On Minimum Audible Sound Fields

The minimum audible field (M.A.F.) has been determined from data taken on 14 ears over the frequency range from 100 to 15,000 c.p.s. The observer is placed in a sound field which is substantially that of a plane progressive wave, facing the source and listening monaurally. The M.A.F. is expressed as the intensity of the free field, measured prior to the insertion of the observer. Another type of threshold data refers to minimum audible pressures (M.A.P.) as measured at the observer's eardrum. The differences obviously to be expected between M.A.F. and M.A.P. values are due to wave motion in the ear canal and to diffraction caused by the head. The M.A.F. data are discussed in relation to the M.A.P. determinations available from several sources. Some possible causes of difference between the two, which are due to experimental procedure and may add to the causes already mentioned, are pointed out. At 1000 c.p.s. the average M.A.F. observed is 1.9×10−15 watts per cm2, corresponding to 2.8×10−4 bars or 71 db below 1 bar. Data are included to show the effect of observer's azimuth with respect to the wave front.