Sound Intensity Distribution Research Articles

The Objective Measurement of Machinery Noises

The accurate measurement of the intensity of machinery noises, especially those of musical character, is very difficult under ordinary conditions on account of the presence of the interference pattern. The sound chamber method has been adapted to this problem, using a rotating reflector for securing uniform average distribution of sound intensity. A condenser microphone with tuned amplifier and slowly responding indicating meter is used for making the measurements which can be repeated on the same day with an accuracy of 0.5 decibel. In place of the tuned circuit in the amplifier, a “loudness circuit” is sometimes used which weights the various frequencies according to the sensitivity of the average ear at a loudness level 60 db. above the 700 cycle threshold. The loudness readings of six vacuum cleaners as taken with this apparatus checked against the average loudness estimate of twenty observers.