Studio Acoustics
Acoustic resonances (‘standing waves’) can occur as the sound reverberates to and fro between parallel non-absorbent surfaces. The character of the sound picked up by a microphone depends on where it is placed in relation to the intensity pattern of these standing waves.
Acoustical treatment
Acoustic resonances can be avoided by sound diffusion. This is usually achieved by introducing deliberate irregularities in the surfaces involved, so that sound waves are scattered when they are reflected.
Standing waves are not normally a serious problem in well-designed television studios, because sound absorbers on the walls reduce the reflected wave energy. Also the presence of technical equipment and scenery provides adequate scattering. However, it can happen that a set contains hard parallel surfaces, so that sound pick-up within it is poor. The best solution here is to alter the set. Opposite walls may be placed non-parallel and made of less acoustically reflecting material—e.g. canvas instead of, say, glass fibre or plywood.
Reverberation time
Reverberation time (RT) is the time taken for a sound in a studio to decay (fade away) through 60 dB. Roughly speaking, this means the time it takes a fairly loud sound to die away to inaudibility. RT is found, approximately, from the Sabine formula (see opposite).
Total sound absorption is the sum of the absorption at all surfaces and is calculated from the area of the surface in square metres, multiplied by the percentage sound absorption for the surface material used.
Reverberation time affects the final quality of the sound considerably. If it is too short, speech tends to sound ‘dry’ (dead, unreverberant) and orchestral music lacks ‘warmth’ and ‘blend’. Too long a reverberation time, however, makes speech distant and ‘echoey’.
Required reverberation times
We normally aim to make a television studio’s reverberation time quite short because reverberation can always be added artificially to the sound output but can never be taken away.
It is difficult to make a studio of any appreciable size with an extremely short RT, because the floor is inevitably hard and hence non-absorbent to sound, but it is practicable to make a small television studio with an RT of 0.3 to 0.5 seconds. This should be acceptable for almost all speech requirements and would also be quite adequate for pop music, where acoustically ‘dry’ conditions are needed to achieve sufficient separation between the individually balanced sources.
Some typical reverberation times (at mid frequencies)
| Open air | nearly zero |
| Average sitting room | 0.5 second |
| Radio talks studio | 0.4 second |
| Theatre | 1.0 second |
| Large TV studios | 0.7 to 1.1 second |
| Concert halls | 1.5 to 2.2 seconds |
| Large Gothic cathedral | up to 10 or 12 seconds (at low frequencies) |
Percentage sound absorption at different frequencies
| 60 Hz | 1000 Hz | 8000 Hz | |
| Rough concrete | 1 | 6 | 12 |
| Heavy fabrics (draped) | 5 | 80 | 60 |
| Wood | 5 | 10 | 15 |
| Plain brick wall | 2 | 4 | 10 |
| Breeze blocks (unplastered) | 13 | 65 | 51 |
| Smooth plaster (painted) | 1 | 2 | 2 |
| Plaster on wood lath | 7 | 13 | 13 |
| Building board (distempered) | 4 | 19 | 22 |
| Building board with 25 mm air space | 15 | 20 | 30 |
| Glass (6 mm) plate | 3 | 3 | 3 |
| A person seated is equivalent to roughly 0.5 m2 of perfect (100%) absorber. | |||
The Sabine formula
1 Sabine unit = 1 m2 of 100% absorber. If a = absorption coeffient and s = area (m2), then
