Field observation of low-to-mid-frequency acoustic propagation characteristics of an estuarine salt wedge

The estuarine environment often hosts a salt wedge, the stratification of which is a function of the tide's range and speed of advance, river discharge volumetric flow rate, and river mouth morphology. Competing effects of temperature and salinity on sound speed in this stratified environment control the degree of acoustic refraction occurring along an acoustic path. A field experiment was carried out in the Columbia River Estuary to test the hypothesis: the estuarine salt wedge is acoustically observable in terms of low-to-mid-frequency acoustic propagation. Linear frequency-modulated acoustic signals in the 500–2000 Hz band were transmitted during the advance and retreat of the salt wedge during May 27–29, 2013. Results demonstrate that the salt wedge front is the dominant physical mechanism controlling acoustic propagation in this environment: received signal energy is relatively stable before and after the passage of the salt wedge front when the acoustic path consists of a single medium (either entirely fresh water or entirely salt water), and suffers a 10–15 dB loss and increased variability during salt wedge front passage. Physical parameters and acoustic propagationmodeling corroborate and inform the acoustic observations.

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Atmospheric noise of a breaking tidal bore

A tidal bore is a surge of waters propagating upstream in an estuary as the tidal flow turns to rising and the flood tide propagates into a funnel-shaped system. Large tidal bores have a marked breaking roller. The sounds generated by breaking tidal bores were herein investigated in the field (Qiantang River) and in laboratory. The sound pressure record showed two dominant periods, with some similarity with an earlier study [Chanson (2009). J. Acoust. Soc. Am. 125(6), 3561–3568]. The two distinct phases were the incoming tidal bore when the sound amplitude increased with the approaching bore, and the passage of the tidal bore in front of the microphone when loud and powerful noises were heard. The dominant frequency ranged from 57 to 131 Hz in the Qiantang River bore. A comparison between laboratory and prototype tidal bores illustrated both common features and differences. The low pitch sound of the breaking bore had a dominant frequency close to the collective oscillations of bubble clouds, which could be modeled with a bubble cloud model using a transverse dimension of the bore roller. The findings suggest that this model might be over simplistic in the case of a powerful breaking bore, like that of the Qiantang River.

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The neural encoding of formant frequencies contributing to vowel identification in normal-hearing listeners

Even though speech signals trigger coding in the cochlea to convey speech information to the central auditory structures, little is known about the neural mechanisms involved in such processes. The purpose of this study was to understand the encoding of formant cues and how it relates to vowel recognition in listeners. Neural representations of formants may differ across listeners; however, it was hypothesized that neural patterns could still predict vowel recognition. To test the hypothesis, the frequency-following response (FFR) and vowel recognition were obtained from 38 normal-hearing listeners using four different vowels, allowing direct comparisons between behavioral and neural data in the same individuals. FFR was employed because it provides an objective and physiological measure of neural activity that can reflect formantencoding. A mathematical model was used to describe vowel confusion patterns based on the neural responses to vowelformant cues. The major findings were (1) there were large variations in the accuracy of vowelformantencoding across listeners as indexed by the FFR, (2) these variations were systematically related to vowel recognition performance, and (3) the mathematical model of vowel identification was successful in predicting good vs poor vowel identification performers based exclusively on physiological data.

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