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Getting Studio Acoustics Right

In major studios or in home setups, a good listening environment is essential. Wilf Smarties presents a practical guide to creating the right space for your sound.


One of the recording areas in Metropolis studios/London, illustrating some of the principles that can be applied to correct control room acoustics. Note the use of non-parallel walls, and an array of wooden panels to act as a diffuser.


There is something cosy about an old house. The walls are solidly built, and the design is nicely idiosyncratic. Contrast this with the cold impersonality of a breeze block and plasterboard job, and it is easy to see why the former type of residence commands a premium on the housing market. The impression of warmth that an old house brings is not merely conveyed by its appearance: brick or stone walls when plastered offer a much more solid, absorptive and irregular surface than anything on an estate, and the pings and resonances associated with those little boxes are pleasingly absent. Another huge plus, from an acoustic point of view, is the superior sound isolation offered by older properties. It follows that it is going to be easier to construct a bedroom studio in a Victorian terrace than in a modern flat.

If you are operating in an overly-rectangular room you will encounter the problem of resonant reflection. Parallel walls trap standing waves with wavelengths corresponding to the room dimensions. These frequencies 'ring' when activated, for example by a handclap. The net effect is that a sound or mix heard in that room will be mutilated by the 'sound' of the room. Well mixed music will sound bad, and a mix which sounded good in that room will sound lousy outside. There has been a tendency in studio control rooms to apply 31-band graphic equalisers on the monitors to 'correct' problems like this. What nonsense! A 'ring' is not merely an unwanted frequency enhancement: it is also a frequency dependent reverberation. I'm afraid the only way to get a decent listening environment is to construct one.

FIRST, TAKE YOUR HOME...



It helps to start with a suitable house or building, but I'm going to deal with the worst case, since many of you will be working under such adverse conditions. In a modern home, the structure simply is never going to be good enough to offer an ideal listening environment, and here the name of the game is damping. Although this can never be a complete solution it is nevertheless: easy to install; cheap; reversible. Acoustic tiles are a trendy and expensive way to apply damping to an overly reflective surface, but being light in weight they do little to prevent sound transmission at low frequencies, where it counts.

A cheap alternative, which can be applied on the floor as well as to walls comes in the shape of a type of carpet underlay, comprising a 3/4" thick mat of compressed carpet scraps. Rolls are typically 1m wide. Applying this to ceilings is tricky because of weight considerations, but once two adjacent walls and the floor have been treated you will already have killed off those annoying standing waves. Start by choosing the walls which are the worst culprits for sound transmission. Floors may be further soundproofed by placing a layer of tongue-in-groove chipboard between two layers of underlay.

Acoustic tiles.

Check your non-wall room boundaries (doors, downlights, windows). Are they airtight? Is the door made of sufficiently dense material? A spot of careful DIY in this area can vastly improve your signal-to-complaints ratio.

After 40 hours and a couple of hundred quid you will have done as much as it is practical to do to such a room. Low frequencies will still get everywhere, but at least the highs and mids will be under much better control.

Tip 1: Remember when mixing through the speakers to apply slightly less reverb than you'd like. You are listening in an unnaturally dead room; play the mix in a normal listening environment and it will pick up more natural ambience. What sounded fine in your home-made control room will now sound muddy.

Tip 2: If you can hear the bass properly through your speakers, this probably means your neighbours are on their way. Unless you live in a detached house or Georgian flat forget trying to tune the bottom end on loudspeakers. Furthermore, because the bass response of the room has not been addressed, it is likely to be less than smooth. Indeed, this is true of many a so-called 'tuned' control room in £500 a day studios. (Check it out by playing a chromatic scale over the bass octaves on a synth — if any of the notes seem louder or quieter than the rest then you've got problems.)

THE IDEAL LISTENING ENVIRONMENT



The ideal listening environment would exhibit the following characteristics:

1. A flat frequency response from 20-20,000 Hertz.

2. A natural, entirely diffuse reverberation whose time falls evenly as the frequency is raised, usually centred around RT60 = 0.6s at 1 kHz (RT60 is the standard measurement of reverberation, defined as the time taken for a sound to decay by -60dB).

3. An even distribution of all frequencies over all listening positions. (Or if not, at least at the mixing engineer's head!)

Some control rooms do approach these conditions, despite the fact that each is by itself very difficult to achieve, but in the real world compromises usually have to be made. The approach we have taken in our semi-detached bedroom has been less to 'tune' the room to meet the above criteria than to eliminate (as far as possible) its interaction with the music. In other words RT60 = next to nothing. However, the frequency response of the monitors in the room will closely follow the manufacturer's specifications, provided you have followed their suggestions for positioning (not too near a wall etc.), down to room-sized wavelengths. Below that the frequency response is bound to go haywire!

AND FOR THE LUCKY FEW...



If you have the resources and space to substantially alter the acoustic space in which you record and/or monitor, a good first rule to follow is never have parallel walls: any angle is better than none, and 10-15 degrees should suffice. Walls can be made to a 'herring bone' design, made from panels typically 2-4 feet long with an included angle between adjacent panels of around 150 degrees. Structures should be rigid and non-resonant. Breeze blocks are OK, as is a wooden cavity wall (either plywood or chipboard) provided the same is filled with dry sand.

Heavy double airlock doors (again non-parallel) should provide room interconnections and, ideally, each acoustic space should be mechanically isolated from all others (except perhaps in respect of the ground level floor). Ceilings also should not be parallel with the floor, and again as rigid as possible. Viewing windows should be double or triple glazed with heavy plate glass, and non-parallel panes.

A reasonable size for a modern control room would be 6 x 6 x 3m high. Sound studios can be virtually any size, but once again the ubiquitous criteria of non-parallelism should be rigorously applied. Smaller booths tend to be heavily damped, and used for close mic voice recording.

ON THE SURFACE



Once the structure has been completed, the business of 'tuning' the rooms (depending on the intended application) can be undertaken, by installing various sound absorbers and diffusers, in order to meet the desired sonic criteria. That for a control room has been given above. (Some sound studios have easily varied acoustics, either by utilising movable screens or, more recently, by using rotating wall panels with a hard and a soft side).

The type of carpet underlay already described constitutes a crude but effective absorber. Contoured acoustic sheets give smoother control over the upper and mid frequencies, which they will not reflect, but do progressively less good as the frequency drops below 500Hz. Below about 250Hz surface treatments cease to have any effect, and other methods of sound absorption must be employed. Low frequencies have to be 'captured', and this can be done by converting sound energy into mechanical motion by means of arrays of suspended, heavy, damped plates (usually carpeted 1" chipboard) which vibrate silently when activated.

An even cleverer and more frequency selective method involves building a mechanical phase invertor designed to reflect sound 180 degrees out of phase, within a narrow frequency band. It is possible to cancel out a variable percentage of energy around a prescribed wavelength. This device, called a Helmholtz Resonator, resembles a ported loudspeaker enclosure, and utilises exactly the same principle that, in the case of the loudspeaker, leads to an extended bass end. Devices such as those described in this paragraph are commonly called 'bass traps'.

Much has been written about control room acoustics, and in the last decade any control room designer who wanted to sound as though they knew what they were talking about constantly referred to 'LEDE': Live End Dead End. The trouble was, no one was entirely sure which end was which. Rather than add to the confusion, I will merely point out that it existed.



"Acoustic tiles are a trendy and expensive way to apply damping to an overly reflective surface, but being light in weight they do little to prevent sound transmission at low frequencies, where it counts."


THE DIFFUSION, DISPERSION, AND DISTRIBUTION OF dBs



Sound diffusion is frequency dependent. Hi-fi loudspeakers utilise cone drivers for bass, which radiates pretty much spherically from any source, and dome tweeters for treble frequencies, which would otherwise tend to project in a tight beam. (Horns and lenses are also used to improve treble dispersion, especially in high-power systems.) High frequencies bounce off hard surfaces and cause 'early reflections', very fast echoes of the original sound. These can cause muddiness if they are perceptibly discrete, but damping a room too heavily will lead to an atypical acoustic (see above 'For Those Of You Who...'). The solution is to ensure that the room does have a perceptible reverberation time, but that this reverberation is diffuse. This can be achieved by a combination of a judicious arrangement of dead acoustic panels and diffusers, intercepting the main reflection paths en route to the listener.

So as to maximise separation between reflected and direct sound at the listening position, it is usual to try to kill the paths which result in a reflection impinging on the listener from the front, while the back wall might sport a large diffuser. This will reflect most of the incident sound energy but disperse it so that, as far as possible, all reflected frequencies are distributed evenly in the listening (horizontal) plane. You can build a diffuser quite cheaply and easily from hardwood planks using a simple formula. (I don't want to clog this article with mathematics, rather illustrate the generalities of, and potentialities for solving, acoustic problems.

If you want to try building a studio utilising bass traps and diffusers etc: I would strongly recommend that you undertake some further reading on the matter. On the other hand, much of the mythology that surrounds studio design is really down to applied common sense, and a summary knowledge of building techniques. Hot tip: plan first, build later!)

CONTROL ROOM LAYOUT



Major items to be placed in any control room include the console, the recorders, the effects and the monitors. These guidlines apply as much to a home studio as to a large commercial operation. I will describe an ideal arrangement: if you can't or won't comply with all of my suggestions, no matter. Even I don't.

Console placement I will leave up to you. You probably won't have much choice in the matter. Remember to leave enough space between the back of the console and the nearest wall to permit sensible speaker placement. I don't think you need to be told how to set up monitors to get a good stereo image at the listening point, but I will give you the equilateral triangle as a clue. Tip: keep the small monitors off the console to avoid mechanical coupling.

Let's now consider the multitrack (if there is one). Where there is a comprehensive remote controller, there is a good case for sticking the multitrack out of sight (acoustically at least). Alternatively you could place it at the back of the room, preferably recessed in an acoustically damped enclosure, to minimise tape transport noise. If there is no remote, however, the recorder must be near to hand.

Effects units also (in my opinion) have to be accessible from the engineer's chair. What is the point of placing an equaliser so far away from the listening position that you can't really hear what it is doing while you are adjusting it? The same argument applies, more or less, to all other effects units. Furthermore, a unit which is conveniently located is likely to get used more frequently than one which is half a control room away. The best arrangement I have found, and one which is being incorporated into more and more studios, is to have a 'producer's desk' situated immediately behind the listening position, the dimensions of which roughly concur with those of the mixing console it faces. Rows of effects units can be set into the front at a suitable inclination, while the top surface can be used for a variety of functions, for example setting up keyboards and computers. The remote unit for the multitrack should be accessible from either the mixing or programming position, so that one person can control everything if need be.

Mixing used to be down to analogue 2-track tape, but as I write the emphasis has switched irrevocably to DAT and digital editing. Since this equipment is a) compact, and b) often shares sequencing/sampling hardware, it needs no separate working environment, and the producer's desk should easily accommodate it.

Some of the above considerations might seem a bit obvious, but anyone with experience of recording studios will know how infrequently they are properly addressed, especially in the areas of FX accessibility and small monitor placement.

TOOLS NOT TOYS



Every month new products are launched in the direction of recording studios and home recordists, each apparently offering more for less than its competitors. Such a reality would require that older products would eventually be consigned to the scrap heap as they became so outmoded as to be nearly worthless. Sometimes this does happen. I remember buying one of the first digital reverbs for £1,000, thinking I was getting a bargain, and selling it a couple of years later for one tenth its original cost.

Digital time domain FX have probably suffered more than most in this respect. A similar fate seems to be awaiting the lower order of multitracks, as affordable digital makes a mockery of all but the best sounding machines. I wouldn't like to be hanging on to 64 tracks of reel to reel digital, either, unless my studio was very busy.

Some products, however, far from suffering this fate, seem to accrue value the longer in the tooth they get, somewhat akin to a fine wine. Others have perhaps depreciated as far as they are likely to. Equipping your studio with items like these has the huge advantage that your investment will be recouped (more or less) when you finally realise that you always wanted to be a chartered accountant after all. Furthermore, the same qualities that make these units attractive purchases to your bank manager will also cause them to be sought out by discerning clients.

Before you go out to buy the latest bit of gadgetry, shop around to see if you can find an 'industry standard' which will to a similar job for a similar price. Chances are it will do the job better, often for reasons not immediately apparent to the novice. Of course, older units may not be in the best condition, and bringing them up to spec, can be expensive. Never buy blind unless you are sure of the dealer's reputation. The following is a short selection of recommended 'golden oldie' purchases, together with a brief explanation of each product's star qualities, and a rough price guide.

Neumann mics.

The U87 is, incidentally, probably the most photographed studio mic. It's big in size and sound, and still in production. Switchable between cardioid, circular and figure of eight polar response patterns. No rarity value, but a S/H one in good nick should never drop in value much below 75% of the current list price. The U47 and U67 are valve microphones, long out of production. Highly prized, and in perfect working order they'll set you back between 1.5 and 2.5K. An appreciating (and appreciated) investment.

Beyer DT100 headphones.

Big blasters. Not only are they accurate, they are loud, and offer a unique combination of sound isolation and comfort (although the ears can get a bit hot in more ways than one). Also, they are user serviceable, with drivers, leads and other fittings being easy to replace. Stand up well to punishment, and will hold their value. Tip: use cannon connectors in your headphone distribution system, so you can use mic cables to extend their range.

Bel BD80.

A 2U, (up to) eight seconds mono sampler/delay. This unit used to retail for four figures, but has bottomed out at around £250-300 S/H. If you are looking to replace a kick drum sample on tape, nothing else comes close to its speed and ease of use. Furthermore, it seems to have just the right combination of trigger speed and secondary peak rejection to avoid multiple triggering/delays occurring. No longer made, but still serviced by the manufacturer for reasonable cost, and a rarity these days.

Urei 1176 LN

Another 2U unit which looks as if it is still being made to a pre-war specification. Not many knobs and switches on this mono limiter/compressor, but it does have a reassuringly large illuminated VU meter. You can't go far wrong with this baby, either as an investment or in use. Buy. Secondhand prices are ludicrously close to list, and apparently (older) black ones are worth more for some reason. No self-respecting studio can afford to be without one.

Lexicon PCM70.

The Lexicon sound is legendary — ABC named their first album after it, so in awe were they of its potential. Nowadays Lexicon have to compete with a host of other manufacturers of digital reverb and FX units, but the PCM70 continues to hold its own, probably because it represents the best features-to-cost ratio of any Lexicon unit. Its programmability is vast, but the ergonomics (few front panel controls and a tiny display) discourage you from doing much in that area. Which is a pity, because it supports dynamic MIDI. I guess most people just tinker around with the presets. Still fetches around a grand S/H.

Yamaha NS10.

Most popular small monitors. I don't know why, unless it's because they perform well when badly sited, ie. sitting on the meter bridge of a console. Can't be avoided.



Previous Article in this issue

Power Players

Next article in this issue

Retro-Sampling


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Apr 1992

Topic:

Acoustics


Feature by Wilf Smarties

Previous article in this issue:

> Power Players

Next article in this issue:

> Retro-Sampling


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