SUMMARYTemporary threshold shift (TTS) after loud noise exposure was investigated in a male and a female beluga whale (Delphinapterus leucas). The thresholds were evaluated using the evoked-potential technique, which allowed for threshold tracing with a resolution of ~1min. The fatiguing noise had a 0.5octave bandwidth, with center frequencies ranging from 11.2 to 90kHz, a level of 165dBre.1μPa and exposure durations from 1 to 30min. The effects of the noise were tested at probe frequencies ranging from -0.5 to +1.5octaves relative to the noise center frequency. The effect was estimated in terms of both immediate (1.5min) postexposure TTS and recovery duration. The highest TTS with the longest recovery duration was produced by noises of lower frequencies (11.2 and 22.5kHz) and appeared at a test frequency of +0.5octave. At higher noise frequencies (45 and 90kHz), the TTS decreased. The TTS effect gradually increased with prolonged exposures ranging from 1 to 30min. There was a considerable TTS difference between the two subjects.

The sensitivity of human hearing to shifts in rippled spectrum patterns of sound was investigated. The test signal was band-limited rippled noise with spectrum ripples of various frequency spacing and bandwidth; this type of sound may be considered as a quantitatively controlled imitation of complex natural sounds. The listener was required to detect a shift in the spectrum ripple phase while keeping the other parameters of the noise constant. For cosine-shaped ripples, the lowest threshold (1.1%) was found at a ripple frequency of 3.5 ripples per octave (rpo), which corresponds to a ripple spacing of 20% of the center frequency. The threshold increased for both lower and higher ripple densities. Qualitatively similar patterns of threshold dependence on ripple density were observed for center frequencies from 1 to 4 kHz. Making the ripples narrower than cosine decreased the thresholds to 0.7%-0.75% for ripple densities of 2-5 rpo. Keeping the ripple width constant at 3.5%-7.5% of the frequency resulted in a monotonic threshold dependence on ripple density: The threshold decreased with decreasing density (down to 0.7%). An excitation-pattern model explains qualitatively the observed dependence of the ripple-phase shift threshold on ripple pattern parameters.

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The resolution of spectral ripples is a useful test for the spectral resolution of hearing. However, the use of different measurement paradigms might yield diverging results because of a paradigm-dependent contribution of excitation-pattern and temporal-processing mechanisms. In the present study, ripple-density resolution was measured in normal-hearing listeners for several frequency bands (centered at 0.5, 1, 2, and 4 kHz), using two paradigms: (i) discrimination of a rippled-spectrum test signal from a rippled reference signal differing by the ripple phase pattern, and (ii) discrimination of a rippled-spectrum test signal from a nonrippled reference signal. For the rippled reference signals, the resolution slightly depended on signal frequency. For the nonrippled reference signals, the resolution depended on the signal frequency; it varied from 8.8 ripples/oct at 0.5 kHz to 34.2 ripples/oct at 4 kHz. Excitation-pattern and temporal-processing models of spectral analysis were considered. Predictions of the excitation-pattern model agreed with the data obtained with the rippled reference signals. In contrast, predictions of the temporal-processing model agreed with the data obtained with the nonrippled reference signals. Thus, depending on the used reference signal type, the ripple-density resolution estimates characterize the discrimination abilities of the corresponding mechanisms.

Rippled-spectrum stimuli are used to evaluate the resolution of the spectro-temporal structure of sounds. Measurements of spectrum-pattern resolution imply the discrimination between the test and reference stimuli. Therefore, estimates of rippled-pattern resolution could depend on both the test stimulus and the reference stimulus type. In this study, the ripple-density resolution was measured using combinations of two test stimuli and two reference stimuli. The test stimuli were rippled-spectrum signals with constant phase or rippled-spectrum signals with ripple-phase reversals. The reference stimuli were rippled-spectrum signals with opposite ripple phase to the test or nonrippled signals. The spectra were centered at 2 kHz and had an equivalent rectangular bandwidth of 1 oct and a level of 70 dB sound pressure level. A three-alternative forced-choice procedure was combined with an adaptive procedure. With rippled reference stimuli, the mean ripple-density resolution limits were 8.9 ripples/oct (phase-reversals test stimulus) or 7.7 ripples/oct (constant-phase test stimulus). With nonrippled reference stimuli, the mean resolution limits were 26.1 ripples/oct (phase-reversals test stimulus) or 22.2 ripples/ oct (constant-phase test stimulus). Different contributions of excitation-pattern and temporal-processing mechanisms are assumed for measurements with rippled and nonrippled reference stimuli: The excitation-pattern mechanism is more effective for the discrimination of rippled stimuli that differ in their ripple-phase patterns, whereas the temporal-processing mechanism is more effective for the discrimination of rippled and nonrippled stimuli.

The influence of fatiguing sound level and duration on post-exposure temporary threshold shift (TTS) was investigated in two beluga whales (Delphinapterus leucas). The fatiguing sound was half-octave noise with a center frequency of 22.5 kHz. TTS was measured at a test frequency of 32 kHz. Thresholds were measured by recording rhythmic evoked potentials (the envelope following response) to a test series of short (eight cycles) tone pips with a pip rate of 1000 s −1 . TTS increased approximately proportionally to the dB measure of both sound pressure (sound pressure level, SPL) and duration of the fatiguing noise, as a product of these two variables. In particular, when the noise parameters varied in a manner that maintained the product of squared sound pressure and time (sound exposure level, SEL, which is equivalent to the overall noise energy) at a constant level, TTS was not constant. Keeping SEL constant, the highest TTS appeared at an intermediate ratio of SPL to sound duration and decreased at both higher and lower ratios. Multiplication (SPL multiplied by log duration) better described the experimental data than an equal-energy (equal SEL) model. The use of SEL as a sole universal metric may result in an implausible assessment of the impact of a fatiguing sound on hearing thresholds in odontocetes, including under-evaluation of potential risks.

In a beluga whale, the positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound source positions. For this investigation, auditory evoked potentials (AEPs) in response to short tone pips were recorded. Latencies of the first AEP wave that presumably reflected the activity of the auditory nerve were measured at different sound source azimuths. For AEPs of equal amplitudes, the difference in AEP latencies was attributed to the difference in the acoustic delays. These delay differences were used to compute the azimuths of sound receiving points. Measurements were conducted at frequencies from 22.5 to 90 kHz in half-octave steps. At all stimulus frequencies, the receiving points were located 24–38 cm caudal of the melon tip, which is near a proximal part of the lower jaw. Thus, the results indicated the latero-mandibular acoustic window. Possible causes for not finding a lateral or ventro-mandibular window are discussed.

The goal of the study was to enlarge knowledge of discrimination of complex sound signals by the auditory system in masking noise. For that, influence of masking noise on detection of shift of rippled spectrum was studied in normal listeners. The signal was a shift of ripple phase within a 0.5-oct wide rippled spectrum centered at 2 kHz. The ripples were frequency-proportional (throughout the band, ripple spacing was a constant proportion of the ripple center frequency). Simultaneous masker was a 0.5-oct noise below-, on-, or above the signal band. Both the low-frequency (center frequency 1 kHz) and on-frequency (the same center frequency as for the signal) maskers increased the thresholds for detecting ripple phase shift. However, the threshold dependence on the masker level was different for these two maskers. For the on-frequency masker, the masking effect primarily depended on the masker/signal ratio: the threshold steeply increased at a ratio of 5 dB, and no shift was detectable at a ratio of 10 dB. For the low-frequency masker, the masking effect primarily depended on the masker level: the threshold increased at a masker level of 80 dB SPL, and no shift was detectable at a masker level of 90 dB (for a signal level of 50 dB) or 100 dB (for a signal level of 80 dB). The high-frequency masker had little effect. The data were successfully simulated using an excitation-pattern model. In this model, the effect of the on-frequency masker appeared to be primarily due to a decrease of ripple depth. The effect of the low-frequency masker appeared due to widening of the auditory filters at high sound levels.