Page 231 - Master Handbook of Acoustics

requires quite dead spaces to realize adequate acoustical separation between tracks. Music producers
often require different reverberation times for different instruments; hence hard areas and absorptive
areas may be found in the same studio. The range of reverberation time realized in this manner is
limited, but proximity to reflective surfaces does affect local conditions.

Generally, spaces used primarily for speech or recording require close to the same reverberation
times, as shown in Fig. 11-15. Television studios have even shorter reverberation times to deaden the
sounds associated with some video equipment, dragging cables, and other production noises. It
should also be remembered that acoustics in television are dominated by the setting and local
furnishings. In many of the spaces represented in Fig. 11-15, speech reinforcement is employed.
Movie theaters are usually quite absorptive; adequate reverberation is already recorded in the

soundtrack, and in particular added room reverberation would reduce the intelligibility of dialogue.
For hearing-impaired listeners, reverberation times should be decreased for better intelligibility.
Many spaces that are used for multiple functions can benefit from adjustable acoustical methods.

Bass Rise of Reverberation Time

The goal in most recording studios, as well as many other rooms, is to achieve a reverberation time
that is consistent throughout the audible spectrum. This can be difficult to realize, especially at low

frequencies. Adjustment of reverberation time at high frequencies is easily accomplished by adding
or removing relatively inexpensive absorbers. At low frequencies, the situation is quite different as
absorbers are bulky, difficult to install, and sometimes unpredictable.
Researchers at the British Broadcasting Corporation (BBC) observed that subjective judgments
indicated a tolerance for a certain amount of bass rise of reverberation time. Investigating this in
controlled tests, Spring and Randall found that bass rise to the extent indicated in Fig. 11-16 was

tolerated by the test subjects for voice signals. Taking the 1-kHz value as reference, rises of 80% at
63 Hz and 20% at 125 Hz were found to be acceptable. These tests were made in a studio 22 × 16 ×
3
11 ft (volume about 3,900 ft ) for which the midband reverberation time was 0.4 second (which
agrees fairly well with Fig. 11-15).