Re-examination of the low-frequency (50-1000 Hz) normal threshold of hearing in free and diffuse sound fields

In two experimental replications involving a total of 17 otologically normal subjects, hearing threshold levels were observed that were from 2.6 to 7.1 dB less sensitive in the 125-1000-Hz frequency range, than those specified in ISO R226-1961. The published data from 12 additional threshold experiments conducted since 1933 were reviewed. Variables that could affect the data, such as free versus diffuse sound fields, pure tones versus 1/3-octave noise band stimuli, psychophysical methods, pulsed versus continuous stimuli, binaural versus monaural listening, age, sex, race, and otological rejection criteria, were analyzed. For frequencies from 50-1000 Hz and ages up to 35, the only corrections found to be necessary were for sound field conditions and binaural versus monaural listening. A second-order polynomial regression line was fitted to the combined, corrected data. The resultant thresholds were 5-6 dB less sensitive than ISO R226 between 50 and 250 Hz, with the divergence dropping to +1.7 and -1.5 dB at 500 and 1000 Hz, respectively.