The reliability of insertion gain measurements using probe microphones in the ear canal

Two methods are now available for measuring the insertion gain as a clinical procedure; the ear canal microphone- and the soft probe tube microphone-system. It was decided to investigate some of the variables affecting the reliability of the insertion gain measurements, such as the measurement location and head movement effects, as well as factors related to the presence of the ear canal microphone in the auditory canal. The last variable exhibited significant systematic differences of -2 to 4 dB in the measured insertion gain with the ear canal microphone compared to the measurement obtained with the probe tube microphone. The intrasubject standard deviations were about the same for both methods and less than 4 dB in the frequency range 0.25 to 4.0 kHz. Finally when making clinical hearing aid recommendations using the probe tube microphone technique the errors in one insertion gain measurement are within 95% probability +/- 2 to +/- 8 dB in the frequency range 0.25 to 4.0 kHz.