Free-Field Thresholds vs. Pressure Thresholds at Low Frequencies

Abstract

In an earlier paper data were reported indicating there was no difference between minimum audible field and minimum audible pressure at low frequencies. No satisfactory explanation was available at that time as to why these results differed from previously published data except the statement that body noises, breathing, etc., had been minimized. Further work has now produced a satisfactory explanation. Furthermore, if a well-sealed volume of 6 to 7 cm3 is maintained invariant over the eardrum and the mechanical isolation of the signal source progresses from good to bad, the difference between MAF and MAP varies from 0 to about 16 db. Data have been obtained for deviation of MAP from MAF as the volume and mechanical isolation are kept constant, the seal being broken by measurable amounts. Finally, to study the effect for the smallest possible volume, the plugged ear was fed acoustically from a high impedance source that was well-isolated mechanically. With this arrangement the results indicate that for the 2-cm3 volume the difference between MAP and MAF can be made less than 4 db by carefully controlling the subject. This control involves the position of the teeth, tongue, arms, etc.