Efficient monaural procedure for the psychoacoustic calibration of earphones.

A new procedure is examined for psychoacoustic calibration of earphones in which the air-conducted outputs of a standard and an unknown earphone are successively equated for loudness to a reference boneconducted tone. The problem to the subject is one of monaural loudness discrimination, with a relatively small variance (differential sensitivity — 1.23 —1.61 dB), and involves only four sources of variance associated with coupling two earphones to the same ear, and a single loudness discrimination judgment for each phone. The mean test-retest difference in the earphone transfer functions varied by 1.33 — 5.89 dB at different frequencies, mid-value — 3.36 dB. Only a few minutes are required to complete a subject's observations at any frequency. Acceptable group means for transferring audiometric standards to an unknown earphone could be obtained at any frequency by requiring as few as nine subjects to make a single monaural loudness discrimination per earphone by this technique. In contrast, the traditional alternate interaural loudness balancing has a somewhat larger variance associated with the judgment per se (differential sensitivity = 1.50 — 2.50 dB), and involves the additional variances associated with collecting two absolute thresholds and coupling the two earphones to a second ear when the earphones are reversed on the head. The mean test-retest difference in the earphone transfer functions by the traditional "ear-reversal" method varied from 4.16 — 7.54 dB at different frequencies, mid-value «— 6.30 dB, nearly twice that of the suggested procedure.