In addition, there is pure tone noise caused by the repetitive passage of the vanes over the exhaust slots in the air motor liner or, in the case of percussive tools, the piston passing the exhaust holes in the cylinder of the tool. Whenever pure tones are present, one encounters resonance from the geometry of the tool that is triggered by the fundamental frequency of the pure tone. A designer should learn which factor is the major cause of noise and deal with it first. The basic requirements for this part of the job are a sound level meter, an octave band analyzer, a narrow band analyzer, a graphic level recorder, a calibrator, and a work area where the background noise is at least 10 dBA lower than the desired sound level. Also, appropriate loading devices must be available so an operator can test the tool at peak power and free speed.
Several techniques can reduce pure tone noise.
One is to use gradual exhaust to reduce the amplitude of the exhaust pressure pulse. Another method is to place partitions or baffles to prevent standing waves. A third is to design a reactive muffler that is tuned to suppress the pure tone frequencies. Suppression of broadband noise requires different techniques, depending on which noise-producing parameters are at issue. When a steady airflow passes through a duct or muffler, there is a steady pressure drop that is related to the magnitude of the flow and the geometry of the air passages.