Free-Field Thresholds Versus Pressure Thresholds at Low Frequencies

Abstract

In 1933 Sivian and White closed their paper in the Journal of the Acoustical Society with the statement that at low frequencies the difference between the thresholds for minimum audible field and minimum audible pressure could not be explained. To investigate this problem further monaural field thresholds and monaural pressure thresholds have been obtained, the latter for cavities varying from 6 cc to over 800 cc. A probe tube, attached to a W.E. 640-AA condenser microphone, was inserted near the ear drum and the sound pressure determined with the probe remained invariant under transformations of volume. Similarly the same probe was used to determine the pressure for free field. When sufficient care is taken to eliminate muscle noises, breathing noises, etc., and to insure monaural listening, there appears to be no significant difference between minimum audible field and minimum audible pressure at low frequencies. This result is based on measurements made with a limited number of subjects; however, to secure more representative data measurements are being continued with a larger number of subjects.