A Project Studio; Design Considerations - White Paper

BUILDING PROJECT - A NEW STUDIO :  DESIGN CONSIDERATIONS

We at White Mark have long been concerned at the unnecessary mystique that surrounds the design of both studios and many other audio environments. To this end there are areas of the web-site dedicated to explaining the philosophies that underlie what we do and why we do it. The theories that underpin the creation of a natural monitoring environment and the support of good spatial (stereo or surround) imaging are well understood and have been described in a number of readily accessible works for some time. 
 

Reverberation time:

The reverberation time of a space is a numerical way of expressing how fast the sound level in that space decays. In order to control (reduce) the reverberation time, absorption is placed along the walls of a room, the greater the amount of the wall given over to absorption the deader the space will seem and the shorter (or smaller) the reverberation time figure will be. The actual definition of the reverberation time figure is the time, in seconds, taken for the sound in a room to decay by 60dB following the cessation of the driving sound from the source. This is known as the RT60. Figures for the idealised values for this figure for various rooms and functions are published but are very subjective. A large scoring stage may be 1.5 to 2 seconds with auditoriums for speech being designed to be closer to 0.5 to 1 seconds depending on size. The aim for any monitoring or performance environment is to produce a room wherein the reverberation time is consistent with the function of the room but, and of paramount importance, is even with frequency. This means that the room must cause the decay of low frequencies to be equal to the decay of high frequencies. Uneven decay in a room results in "coloration" and in an unnatural perception of the balance of sounds in the music or speech being monitored or recorded.
 

Imaging: 

The ear senses the position of a sound in a number of ways. Firstly, and most crudely, the relative level of a sound at each ear is sensed and this helps place it left to right. Secondly the relative arrival time of a sound at each ear is sensed and this refines the lateral positioning information. These are the two most commonly understood locational mechanisms and are catered for by the symmetrical positioning of the stereo loudspeakers and the use of pan pots and delays in the creation of spatial effects. 

A third and critical mechanism has been understood for some time and that is the effect of the outer ear's folds on the sound. The outer ear (or Pina) has a number of folds that cause multiple reflections of an arriving sound to be directed towards the ear-drum. The effect of multiple sounds adding together is to produce a comb-filter response in the ear that varies with the number of reflections and their relative delay. Thus it can be seen that the direction from which a sound arrives will cause slightly different amounts of delay within the ear and thus different comb-filter effects. The signal processing within the brain can interpret these changes very accurately and add another level of accuracy to the positional assessment of the sound's source. This mechanism allows for positioning to be done in the vertical plane as well as left to right and behind. 
 

Coloration:

As well as the unevenness of decay with frequency in a room, unwanted reflections can also cause serious coloration. The effect of a direct sound arriving at the ear closely followed by another copy of that sound that has bounced off a wall can easily by very destructive. If it has taken a short time longer for the reflected sound to arrive then it will be delayed with respect to the direct sound. The combination of the two sounds will therefore result in cancelling of those frequencies at which the time delay causes the two waves to be out of phase and reinforcing of those frequencies that it causes to be in phase. Thus a series of distortions will occur that result in boosts and cuts across the frequency spectrum that resemble the teeth of a comb and give the effect its name of comb filtering. 
 

Design targets:

Thus it can be seen that the design aspiration for any room should be to produce an environment that decays evenly with frequency and has as few reflection possibilities as possible for sound leaving a loudspeaker en route to the ear. These simple aims will help to satisfy the criteria for good design that are indicated by the need for both uncoloured frequency monitoring and for good imaging. In major recording environment design these targets are achieved by a combination of careful calculation of wall geometry, the placement of large arrays of absorptive material, usually finished with cloth surfaces, and the use of diffuse surfaces to put energy back into the room without allowing the production of colouring comb filtering. In smaller rooms, where space and budget do not allow there to be major building works, these design aims seem often to be lost in a welter of proprietary products each with differing claims and limiting price tags. 

The many more modest facilities that are constructed are faced with difficulty in realising a design that meets these same requirements at a reasonable budget and in a way that allows reasonable aesthetics to be achieved. The assessment of potential proprietary treatments to apply to a room is very difficult given the information available and the temptation is to source materials directly. This however is also fraught with difficulty as the availability of suitable absorption foams etc. in moderate quantities is limited and the design of suitable methods of fixing is not easy. 

The first and major requirement for control of the sound response of a room is the provision of some absorption on its surfaces. As described above, the aim should be to be to reduce the decay time of the room evenly with frequency and to reduce the occurrence of reflections in the region of the monitoring position. Thus the type and placement of absorber are both very important. 

Design of absorption is somewhat similar to the design of loudspeakers from a frequency perspective. It is not practically possible to use the same mechanisms to absorb high frequencies as those used to absorb bass frequencies. All frequencies are absorbed by removal of the energy they possess in the form of movement of the air. High frequencies cause relatively small high-speed vibrations whereas low frequencies cause larger, slower velocity air movement. High frequencies decay rapidly in air that is impeded in movement by being caught in the fibres of an open weave cloth or foam. Thus a layer of mineral wool or foam (or even heavy curtains) is very effective at removing the energy from a room at the higher end of the spectrum. As the frequency drops however the effectiveness of the simple foam treatment falls away. 

If a sound wave is considered as it hits a wall and reflects back into the room then a number of features become apparent. At the wall, the speed of the air must always be zero (it's a fact, believe us!) and therefore at a distance from the wall given by ¼ of the wavelength of the reflected sound the air velocity will be at a maximum. Placing an area of open weave foam, or similar, at this position will therefore optimally slow the air down and hence absorb this frequency of sound preferentially. The aim is however to evenly absorb a range of frequencies. Therefore, by sloping the layer of absorber at an angle not parallel to the wall, this maximum absorption wavelength will be spread out across the spectrum. 

Still lower down the range of frequencies, the above mechanism could be seen to cause us to mount panels at a distance from the wall that would result in the room being reduced in size to a point that became impractical. Fortunately another mechanism for absorption comes into play as the bass end of the spectrum is approached. The open celled nature of the foam used can become "invisible" to the lower frequency wave and the panel as a whole becomes movable like a drum skin. If the nature of the foam used, together with its mounting, then behaves as a lossy membrane, then lower frequencies will be absorbed. 

Thus by use of carefully chosen open celled absorption mounted at an angle to walls at critical positions in the room, wide band control of the reverberation characteristics of the space can be achieved. Coloration is minimised by the broadband nature of the method used and by placement of the panels to avoid comb-filtering reflections from the monitors. 
 

For further information on Absorbtion panels or Diffusing panels see:

• White Mark Products: Acoustic Absorption Panels
• White Mark Products: Acoustic Diffusor Panels