In general very few auditoriums contain a sufficient amount of sound absorbing material unless they
have been specially treated. The efficiency of a sound absorbing material depends, not only on the
nature of its composition, but also on its thickness, and the way it is installed. Usually a material
increases in absorption efficiency, particularly at the lower frequencies, with an increase of thickness
and with an increase of air space between the wall and the material. In talking motion picture houses,
the most effective placement of absorbing material is usually on the rear and side walls, and the front
section of ceiling above the proscenium.
Since the reverberation time depends on the absorption in the room, the reverberation varies
considerably with the audience unless the seats are heavily upholstered. The use of heavily
upholstered seats is very desirable since it keeps the reverberation and consequently the sound
loudness constant, irrespective of the audience present.
The absorption treatment of the walls and ceiling inside the operating booth, which has heretofore been
neglected, is very desirable. Such treatment would result in the following:- a - reduction of noise in the
booth; b - reduction of noise transmitted to the auditorium; c - increased working efficiency in the booth;
and d - possibility of the complete control of the volume by the operator because of the improved
hearing conditions in the booth.
162. Transmission of Sound. -A theatre with good acoustics has its walls insulated against the
transmission of outside noises into the auditorium. The transmission of sound is of two kinds, aerial
and structural. Small openings due to doors, windows, port holes, etc., transmit sound to a great degree.
Thus, all the joints between walls, doors, windows, etc., should be made as air-tight as possible.
Transmission of sound through structures, such as the noise from vibrating motors and machinery, can
be minimized by using massive walls and floors, and by separating all vibrating bodies from their
supporting structures with sound insulating materials such as cork, felt, lead, and rubber, used singly or
in combination.
Massive walls are not always necessary to obtain sufficient sound insulation.
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