Frequency response of headphones measured in free field and diffuse field by loudness comparison

Abstract

Coupler measurement of headphone frequency response may be incorrect at high and low frequencies [T. Hirahara and K. Ueda, J. Acoust. Soc. Am. Suppl. 1 87, S142 (1990)]. Since sound pressure near the eardrum is difficult to measure physically, a psychophysical method was employed to assess real ear frequency response of headphones. Loudness comparisons were performed by four subjects, under two experimental conditions: free field (anechoic room) and diffuse field (reverberation room). These conditions were run to check the influence of field characteristics on the reliability of the measurements. Each subject adjusted the headphone level of critical band noise bursts until they were equally loud as those from a reference loudspeaker (7 dB SPL). Measurement scatter was smaller in the diffuse field than in the free field. One hypothesis to explain this result is that head movement causes less change in level in a diffuse field than in a free field. Support for this comes from measurement of the SPL distribution in the two rooms, which revealed that the sound-pressure variation was smaller in the diffuse field than in the free field. To examine the reliability of loudness judgments at high frequencies, another method—hearing thresholds by Békésy tracking—was employed. Each subject's threshold was measured with both loudspeakers and headphones. After compensation was made for loudspeaker transfer functions, headphone frequency response was deduced from the results. This method led to high-frequency responses similar to those from loudness comparison. Below 6 kHz, however, there were only slight differences, because equating loudness for loudspeaker sound and for headphone sound does not strictly correspond to equal sound pressure at the eardrum.