Estimation of Equivalent Freefield Physiological-Noise Levels by Loudness-Balancing Technique

Abstract

There is evidence to suggest that the pressure threshold of hearing at low frequencies is linked with the levels of physiological noise generated by rigid head motion and the inertia of the earphone. The levels in the inner ear due to the inertia of the conductive mechanism itself are, therefore, pertinent to the freefield threshold of hearing, and have been investigated as follows. The subject, whose head is driven by a pure-tone vibrator, adjusts the level of excitation for loudness balance with the ears (a) uncovered and (b) covered by a supraaural or circumaural enclosure. With a supraaural device simulating an MX41AR-TDH39 earphone, the excitation has to be reduced by approximately 18, 30, 25, 25, and 10 dB at 40, 62.5, 125, 250, and 500 cps, respectively. With a 150-cm3 circumaural cavity, the corresponding values are 4–10 dB lower. Subtracting these data from measured levels of physiological noise in aural enclosures and assuming the noise to be generated solely by rigid head motion, one arrives at approximate equivalent freefield physiological-noise levels of 40, 15, 10, −10, and −10 dB re 0.0002 dyn/cm2 at 40, 62.5, 125, 250, and 500 cps, respectively.