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80
BRITISH KINEMATOGRAPHY
Vol. 16, No. 3
high frequency end it is not practicable to maintain a straight line curve owing to the effect of absorption by the air.
Resonant Absorption
Having now laid down our requirements, the next step is to ascertain how to obtain them. It is necessary, in almost every case, to add sound absorbing material in order to bring the reverberation time of the studio to the required value, and for this purpose the usual porous absorbents such as rock wool, glass silk, Acousti-Celotex tiles, and the like are satisfactory for the frequencies above 500 c/s. On account of the rapid falling off of the co-efficient of absorption of these materials for frequencies below 500 c/s, some other method of absorption in this lower frequency band must be employed. The most simple method of obtaining absorption in the lower frequency range is to make use of resonant absorption. It is fortunate that in most buildings there exists
a considerable degree of resonant absorption, in such elements as floors and ceilings. This property will be illustrated more fully later.
Fig. 6.
8.8.C. Studio
No. 2, Maida Vale.
Change of
reverberation
characteristic
with progress of
acoustic
treatment.
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Frcqutncy in c/i
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Briefly, the principle of resonant absorption is that, for example, a wood panel, which is supported at its edges, and possesses mass, elasticity and internal resistance, forms a resonant circuit. How flat the resonant curve is depends on the internal resistance of the material. When a sound wave, at or in the region of the resonant frequency of the panel, strikes it, it is caused to vibrate ; energy is absorbed and is dissipated in the internal resistance of the panel, and hence the sound energy in the room is reduced in a similar manner to that when a sound of higher frequency strikes a porous absorbent. The subject is not as simple as has just been described, and actually is rather complex, but time does not permit of delving more deeply into all the factors involved.
Investigation of Acoustic Treatment
When the studio with the corrugated walls, which has been previously mentioned, was due for retreatment, the opportunity was taken to find out the behaviour of the various elements, and to do so, the retreatment was carried out in a piecemeal manner. The studio was first stripped of the existing treatment, leaving only the bare plastered walls, the screeded concrete floor and the lath and plaster ceiling. A reverberation curve was then taken and this has been reproduced in Fig. 6, curve A. It will be seen that the curve is substantially flat between about 60 and 2,000 c/s, but that the reverberation time is excessive ; the optimum value for a studio of this size is about H seconds.