Building the perfect inner wall
Not all of us can design a studio from the ground up, and boring necessities like perpendicular walls can cause nasty reflections. Ever the DIY whizz, Stuart Litobarski guides you through that holy grail of modern studio design, the reflection-free inner room
With its multiple chambers and anterooms, studio construction bears more than a passing resemblance to a Russian doll. In this article we're going to focus on the influence of room shape, and how the inner room affects control-room design. That'll still leave the massive outer shell, with its all-important low-frequency modal distribution. But that sort of major DIY we'll save for a rainy day.
Designing dedicated studio control rooms from scratch has traditionally involved building a soundproofed outer shell, from fairly massive building materials, and then fitting this out with timber and plasterboard carcassing, in a bid to create a reflection-free zone design. Bass sound is contained by the outer massive shell, while the mind-bending psycho-acoustic frequencies are controlled by the inner carcassing. What happens in practice is that the front wall of the control room is given a Z-bend form, while the rear wall is kept flat, with diffusers installed. I am reliably informed that some good results can also be obtained when QR diffusers are mounted on the front wall, although you would usually expect to find an observation window there.
Even the traditionally straight-laced BBC are moving over to reflection-free designs, which they dub 'Controlled Image Design'. Two music control rooms were recently completed for them by Neil Grant Associates.
The contouring of the front wall and ceiling of the inner carcassing deflect the all important psycho-acoustical frequencies away from the centre of the room. This delays the arrival of the first reflected wave at the operator's ears. And so a maximum acoustic space is created, effectively anechoic, or free from reflections. In this chamber, a recorded acoustic may be perceived within the mix, untainted by the acoustic environment of the control room itself.
The first reflection will come from the rear wall, which is consequently made heavily absorbent or diffuse, to diminish the reflection still further. This solution is not quite so simple in practice, because the tightly splayed front walls and ceiling diffract and deflect only the geometric treble frequencies. Lower frequencies must be treated differently.
The room must be very carefully designed so that mid-frequency focussing effects are avoided. The front wall of the control room needs very accurate angles, with selectively-located deep absorption being used to control the reflection pattern. While practical results indicate that acoustic treatment on the front end should be quite deep, this should not defeat one of the main advantages of the reflection-free room - namely that a large, reflection-free listening zone can be created without the room being made excessively dead or dry.
A naturally live control room acoustic is quite desirable with this kind of design. The monitor loudspeakers are soffit-mounted, high up into the edge between the wall and ceiling, to further control reflections. As a result, the exact location of the mixing desk assumes paramount importance.
'Pinking up' a studio control room used to be the standard method for setting up the monitoring, and checking the room acoustic. The noise source may be split into third-octave bands for improved signal-to-noise rejection. The method involves injecting a pink-noise source through each monitor in turn, and then adjusting the overall sound level in the room to be the same at each frequency, using a graphic equaliser installed in each monitor chain. The graphic equaliser usually operates in third-octave bands in the frequency domain, since this is the narrowest bandwidth to which the human ear can respond.
But this technique operates only in the steady-state frequency domain, and is becoming rather outmoded these days. So you'll have to get the acoustic design correct before you build your-room.
TEF analysers operate in the more accurate time domain and are now an important tool in the studio acoustician's arsenal. TEF is excellent for architectural acoustics, where it is extremely accurate in pinpointing regions that might need acoustic treatment. If you can hire one of these, preferably with an operator to go with it, you will be able to locate any interfering objects, or other angular obstructions, which may cause discrete reflections in the time domain. The effect of these is to cause smearing in the frequency domain, causing ripples throughout each frequency band. Another name for this phenomenon is 'comb filtering'. Or, put in digital signal processing terms, the 'Gibbs phenomenon'. In extreme cases it can cause an imbalance in the frequency response of the room. This is why it's extremely important that there are no sharp edges in any of the peripheral sound paths between the monitors and the operator's ears.
Loudspeaker designers prefer a system known as MLSSA (pronounced 'Melissa'). This uses a pseudo-random noise source which has the remarkable property of a flat frequency spectrum. That makes it ideal for showing up dips and humps in the frequency response of loudspeakers. The synthetically coded nature of the MLSSA test signal means that the signal-to-noise ratio is dramatically better than when using a narrow-band sine-wave, by a factor of up to 40,000 times. This has been made possible because the MLSSA test signal is unique, and easily distinguished from natural background noise.
TEF is fine for checking out and fine-tuning the finished room, but how do you define the room at the design stage, on paper? Ray-tracing, and the more accurate image-source method, have been used for some time to design concert hall sound systems, and with considerable success. CATT Acoustics of Sweden, working in collaboration with others throughout the world, have now perfected a computer-based auralisation system.
Auralisation is a technique that acousticians are just starting to use, to give an impression of what a room would sound like based only on the paper design. You can listen to a number of proposed designs without having to build them first. It allows you to enter into the computer a 3-D CAD sketch of a proposed room or hall, together with information on the surface materials including diffusers. When you run the program, you can generate in a few seconds the necessary FIR filter coefficients representing the acoustics of the room. These are used in a digital filter. The filter processes an anechoic recording of sample music, using the same time and frequency characteristics of your room, to give an aural impression of what the music will sound like.
What is entirely new with the CATT system is that auralisation can now be run on a fast 486 PC, with the audio processing being carried out on a top-end SoundBlaster audio card. You can listen directly to the sound of your proposed studio design, by auralising it as it comes off DAT.
In studio applications, ray-tracing is limited by the laws of physics to psycho-acoustic frequencies, ie. to large spaces relative to sound wavelength, or to high frequencies in a small room. Because the geometric laws for sound waves are most accurate at high frequencies - that is, above the diffuse or scattering frequency range - CATT may ideally be used to define the reflection-free angles for the carcassing at the front end of the control room.
CATT can do many more acoustic calculations than simply auralisation, and initial indications suggest that CATT should be a valuable tool in designing reflection-free control rooms. Applying this system to recording studios will require a good library of the directivity indices of the most widely-used monitor loudspeakers, but this should become available quite soon.
Reflection-free contouring is perfectly okay for the front half of a home studio. The monitor loudspeakers may be soffit-mounted, but they don't have to be to complete the reflection-free design. Speakers may be mounted away from the desk on stands, at such an angle as to minimise HF 'splash' off the desk top. You may already have suitable top-end loudspeakers, which are designed specifically with soffit-mounting in mind. If so, they can be upgraded using their own recommended power-amps and electronic crossovers.
"Too much pegboard or eggbox finish can make a studio look jaded"
The loudspeakers, when mounted in soffits, are situated well up into the corners, to keep the direct sound well clear of the ceiling. The front upstand of the desk should be angled, or treated with absorbent facing, while the control-room observation window should be angled to preserve the reflection-free contouring of the front wall.
The front wall may be designed on paper by hand. You draw a plan and a mid-section view of your proposed studio, then draw lines projecting from the monitors as fan-shaped rays. Using basic 'O'-level Physics, you apply Snell's law to the first reflections (I changed to Latin in the third form - Ed) to calculate the angles at which your front-room panels should be mounted. The sound rays will reflect at the same angle as they strike the panel. This technique will be effective from high frequencies right down to the diffusing frequency range. The exact cut-off frequency of the reflection-free design will be found after experimentation. You would typically build the front-wall design out of plasterboard and timberwork. The sketches shows a basic outline you might base this on, both plan view and midsection.
If you splay the side walls, they may need to be stepped in stages, so that the room doesn't become too eccentric. The floor may also be raked, although this is not very practical in a home studio.
When installing air-conditioning in a control room, keep diffuser grilles well away from the direct sound paths. Reflections from these grilles can give rise to nasty acoustic defects in the listening environment. And don't forget to angle the front faces of FX racks.
Opposing parallel surfaces in the room should have different types of treatment on them, to ensure adequate global diffusion over the room as a whole. For example, if the floor is carpeted the ceiling could be fairly hard, using a material such as plasterboard, but it should be angled, especially above the mixing desk, to reduce HF reflections.
A reflection-free zone room may be combined with a live-end dead end, or 'LEDE', configuration. Which end is which? For sure, noisy equipment should be positioned in dead absorbent areas. Also, too much trapping around the monitors can soak up valuable power. The floor, for reasons of high traffic ruggedness, may be constructed in a parquet finish. With a surface like this you can put quite a lot of absorption into the ceiling, as it's a good area to invest lots of treatment. Corners are an efficient and cost-effective position for bass traps. The general rule is to make the front end 'live', and the rear-end dead and diffuse.
This leaves our main flat surfaces to be acoustically treated. Treat the middle of the side-walls, and the ceiling, to kill the first-order reflections, and to double the room size at a stroke. Repeat the exercise with the centre front-wall and centre rear-wall, and you preserve bilateral dissimularity - that is, the stereo sound from the left-channel speaker doesn't bounce off the rear wall and into the right ear, and so become mixed with sound from the right-channel speaker, and vice versa. This would destroy the point of having two separate channels in the first place. Or of having two separate ears, for that matter!
Diffusion may be applied to the left- and right-hand walls, by scattering your treatment in modules or patches along the walls, adding alternately high-frequency absorption and bass absorption.
The rear wall is a special case. It should contain mega diffusion, to maintain bi-lateral dissimularity. QR diffusers are now readily available off the shelf, and at budget prices. If you can't afford QR diffusers, the alternative is for the rear end to contain plenty of absorption. An absorbent area of rear wall is also an ideal location to site noisy machinery, as the noise will be soaked up. Placing the machines in an absorbent alcove will further reduce the noise, which could be surprisingly obtrusive where fans and motors are concerned. The alternative is to place machinery in a separate machine room.
Sharp edges of any kind should be taboo in the control room. In a practical sense, try wherever possible to make the shape of the surfaces as varied as possible, as this will help to break up the sound in the room and create a more diffuse and even room acoustic. A fabric facing to the timber carcassing takes care of acoustic treatment, and should be made removable for cleaning and studio fine-tuning.
If you have separate studio and control rooms, then you may want to include an observation window. The window need not involve complicated construction, or require steeply angled glazing, except to avoid lighting glare and HF splash. But note that the desk upstand and rear wall should not be parallel to the glass. In the case of reflection-free designs, the observation window would form an integral part of the front-wall design, with its inclination being set accordingly. In this case the window may be usefully situated low to the front of the desk, the front upstand of the desk being acoustically neutralised by sloping or absorbency. Sight lines over the top of the desk may look downwards into the studio room, with the control-room floor being raised up to facilitate this.
The problem with glass, which distinguishes it from the rest of the front wall, is that it cannot receive any porous acoustic treatment at all. Although windows fabricated in QR diffuser designs have been fashionable, some of the latest studios feature an observation window sited on a side wall. And special anti-glare glass, although quite expensive, is available from Germany.
These simple changes alone could yield major benefits.
In professional control rooms, it is common to mount the monitoring loudspeakers on one long wall. In the smaller home studio it may be more practical to mount the speakers on the short wall. This will allow the wide-band absorbers to be installed on the long walls, and on the ceiling, in such a way as to soak up the first reflections. Maximum cost benefits are then obtained with acoustic treatment.
Treatment may be added initially in the form of thick carpet with underfelt, neatly preserving the domestic appearance of the room for multipurpose usage. If possible, leave some uncarpeted areas; a full floor of carpet can easily mop up too many high frequencies.
If you decide to soffit-mount your monitor loudspeakers, you should build the supporting enclosures separately from the main carcassing of the front wall. The composite front wall of the room may be likened to the front face of a guitar, with a modal response in its own right. The modal vibration pattern can adversely modify the wave-front being propagated in each psycho-acoustic frequency band. Structurally, the loudspeaker must be totally disconnected from the front wall, using an entirely separate supporting frame. Any adjoining air-gaps may be sealed with Neoprene foam rubber.
Bear in mind that you must meet all the relevant classes of government fire codes. Things like power amplifiers can generate an awful lot of heat from the cooling heatsinks on which the output transistors are mounted. In the event of an electrical fault, you must be sure that your insurance will cover you. If in doubt, contact the local building inspector or fire control officer, who should be able to provide some free expert advice on your installation. You should not need any special planning permission to install reflection-free carcassing, provided that it does not affect the structural loading in any way - which, being lightweight, it should not. You need to take special care not to obstruct any cooling fans, as these rely on a continuous flow of cool air to maintain internal electronic circuitry at a safe working temperature.
Don't forget the overall impression or design impact that your finished studio will have on the people who will be visiting it. A 'techy' sci-fi appearance is often appropriate, because it invokes an impression of the classic recording studio. However, too much pegboard or eggbox finish can make a studio look rather jaded. It is quite feasible these days to hide acoustic treatments behind decorative fabric finishings which are acoustically transparent, as the photo courtesy of Fabritrak illustrates. These modern plastic tracking systems are quick and easy to fit. Polished wood floors and soft furnishings are all acoustically friendly. Installing low-voltage lighting in the carcass panelling of your reflection-free studio will give your studio a more professional feel, and reduce the possibility of lighting glare from observation windows.
"A full floor of carpet can easily mop up too many high frequencies"
Stuart Litobarski is an Electronic and Acoustic Engineer. His company, Soundwave Acoustics, specialises in studio design and acoustics consultancy.
Feature by Stuart Litobarski
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