Audibility thresholds for a pure tone in the presence of a pulse jam in the bottlenose dolphin

Abstract

The effect of a pulse jam on the audibility of pure tones in the bottlenose dolphin (Tursiops truncatus) is investigated. The pulse jam consists of a sequence of pairs of identical short pulses with a pulse spacing of 50 µs in each pair and with a pair repetition rate of 300 s−1. The test signal is represented by pure tones in the frequency range 20–100 kHz. The audibility thresholds for the test signals are measured at 10-kHz steps, both in the presence of the pulse jam and in its absence, on the basis of the conditioned-reflex method with food reinforcement. The resulting dependence of the threshold shifts (TS) due to the pulse jam on the frequency of the test signal has a complex form. This dependence can be separated into three components: (1) the oscillations of the TS curve that correlate with the extrema of the spectral density of the jam, so that the peaks and dips of the TS curve correspond to the maxima and minima of the spectral density, respectively; (2) the component monotonically decreasing as the frequency grows up to 80 kHz, which distinguishes the TS curve under consideration from the rising curve obtained for masking by random noise; and (3) the frequency-independent component of the TS curve. The following auditory features are associated with these components: component 1 determines the timbre of the pulse jam; component 2 is presumably related to the pitch corresponding to the frequency 1/τ; and component 3 exhibits a rather strong auditory feature of random noise due to the random neural activity caused by the pulse jam in the whole auditory filter band.