Earcanal Pressure Generated by Circumaural and Supraaural Earphones

The hearing thresholds of four subjects have been measured at nine audiometric frequencies from 0.125 to 8 kcps with a probe microphone placed in the external ear. The probe-tube pressures at hearing threshold are found to be substantially the same with the four different earphones used to generate the tone. The earcanal response curves of three circumaural and two supraaural earphones coupled with a group of ten subjects have been measured over a 0.2- to 15-kcps frequency range. The groups of curves have been normalized with respect to reference response curves obtained with suitable couplers. The five groups of earphone response curves are found to have much in common with one another and with curves of earcanal pressure generated by a free sound field, using the same group of subjects; there are also significant differences. The relationships are thought to shed much light on the acoustic behavior of earphone systems. Average response curves for the five earphones permit the transfer of coupler pressure at hearing threshold; data for TDH39/MX41AR and HA10 earphones are in good agreement with published subjective data. Intra-subject range (average range of pressure with repeated measurements on a single subject) varies from 0.2 to 10 dB, depending on earphone type and frequency. In conclusion, it is suggested that existing earphone systems, though well-adapted for speech communication, may not provide ideal coupling for more-exacting applications.