Sound Integrating Machine


    EXPLORING the sound field around a small source such as a bell or loud–speaker is usually a laborious and time–consuming task because it involves making many measurements in all directions around the radiating source. In the Bell Laboratories Record of July it is shown how this effort can be avoided by doing the work automatically with a sound–integrating machine. The apparatus to be tested is rotated on a turn–table while a small condenser microphone, which is mounted on the end of an arm, is swept backwards and forwards over it. This arm is oscillated in a vertical plane through an angle of 180° by a cam which moves it progressively more slowly as it approaches the ends of its excursion, so that equal radiating areas are traversed in equal times. The output of the microphone is amplified and applied to an analyser to determine the sound intensity in different frequency bands. A meter reading gives the average intensity of the sound in a selected band: and multiplication by a factor, involving the area of a hemisphere the radius of which is the length of microphone arm, gives the total power radiated in that particular band. This integrator measures sound outputs in about one fiftieth of the time previously required to make separate observations at many points about the source. It has been used extensively in developing telephone set housings.

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    Sound Integrating Machine. Nature 148, 368–369 (1941).

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