Volume 122, Issue 4, October 2007
Index of content:
- PSYCHOLOGICAL ACOUSTICS 
122(2007); http://dx.doi.org/10.1121/1.2770545View Description Hide Description
Echo suppression plays an important role in identifying and localizing auditory objects. One can distinguish between binaural and monaural echo suppression, although the former is the one commonly referred to. Based on biological findings we introduce and analyze a mathematical model for a neural implementation of monauralecho suppression in the cochlear nucleus. The model’s behavior has been verified by analytical calculations as well as by numerical simulations for several types of input signal. It shows that in the perception of a pair of clicks the leading click suppresses the lagging one and that suppression is maximal for an interclick interval of . Similarly, ongoing stimuli will be affected by the suppression mechanism primarily a couple of milliseconds after onset, resulting in a reduced perception of a sound shortly after its start. Both effects match experimental data.
122(2007); http://dx.doi.org/10.1121/1.2769829View Description Hide Description
The decrease in absolute threshold with increasing stimulus duration (often referred to as “temporal integration”) is greater for listeners with normal hearing than for listeners with sensorineural hearing loss. It has been suggested that the difference is related to reduced basilar-membrane (BM) compression in the impaired group. The present experiment tested this hypothesis by comparing temporal integration and BM compression in normal and impaired ears at low levels. Absolute thresholds were measured for 4, 24, and pure-tone signals, with frequencies of 2 and . The difference between the absolute thresholds for the 4 and signals was used as a measure of temporal integration. Compression near threshold was estimated by measuring the level of a off-frequency pure-tone forward masker required to mask a pure-tone signal presented at sensation levels of 5 and . There was a significant negative correlation between amount of temporal integration and absolute threshold. However, there was no correlation between absolute threshold and compression at low levels; both normal and impaired ears showed a nearly linear response. The results suggest that the differences in integration between normal and impaired ears cannot be explained by differences in BM compression.
Hearing loss from interrupted, intermittent, and time varying non-Gaussian noise exposure: The applicability of the equal energy hypothesis122(2007); http://dx.doi.org/10.1121/1.2775160View Description Hide Description
Sixteen groups of chinchillas were exposed to various equivalent energy noise paradigms at or SPL. Eleven groups received an interrupted, intermittent, and time varying (IITV) non-Gaussian exposure quantified by the kurtosis statistic. The IITV exposures, which lasted for , for , were designed to model some of the essential features of an industrial workweek. Five equivalent energy reference groups were exposed to either a Gaussian or non-Gaussian , continuous noise. Evoked potentials were used to estimate hearing thresholds and surface preparations of the organ of Corti quantified the sensory cell population. For IITV exposures at an equivalent energy and kurtosis, the temporal variations in level did not alter trauma and in some cases the IITV exposures produced results similar to those found for the continuous exposures. Any increase in kurtosis at a fixed energy was accompanied by an increase in noise-induced trauma. These results suggest that the equal energy hypothesis is an acceptable approach to evaluating noise exposures for hearing conservation purposes provided that the kurtosis of the amplitude distribution is taken into consideration. Temporal variations in noise levels seem to have little effect on trauma.
Phantom echo highlight amplitude and temporal difference resolutions of an echolocating dolphin, Tursiops truncatus122(2007); http://dx.doi.org/10.1121/1.2769973View Description Hide Description
A dolphin’s ability to discriminate targets may depend greatly on the relative amplitudes and the time separations of echo highlights within the received signal. Previous experiments with dolphins varied the physical parameters of targets, but did not fully investigate how changes in these parameters correspond with the scatteredacoustic wave forms and the dolphin’s subsequent response. This experiment utilizes a phantom echo system to test a dolphin’s detection response to relative amplitude differences of secondary and trailing echo highlights and the time separation differences of all the echo highlights both within and outside the animal’s integration window. By electronically manipulating the amplitude and temporal separation of the echo highlights, the underlying acoustic classification cues are more efficiently investigated. The animal successfully discriminated between a standard echo signal and one with the secondary highlight amplitude lowered by from the standard. Furthermore, the animal successfully discriminated between a standard echo signal and one with the trailing highlight amplitude lowered by from the standard and also a standard echo signal and one with a time separation of between the secondary and trailing highlights.
122(2007); http://dx.doi.org/10.1121/1.2775424View Description Hide Description
The underwater sound localization acuity of harbor seals (Phoca vitulina) was measured in the horizontal plane. Minimum audible angles (MAAs) of pure tones were determined as a function of frequency from for two seals. Testing was conducted in a -diam underwater half circle using a right/left psychophysical procedure. The results indicate that for both harbor seals, MAAs were large at high frequencies (13.5° and 17.4° at ), transitional at intermediate frequencies (9.6° and 10.1° at ), and particularly small at low frequencies (3.2° and 3.1° at ). Harbor seals seem to be able to utilize both binaural cues, interaural time differences(ITDs) and interaural intensity differences (IIDs), but a significant decrease in the sound localization acuity with increasing frequency suggests that IID cues may not be as robust as ITD cues under water. These results suggest that the harbor seal can be regarded as a low-frequency specialist. Additionally, to obtain a MAA more representative of the species, the horizontal underwater MAA of six adult harbor seals was measured at under identical conditions. The MAAs of the six animals ranged from 8.8° to 11.7°, resulting in a mean MAA of 10.3°.