NBS-9A coupler-to-eardrum transformation: TDH-39 and TDH-49 earphones

A transformation was derived to convert sound-pressure levels referenced to an NBS-9A coupler to corresponding eardrum sound-pressure levels. It was found that the same transformation may be used for male and female adults and for both TDH-39 and TDH-49 earphones in MX-41/AR supra-aural cushions. It was determined that from 250-2000 Hz the transformation provides an eardrum sound-pressure level estimate that is within 6 dB of the actual eardrum level for 95% of individuals. However, in the 4000-6300-Hz range, the accuracy deteriorates to +/- 12 dB of actual eardrum level for 95% of individuals.