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Auditorium Noise Level
The exclusion of noise arising from without and the reduction of extraneous noise arising within the auditorium are important factors. Not infrequently kinemas are situated in busy main streets and street noise is a cause of complaint. Investigation has shown that the equivalent loudness of outdoor noise can vary between 70 phons in a local shopping road to 100 phons in a main road carrying heavy motorised traffic. These values represent the loudness at the kerb-side, and, although there is a progressive diminution in intensity as the distance from the sound source increases, reflection of noises from one side of a street to the other enhances the general loudness and promotes its continuity. This increase in loudness may under some circumstances reach 5 phons.
Noise from street sources can be introduced into the auditorium through
Fig. 8.
Successive Reflections of Sound
Wave in badly shaped
Auditorium.
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â– three different channels, viz., by direct path through cracks around doors, by flexur.al movement of walls and floors which vibrate as diaphragms and readily transmit low and medium frequencies, and by vibrations set up within the whole structure which transmit the higher frequencies.
The reduction of street noise by direct path is most satisfactorily attained by insulating the auditorium by means of entrance lobbies with close fitting doors, ;and by avoiding any exits which open directly from the auditorium on to the street.
The quantity of noise transmitted by flexural movement and vibrations is largely a function of the kind of material employed in the structure ; and the mass or weight is a reasonable guide to the insulation properties of the material. Fig. 9 indicates this relationship between weight of structure and sound reduction, and generally speaking, the relationship is such that each time the weight per Unit area of the material is doubled the insulation value increases