MEASUREMENTS OF ATTENUATION OF SOUND IN FOGGY AIR AT LOW AUDIBLE FREQUENCIES

Abstract

The anomalous sound absorption in water fog at low audible frequencies observed by Knudsen, Wilson, and Anderson has been further investigated by the method of standing waves.Measurements were made in the frequency range of 25-250 c.p.s. for artificial fogs of several different concentrations and average droplet sizes. Artificial fogs were produced by a specially designed water sprayer and introduced into the measuring tube. The fog inside the main tube was almost turbulent-free and both the droplet size and concentration were made controllable. In order to sample the droplet size, fog droplets were allowed to settle by gravity cn a horizontal vaseline-coated glass disk which was partly inserted in the tube. This disk rotated at a constant speed while photomicrographs were taken with a 16mm. motion picture camera synchronized with an Edgerton's strobolux. The fog concentration was determined by measuring the total liquid water content in a known volume of the foggy air. The fogs thus produced had average radii of 5-9 microns and a concentration 5-14×103/c.c. These measurements were made simultaneously with the acoustic measurement. Standing wave pressure variations along the tube were registered by a logarithmic level recorder, both with and without the fog. The observed attenuation coefficients were computed from the observed first pressure minimum and the first pressure maximum of the standing wave. The maximum attenuation coefficients thus computed varied from 8.8db/sec to 20.8db/sec. The positions of maximum absorption were found to vary from 35 to 50 c.p.s.The theoretical values based on both Oswatitsch's and the modified theory proposed by the present autheor are compared with the experimental values. The experimentally obtained attenuation coefficients due to evaporation and condensation processes are found to be higher than those computed by both theories throughout the lower frequency range but in better agreement with the modified theory than with that of Oswatitsch.