Aural Harmonics: Preliminary Time-Intensity Relationships Using the Tone-on-Tone Masking Technique

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Abstract
Variation in sensitivity to an objective 2000-Hz harmonic (f2) is produced at different intensities with a 1000-Hz fundamental (f1) masker. This continuous f1 pure tone is presented monaurally at 57, 61, 65, 67, 69, 71, or 74 dB SPL to each of seven normal ears. Simultaneously, the listeners trace Békésy-type thresholds for the interrupted f2 in the same ear. This objective f2 is then phase shifted, in 60° steps, through 360°. The threshold shift (TS) obtained has been described previously by the general equation, TS = A1+A2×sin (X+Φ). The present purpose is to confirm this observation by examination of a range of f1 intensities and to analyze separately any systematic changes in the three dependent variables; A1, A2, and Φ. Explanations of the effect of f1 intensity upon these parameters are derived from a model that makes three major assumptions: (a) The ear generates an aural harmonic (AH); (b) the AH sums vectorially with the objective f2; and (c) the listener maintains a threshold response. The mean TS results fit the above equation at all f1 intensities sampled here. Detailed analysis of the three dependent parameters reveals: (1) Linear increases in A1 (slope 2.0–2.5 dB) for f1 intensities above 60 dB; (2) complex changes in A2 with relatively large individual differences; and (3) independence of Φ from increases in f1 intensity. The simple model is useful in explaining the mechanisms underlying these parameters in the range of f1 intensities from about 45–50 dB to 74+ dB SPL.

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