating big dips in the frequency response).A small room (as shown in this screen) will have relatively few modes below a few hundred Hertz, and if some of its dimensions are close to or equal to each other, these modes will pile up at some frequencies (resulting in a huge peak), with large gaps between them (creating big dips in the frequency response).A larger room with well-chosen dimensions will have a more uniform spread of modes, down to a lower frequency, resulting in a much flatter frequency response — even before acoustic treatment has been fitted.The room from which the results in this screen were taken is likely to have peaking problems at around 60Hz, 90Hz, and 170/180Hz, where various modes 'pile up', and troughs in the frequency response in the gaps in between.The room from which the results in this screen were taken is likely to have peaking problems at around 60Hz, 90Hz, and 170/180Hz, where various modes 'pile up', and troughs in the frequency response in the gaps in between.

Tuned Bass Traps

The bass traps so far described work over a wide frequency range, but it is possible to build tuned traps that 'suck out' energy from a relatively narrow, quite specific frequency range. These require a certain amount of mathematical and DIY skill, so I don't propose to go into great detail here, though if you're interested there's plenty of information and all the necessary formulae on the Internet.

One of the earliest studio trap designs can be recognised by its front face of a panel with a matrix of holes drilled into it (not standard pegboard, as the hole size and spacing, as well as the depth of air behind the panel, is critical to its tuning). This is the very well-established Helmholtz resonator, which is essentially a tuned cavity, rather like an organ pipe with some damping material inside. To see how this works, blow over a bottle and listen to the note you get out of it. Then stuff a little cotton wool inside and try again. Note that the damping prevents the bottle from resonating. In fact if you put a lot of such bottles in a room, they'll absorb energy at the frequency the bottle used to resonate at before you put in the damping material. A Helmholz resonator is, in effect, a flat, multi-necked bottle tuned to a specific frequency and then filled with damping material. While it isn't hard to calculate the resonant frequency of such a trap, figuring out its bandwidth isn't so trivial, as the way it is constructed and the amount of damping inside affects its performance in this respect. You also need quite a large one to be effective at low frequencies.

Another popular tuned absorber is the panel trap, which relies on a flexible membrane fixed over the front of a sealed box. The mass of the membrane and the dimensions of the box dictate the tuning, though, again, estimating the bandwidth covered isn't so easy because a lot depends on the self-damping of the panel material and on damping material used inside the box. It is usual to put glass-fibre or Rockwool inside the box, close to (but not touching) the panel. Such traps reflect at mid- and high frequencies as the membrane is usually made from plywood or a similar flexible material. Both Helmholz and panel traps are mounted flat against the wall, but placing them near to (not across) corners can help them work more efficiently if they are tuned to low frequencies.

To use a tuned trap effectively you need to be able to make accurate room acoustic measurements, a luxury that most of us don't have. Having said that, there are now quite a number of reasonably affordable acoustics measurement computer programmes, and if you are really interested in this subject they are interesting to work with. However, it is incredibly easy to get very misleading results (even if you can get the right frequency, it is easy to 'take out' too narrow or too wide a range of frequencies that will probably cause more problems than it solves) and make incorrect assumptions, if you are not very careful — there really is a degree of black art still in the world of acoustics, and experience is important in assessing room acoustics measurements, especially in small rooms!

So we tend to use broad-spectrum trapping for our Studio SOS visits which, after all, are only quick and dirty fixes in many cases. There's a big difference between the acoustic requirements (and the achievable objectives) of a top commercial studio and a home-based studio set up in an existing bedroom or garage.

Standing Waves

Room For Improvement Standing waves are created when sound energy bounces back and forth between solid surfaces, and the incident and reflected waves are in phase. This diagram illustrates how frequencies related to 0.5, 1, 1.5, 2 wavelengths (and so on) result in high sound pressure levels at the walls and at certain other points across the room, and null points at others. For instance, at the lowest frequency Axial mode (F) shown at the top of the diagram there will be a null halfway across the room, while at double this frequency (2F) there's a null a quarter of the way across the room, a peak halfway across, and another null three-quarters of the way across the room. This also explains why it's no good using EQ to 'flatten' a loudspeaker response — it would only be flat at one point in the room. Only by absorbing the modal energy is it possible to flatten the whole room.

Room Layout

You may not have any control over your room's dimensions, but you do have a choice about where to set up your equipment and where to place your acoustic treatment. As a rule, unless you have a large room, set up your speakers across the narrowest wall and get your monitors and listening position as symmetrical as possible with respect to the walls. Also make your acoustic treatment symmetrical side to side, as much as is possible. If you have the speakers pointing across a small room, the chances are that the bass end will vary wildly as you move around the room, and your listening position will be close to the halfway point between front and back where the problem of inconsistent bass end is at its worst.

Set up your speakers so that the tweeters are close to your head height. Although most speakers are designed to be used with the tweeters aimed directly at your head, it is worth experimenting with their 'toe' angle to try to maximise the imaging stability and the size of the listening area sweet spot. Sometimes you'll find they work better when turned outwards to point just behind your head, though sometimes they may give better imaging if aimed to meet slightly in front of the listening position — it all depends on the dispersion of the speakers and reflections from local surfaces such as the desk, mixer, computer monitors and so on. Unless the speakers are designed to be used on their sides, stand them upright, as this will give the widest sweet spot and the most even frequency response.

Using monitors with a less generous bass response may help you make better (less misleading) mixes in a small or marginally treated room, and nearfield monitors improve the ratio of direct-to-reflected sound, so you can expect an improvement in imaging, in comparison with midfields. Nearfield monitoring won't avoid the need for bass trapping, though, as the cancellation effects due to reflections still occur no matter where the monitors are positioned.

Putting a powerful subwoofer in a small room is also no solution, and can actually make matters much worse, as too much bass extension in a badly treated room is worse than using speakers with a limited low-end response and leaving the low EQ controls alone. There is also the issue of properly matching the subwoofer's output to the satellite monitors — which is not a trivial task. Any mismatching in level, phase or crossover region will make the low-end accuracy worse, not better.

In rooms where a subwoofer is appropriate, a good way to find the best place for it is to first stand the subwoofer where you normally sit when you're mixing, then crawl along the front and side walls of the room checking out potential places where the sub might work properly (if you don't have enough space to do this, it is a good indication that a subwoofer isn't a good idea!). Run your chromatic bass test sequence, and if you can find a spot where the notes sound even in level you've found the best place for your sub.

There is a trend towards developing monitors that can equalise themselves to compensate for room acoustic problems. Personally, I'm very sceptical about this approach, because even if you can get the response close to flat in your normal seating position, it is likely to vary even more wildly as you move around the room. While some subjective improvement to peaks can be made using EQ, providing you never move from the sweet spot, the amount of boost required to make up for deep dips in the response may well be more than the speakers can handle at normal monitoring levels. Moreover, any such boosts would become far more pronounced at other places within the room — where your client or bandmates may be standing looking very unimpressed! Another point to consider is that equalisation is a frequency-domain process, whereas reflected sound occurs in the time domain, and any flutter echoes or resonances that continue after the source sound has stopped will still continue to do so whether you equalise or not.

The Live Room

Live room requirements can be very different from those of the control room, but unless you have a decent-sounding room it's often a good idea to record things as dry as possible. Regular Studio SOS readers will be familiar with this setup, where the duvet prevents reflections from the back wall coming into the mic alongside the vocals, while the Reflexion Filter prevents sound getting out into the room to cause the reflections in the first place.Live room requirements can be very different from those of the control room, but unless you have a decent-sounding room it's often a good idea to record things as dry as possible. Regular Studio SOS readers will be familiar with this setup, where the duvet prevents reflections from the back wall coming into the mic alongside the vocals, while the Reflexion Filter prevents sound getting out into the room to cause the reflections in the first place. When it comes to treating the live room, much depends on what you want to record. Again, domestic rooms tend to be a bit on the small side to add any worthwhile ambience to the sound (other than perhaps some early reflections from a wood floor when recording acoustic guitar), so in most cases it is preferable to err on the dead side: remove the sound of the room as far as possible and add ambience later using artificial reverberation.

You can use the same mid/high and bass trapping techniques as discussed for control rooms if necessary, though I find keeping the walls of my live room piled high with unused studio gear, PA speakers and so on gives me excellent diffusion! A good localised vocal recording area can be created by draping a heavy blanket or duvet across a corner and then setting the mic up so that the singer has their back to the drape, which avoids over-the-shoulder reflections reaching the microphone.

A further improvement can be made by setting up a small screen behind and to the sides of the mic, using something like the SE Reflexion Filter or the RealTraps Portable Vocal Booth. These same devices can be used to improve separation and reduce room coloration when recording acoustic instruments or guitar amplifiers.

Summing Up

We've given a lot of advice and background information over the course of this article, but a simple set of effective DIY measures can be summed up fairly briefly.

  • In a typical small room, one 2 x 4-foot mid/high absorber panel might be placed at each side of the listening position, so that it also covers the mirror points (the points where you can see, from your mixing position, the monitors in a mirror held flat against the wall).
  • Where possible, also put a further panel on the ceiling above your head, again covering the mirror points. If it's practical, you can also put panels on the walls behind the monitors.
  • Arrange scattering and absorption at the rear of the room, combining furniture and shelving with further trapping if you need it.
  • Place bass traps in whatever corners are accessible, after first evaluating the need using the chromatic sine-wave sequence I referred to near the start of this article.
  • If some bass unevenness persists after you've installed bass traps, run the chromatic scale again while moving the monitors a few inches forwards or backwards (and side to side) and see what position gives you the most even sound at the listening position.

Making a big improvement to the acoustics of a project studio needs only a little knowledge and DIY skill, but it's a world away from professional studio design, where specifications are much tighter and where larger monitors may generate lower frequencies, which in turn require more trapping. Nevertheless, the maths-free DIY approach does work surprisingly well, it is affordable, and there are lots of companies making suitable acoustics products, often with very valuable practical advice available on their web sites. Some even have the facility for you to type in your room dimensions and they'll come back with a recommended package of treatment, along with the best place to put it. There's also a lot of practical advice contained in articles on the Sound On Sound web site, so don't be afraid to give it a go — you may be very surprised at the difference it makes. .


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