Free-Field Calibration of Earphones

Probe-microphone measurements of the real-ear response of an earphone are subject to inherent problems of interpretation. The cushion-enclosed sound field is homogeneous only in the frequency region below 1 kHz, and measurements outside this band can differ by more than 15 dB from one probe position to another. Probe measurements at the earphone diaphragm do not provide enough information on earphone response, because the earphone can create or modify resonances in the outer ear that affect the pressure delivered to the eardrum. Measurements at the canal entrance provide too much information, because they include pinna gain characteristics that are not functions of earphone performance. In the present experiments, the response of an earphone is derived by subtracting, from earphone-induced sound pressures at the eardrum, the sound pressures produced at the same point by a loudspeaker that the pinna faces. The speaker is used to establish a reference of outer ear acoustic gain before disturbance by the earphone, less the effects of head diffraction. The eardrum position was selected for the probe-tube measurements after a preliminary experiment showed that a supraaural earphone can affect ear canal gain by constricting the canal entrance and lowering the canal's resonant frequency. The derived response curves were in agreement with earphone frequency-response data derived from subjective judgments by the corresponding subjects.