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Recording Studio Design
Small sound reproduction rooms pose particular problems in acoustic design. Go into a small untreated bare room and clap your hands, and you usually get a distorted sound. The sound direct from your hands is quickly followed by reflections from the walls, ceiling and floor. Figure 1 shows a plan view of a small rectangular room, highlighting a first order reflection from the right side wall in red. In small rooms, first order reflections tend to be loud and quick to arrive. These reflections can cause the sound to be coloured, or to put it another way, the timbre of the sound will change. In extreme cases it can also cause the sound image to appear to come from the wrong directions. Consequently, to avoid colouration and imaging problems, the acoustic design of post-production studios needs to first concentrate on the treatment of the first arriving reflections.
Figure 2. Photo by Paul Ellis of The M Network Ltd courtesy of Harris Grant Associates. (This photo is not to be reproduced without permission)
There are two basic forms of acoustic treatment to deal with early reflections, namely absorption and diffusion. Figure 2 shows a rear view of a room with considerable acoustic treatment. One solution to the early reflections is to apply absorption to the wall, which will remove reflections as shown on the left wall in Figure 1. This absorption can be a specialist product such as ones made of mineral wool, open cell foam, or a recycled material, but it can also be a more commonplace object such as curtains, a sofa or carpet. If there is too much absorption, however, the room will sound 'dead'. It would be like listening outdoors, where only the direct sound from the sound source is heard. While a few people favour such acoustic non-environments for mixing, for most people these are rather oppressive spaces too far removed from normal listening conditions.
Diffusers are used to spatially disperse reflections, and are shown dealing with the first order rear wall reflection in Figure 1. Some dispersion can be caused by appropriately placing book cases and other furniture, but specialist diffusing surfaces should be favoured where possible, as they achieve greater dispersion in a more controlled manner. By using diffusers, first order reflections are dispersed to be heard latter by the listener, after reflection off other surfaces. Indeed, by removing and delaying early reflections, diffusion and absorption can make a small post-production studio sound like a larger room. Consequently, design is about locating the reflection points for first order reflections, and applying appropriate treatment there.
Figure 3, A BAD panel, (BAD is a registered trademark of RPG Diffusor Systems Inc.)
While it is possible to treat surfaces separately with diffusers and absorbers, it can be more efficient and cost effective to use a hybrid surface. A Binary Amplitude Diffuser is such a surface. It offers partial absorption while dispersing and diffusing any reflections. It was developed by Jamie Angus who is now Professor of Audio Technology at Salford University. A curved version of this panel is used extensively in Figure 2, although they are hidden behind cloth wrapping. The BAD panel is a piece of absorbent covered by a perforated sheet. The locations of the perforations form a distinctive pattern as shown in Figure 3. The arrangement of the patches is determined by special number sequence. Indeed, it is a strange coincidence that this diffuser exploits number sequences similar to those used in digital audio error coding systems.
While flat BAD panels produce good budget performance, much more dispersion can be achieved if the surface is also corrugated. Most forms of corrugation will create the dispersion, but the best possible performance is usually achieved if a numerical optimisation is used in the design process. In a numerical optimisation, a computer is tasked to search for the best diffuser shape. The computer keeps altering the diffuser's shape and testing the performance of the dispersion produced. This is an iterative process and continues until the best shape is found. This is all done virtually in the computers memory; even so, it can be a rather slow and tedious process. Figure 2 showed a post-production studio using a curved optimised diffuser on the ceiling; this ceiling was designed by myself. Read more about diffuser design
So far, the treatments discussed have concerned mid and treble sounds. The bass sounds in small rooms are often also problematical. Any room has resonances. Resonances are frequencies or note pitches at which the room creates a loud sound. Go up or down in pitch from that note, and the sound will go quieter. If you were to play a perfect glissando on a bass instrument in a small untreated room, you would hear the note go louder and quieter due to room resonances. Resonance is the phenomena that caused the millennium bridge in London to wobble. To cure the millennium bridge they added damping, similarly room resonances must be treated with damping. This means adding specially tuned resonant acoustic absorbers, often referred to as bass bins or traps. If you blow over the top of a beer bottle you get a resonant note because the bottle is acting as a Helmholtz resonator. If a cloth is placed in the neck of the bottle, a Helmholtz absorber is formed which removes sound at the pitch of the resonant note. This is the principle of a Helmholtz absorber, which is one of the most effective bass trap designs.
Another solution to bass resonances is to correctly proportion the room and to find the best places for the recording engineer and the loudspeakers. By setting the room in the correct proportions, for example by not having the length, width and height all the same, the audible effect of resonances can be reduced. There are various methods for determining the correct ratio between the room dimensions. Read more about this here. For instance, Salford University and RPG Diffusor Systems Inc. pioneered the use of a computer based optimisation, where a computer is tasked to search for the best dimensions. It is also possible to use a computer to find the best positions for the loudspeakers and listeners as well. Alternatively, good positions can be found using listening tests and trial and error. It is vital to go through this process, as poor positioning can have a serious detrimental effect on sound quality.
This article was originally written for the APRS
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