Pure-tone Signal Research Articles

Quantifying the complex transmission of substrate‐borne vibrations with scanning laser vibrometry

AbstractSubstrate‐borne vibrations are ubiquitous in nature and are used by diverse taxa to communicate and to obtain information about their environments. However, substrate‐borne vibrations remain understudied compared with other sensory and signaling modalities, in part due to human sensory biases. In addition, understanding and quantifying the transmission of vibrations remains a challenging task due to it being dependent on both signal properties and properties of the substrates that the signals transmit through. Here, we provide methods for playing back and measuring the transmission of vibrations throughout a substrate. Using linear resonant actuators, we conducted playbacks of pure tones and frequency sweeps on wooden dowels and on the stems of potted Ptelea trifoliata L. (Rutaceae) plants. We used scanning laser Doppler vibrometry to measure the signals at multiple locations along the length of the dowels and plant stems. We demonstrate that playback of a frequency sweep yields more data in a shorter amount of time than multiple playbacks and measurements of pure tone signals. Our results are also consistent with previous findings showing that signals produce frequency and location specific minima and maxima (nodes and antinodes) throughout the substrates, rather than simply attenuating with distance. This results in filtering of signals, such that their spectra are unique at any given measurement location—illustrating the importance of measuring vibrations at multiple locations. We discuss the implications of such filtering phenomena for vibrationally signaling animals and the biotremologists that study them.

Adopting a reductionist approach to advance acoustic deterrents in fish conservation

IntroductionDeterrents that use acoustics to guide fish away from dangerous areas (e.g., water intakes) depend on the elicitation of avoidance in the target species. Background noise is often neglected when testing acoustic deterrents, but it is important to account for its effects as freshwater environments present a wide variety of ambient soundscapes.MethodsUsing the widely studied goldfish (Carassius auratus) as a suitable experimental model, this study adopted a reductionist approach to investigate the relationship between the startle response to a pure tone signal and background noise. Under laboratory conditions, the startle responses of individual goldfish exposed to 120 ms tones at 250 Hz and four Sound Pressure Levels (SPLs: 115, 125, 135, 145 dB re 1 μPa) were quantified in the presence (treatment) and absence (ambient - control) of band-limited random noise (105 dB re 1 μPa).Results and discussionWhen observing the dose-response relationship, the proportion of fish that startled to the signal increased with SPL in both the treatment and control, although there was no difference between them, suggesting that the signal-to-noise ratio was not influential under the conditions tested. However, further analysis using Signal Detection Theory indicated that the was higher in the noisy treatment than the control when responding to both false alarms (startle during a pre-signal period) and hits (startle to an external stimulus during the pre-signal period). Furthermore, fish were better able to discriminate (d?) external stimuli over time (during the pre-signal period) in the treatment than control. There is a need to consider the role of background noise when designing acoustic fish deterrents that depend on the exhibition of avoidance behaviors.

Binaural masking level difference for pure tone signals

The binaural masking level difference (BMLD) is a psychoacoustic method to determine binaural interaction and central auditory processes. The BMLD is the difference in hearing thresholds in homophasic and antiphasic conditions. The duration, phase and frequency of the stimuli can affect the BMLD. The main aim of the study is to evaluate the BMLD for stimuli of different durations and frequencies which could also be used in future electrophysiological studies. To this end we developed a GUI to present different frequency signals of variable duration and determine the BMLD. Three different durations and five different frequencies are explored. The results of the study confirm that the hearing threshold for the antiphasic condition is lower than the hearing threshold for the homophasic condition and that differences are significant for signals of 18ms and 48ms duration. Future objective binaural processing studies will be based on 18ms and 48ms stimuli with the same frequencies as used in the current study.

Learning to detect auditory signals in noise: Active top-down selection and stable change in signal representations.

Training can improve detection of auditory signals in noise. This learning could potentially occur through active top-down selection mechanisms or stable changes in signal representations. Here, participants were trained and tested (pretest vs. posttest design) on abilities to detect pure tone signals in noise. Auditory evoked potentials (AEPs) to tones were gathered under dichotic listening conditions where participants attended to nontonal stimuli in the opposite ear. Improvements in detection sensitivity were observable regardless of tested tone frequency. This was true in generalization between 861 Hz and 1058-Hz tones (Experiment 1a), and when testing a frequency range > 1 octave (Experiment 2). Such learning was not apparent without training (Experiment 1b). In contrast to behavior, AEP amplitude increases from pre- to posttest were partially specific to trained tone frequencies, even when selective attention was diverted to the opposite ear of tone presentation. Placed in the context of previous work, results suggest that changes in active top-down selection mechanisms and stable signal representations both play a role in auditory detection learning. The mismatch between AEP and behavioral effects suggests a need to consider how these different learning processes can impact detection performance in the variety of listening scenarios a listener may face. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Objective Signal Analysis for Investigating Feasibility of Active Noise Cancellation in Hearing Screening.

With the development of active noise cancellation (ANC) technology, ANC has been used to mitigate the effects of environmental noise on audiometric results. However, objective evaluation methods supporting the accuracy of audiometry for ANC exposure to different levels of noise have not been reported. Accordingly, the audio characteristics of three different ANC headphone models were quantified under different noise conditions and the feasibility of ANC in noisy environments was investigated. Steady (pink noise) and non-steady noise (cafeteria babble noise) were used to simulate noisy environments. We compared the integrity of pure-tone signals obtained from three different ANC headphone models after processing under different noise scenarios and analyzed the degree of ANC signal correlation based on the Pearson correlation coefficient compared to pure-tone signals in quiet. The objective signal correlation results were compared with audiometric screening results to confirm the correspondence. Results revealed that ANC helped mitigate the effects of environmental noise on the measured signal and the combined ANC headset model retained the highest signal integrity. The degree of signal correlation was used as a confidence indicator for the accuracy of hearing screening in noise results. It was found that the ANC technique can be further improved for more complex noisy environments.

Association of Performance on Dichotic Auditory Tests With Risk for Incident Dementia and Alzheimer Dementia

Age-related hearing difficulties can include problems with signal audibility and central auditory processing. Studies have demonstrated associations between audibility and dementia risk. To our knowledge, limited data exist to determine whether audibility, central processing, or both drive these associations. To determine the associations between signal sensitivity, central auditory processing, and dementia and Alzheimer dementia (AD) risk. This follow-up observational study of a sample from the prospective Adult Changes in Thought study of dementia risk was conducted at Kaiser Permanente Washington, a western Washington health care delivery system, and included 280 volunteer participants without dementia who were evaluated from October 2003 to February 2006 with follow-up through September 2018. Analyses began in 2019 and continued through 2021. Hearing tests included pure tone signal audibility, a monaural word recognition test, and 2 dichotic tests: the Dichotic Sentence Identification (DSI) test and the Dichotic Digits test (DDT). Cognition was assessed biennially with the Cognitive Abilities Screening Instrument (range, 1-100; higher scores are better), and scores of less than 86 prompted clinical and neuropsychological evaluations. All data were reviewed at multidisciplinary consensus conferences, and standardized criteria were used to define incident cases of dementia and probable or possible AD. Cox proportional hazard models were used to determine associations with hearing test performance. A total of 280 participants (177 women [63%]; mean [SD] age, 79.5 [5.2] years). As of September 2018, there were 2196 person-years of follow-up (mean, 7.8 years) and 89 incident cases of dementia (66 not previously analyzed), of which 84 (94.4%) were AD (63 not previously analyzed). Compared with people with DSI scores of more than 80, the dementia adjusted hazard ratio (aHR) for DSI scores of less than 50 was 4.18 (95% CI, 2.37-7.38; P < .001); for a DSI score of 50 to 80, it was 1.82 (95% CI, 1.10-3.04; P = .02). Compared with people with DDT scores of more than 80, the dementia aHR for DDT scores of less than 50 was 2.66 (95% CI, 1.31-5.42; P = .01); for a DDT score of 50 to 80, it was 2.40 (95% CI, 1.45-3.98; P = .001). The AD results were similar. Pure tone averages were weakly and insignificantly associated with dementia and AD, and associations were null when controlling for DSI scores. In this cohort study, abnormal central auditory processing as measured by dichotic tests was independently associated with dementia and AD risk.

El examen del audífono digital

Entre los profesionales dedicados a la adaptación protésica existe la idea generalizada de que la respuesta de los audífonos digitales no puede ser examinada. Esta idea es errónea toda vez que disponemos de la tecnología para poder llevar acabo estos estudios. En este artículo presentamos la metodología necesaria para el examen de audífonos digitales mediante el uso de señales de tonos puros y compuestos. Esta metodología permite el análisis de forma interactiva de la respuesta de los filtros de reducción de ruido característicos de los audífonos digitales. Así mimo se lleva a cabo una revisión del análisis de las respuestas mediante el uso de técnicas de espectografía.Traducido con autorización del autor por Franz Zenker. Referencia original del artículo: Frye GJ. Testing Digital Hearing Instruments [en linea]. Hearing Review. 13 link

Tenors Not Sopranos: Bio-Mechanical Constraints on Calling Song Frequencies in the Mediterranean Field-Cricket

Male crickets and their close relatives bush-crickets (Gryllidae and Tettigoniidae, respectively; Orthoptera and Ensifera) attract distant females by producing loud calling songs. In both families, sound is produced by stridulation, the rubbing together of their forewings, whereby the plectrum of one wing is rapidly passed over a serrated file on the opposite wing. The resulting oscillations are amplified by resonating wing regions. A striking difference between Gryllids and Tettigoniids lies in wing morphology and composition of song frequency: Crickets produce mostly low-frequency (2–8 kHz), pure tone signals with highly bilaterally symmetric wings, while bush-crickets use asymmetric wings for high-frequency (10–150 kHz) calls. The evolutionary reasons for this acoustic divergence are unknown. Here, we study the wings of actively stridulating male field-crickets (Gryllus bimaculatus) and present vibro-acoustic data suggesting a biophysical restriction to low-frequency song. Using laser Doppler vibrometry (LDV) and brain-injections of the neuroactivator eserine to elicit singing, we recorded the topography of wing vibrations during active sound production. In freely vibrating wings, each wing region resonated differently. When wings coupled during stridulation, these differences vanished and all wing regions resonated at an identical frequency, that of the narrow-band song (∼5 kHz). However, imperfections in wing-coupling caused phase shifts between both resonators, introducing destructive interference with increasing phase differences. The effect of destructive interference (amplitude reduction) was observed to be minimal at the typical low frequency calls of crickets, and by maintaining the vibration phase difference below 80°. We show that, with the imperfect coupling observed, cricket song production with two symmetric resonators becomes acoustically inefficient above ∼8 kHz. This evidence reveals a bio-mechanical constraint on the production of high-frequency song whilst using two coupled resonators and provides an explanation as to why crickets, unlike bush-crickets, have not evolved to exploit ultrasonic calling songs.

Multisensory perceptual interactions between higher-order temporal frequency signals.

Naturally occurring signals in audition and touch can be complex and marked by temporal variations in frequency and amplitude. Auditory frequency sweep processing has been studied extensively; however, much less is known about sweep processing in touch because studies have primarily focused on the perception of simple sinusoidal vibrations. Given the extensive interactions between audition and touch in the frequency processing of pure tone signals, we reasoned that these senses might also interact in the processing of higher-order frequency representations like sweeps. In a series of psychophysical experiments, we characterized the influence of auditory distractors on the ability of participants to discriminate tactile frequency sweeps. Auditory frequency sweeps systematically biased the tactile perception of sweep direction. Importantly, auditory cues exerted little influence on tactile sweep direction perception when the sounds and vibrations occupied different absolute frequency ranges or when the sounds consisted of intensity sweeps. Thus, audition and touch interact in frequency sweep perception in a frequency- and feature-specific manner. Our results demonstrate that audio-tactile interactions are not constrained to the processing of simple sinusoids. Because higher-order frequency representations may be synthesized from simpler representations, our findings imply that multisensory interactions in the temporal frequency domain span multiple hierarchical levels in sensory processing. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Octave Error Reduction in Pitch Detection Algorithms Using Fourier Series Approximation Method

ABSTRACTPitch detection has been an area of research in the field on Music Information retrieval, which has always received great attention. Pitch is defined as the frequency of a pure tone signal which the brain perceives while listening to a signal. Pitch detection methods ideally identify the fundamental frequency (f0) or fundamental time period (T) of a signal and f0 is defined as the inverse of T. Several pitch detection algorithms have been proposed in the past and each one has its advantages and disadvantages. One of the prominent problems seen across all pitch detection algorithms is to identify the correct octave to which the pitch or f0 belongs to. Ideally, we are required to find the fundamental harmonic. Due to the structure of vocal tract and the acoustic nature of the instruments and due to various other factors like noise, the speech, and music signals are not perfectly periodic in nature and are termed as quasi-periodic signals. In this paper, we have proposed a method using Fourier series to approximate the quasi-periodic signals with a periodic signal, which analyses the strengths of various harmonics of the signal to arrive at a better estimate of the pitch for monophonic sounds. The brain perceives a periodic signal even while listening to a quasi-periodic signal. Hence, the idea is to approximate the quasi-periodic signal with a perfectly periodic signal and estimate the fundamental frequency of the signal which gives the best approximation. The method was tested on synthetic signals and real music signals and found to improve the pitch detection accuracy.

Effects of temporal coherence and signal-frequency uncertainty for tone detection in a random-frequency multi-tonal masker

Detection thresholds tend to be lower when the spectral and/or temporal characteristics of the signal are predictable than unpredictable, particularly in an unpredictable masker. The present study evaluated the detrimental effect of frequency uncertainty and the beneficial effect of temporal coherence for a pure-tone signal presented in a multi-tonal masker with unpredictable frequency sequences. Stimuli were composed of 80-ms tone bursts. The signal was 1, 2, 4, or 8 sequential tone bursts, and burst frequency was either fixed across trials or randomly varied across trials but fixed across bursts. The masker was composed of four streams of tone bursts; the frequency of each burst was randomly drawn from a uniform distribution 250-4000 Hz, with the caveat that synchronous masker and signal bursts were separated by 1/5 oct or more. Signal and masker bursts were synchronously gated, and the masker played continuously throughout a threshold estimation track. Thresholds tended to improve with increasing numbers of signal bursts and worsen with increasing signal frequency uncertainty. These factors appeared to interact; signal frequency uncertainty was less detrimental when the signal was composed of larger numbers of bursts. Preliminary models of detection will be discussed.

Noise Induced Hearing Enhancement: Clinical and Machine Learning Studies

The addition of a certain amount of background noise has been shown to improve the hearing of pure-tone sounds. The underlying mechanism of this counter-intuitive phenomenon is believed to be stochastic resonance, where the threshold level of a pure-tone signal is lowered by adding an appropriate amount of background noise. Here, to investigate whether background noise can aid the hearing of more complex sound such as the human voice, we perform hearing tests with Korean syllables, and find that syllable recognition is enhanced with noise for rarely used syllables. Similar tests with a machine-learning speech recognition shows that the same enhancement arises only when the system is insufficiently trained, corresponding to hearing a rare syllable. The overall phenomenon looks similar to the stochastic resonance but any successful model should explain that the enhancement arises in perceptual processes as it depends on the level of training and the syllables.

Effect of temporal asynchrony on children’s detection performance in a random-frequency, multi-tonal masking task

This study examined the degree to which children can use temporal onset/offset asynchronies to improve performance in a random-frequency, multi-tonal masking task. Normal-hearing school-age children (5-10 years) and adults were tested in two experiments. In Experiment 1, masked thresholds were measured in a 3IFC adaptive procedure. A 1000-Hz pure-tone signal was presented with a random-frequency, 4-tone masker played at 60 dB SPL. Onset/offset asynchrony was manipulated across conditions by fixing the signal duration at 200 ms and examining performance for masker durations of 200, 280, and 400 ms. The signal, when present, was temporally centered in the masker, resulting in asynchronies of 0, 40 and 100 ms. In Experiment 2, asynchrony was manipulated by holding masker duration constant and varying signal duration to be 200, 320, or 400 ms. Preliminary results suggest that children can use temporal onset/offset asynchrony to segregate a fixed-frequency signal tone from a tonal masker. Potential development...

Adults, but not children, benefit from a pretrial signal cue in a random-frequency, two-tone masker.

This study examined the benefit of a pretrial cue, a preview of the signal, on children's (5-10 years) and adults' detection of a 1000-Hz pure-tone signal in a broadband noise or a random-frequency, two-tone masker. No cuing effect was observed with the noise masker, regardless of listener age. In contrast, all but one adult benefited from the cue with the two-tone masker (average = 9.4 dB). Most children showed no cuing effect (average = 0.1 dB) with the two-tone masker. These results suggest that, unlike adults, the provision of a pretrial cue does not promote frequency-selective listening during detection for 5- to 10-year-olds.

Dynamic binaural sound source localization with interaural time difference cues: Artificial listeners

When an array of two acoustic sensors is used to localize sound sources based on time differences alone, possible solutions form a cone of confusion. This study, together with a similar one for human listeners, demonstrates that azimuth/vertical planes localization of a sound source using only time difference information is feasible when self-motion measurement of the listener is available. In particular, the case of a static sound source playing that broadcasts low frequency pure tone signals was investigated. A dummy head is mounted on top of a rotating chair to mimic the head and body motion of human beings, as well as to collect audio signals. A gyroscope was mounted on top of the dummy head to collect self-motion data. A mathematical model was constructed to describe the interaural time difference (ITD) change over time, and an Extended Kalman Filter (EKF) was used to estimate the spatial angles of the sound sources with respect to the listener using the developed mathematical model and measured data. The effectiveness and robustness of the developed algorithm are shown by both the numerical and experimental results, which reveal the quick convergence of the estimated spatial angles toward their real values given noisy measured data. The possibilities of using other spatial hearing cues were also discussed.

Dynamic binaural sound source localization with ITD cues: Human listeners

In real life, human listeners rarely experience cone of confusion errors in localization of sound sources due to the limitation of interaural time difference (ITD) as a spatial hearing cue. Previous work on robotics suggested that the confusion is disambiguated as successive observation of ITD is made over time, when self-motion of the microphone system was allowed. In this behavioral study, we investigated whether horizontal/vertical planes localization with time difference is possible when self-motion is allowed for human listeners. In particular, the case of a static sound source playing low frequency pure tone signal was studied. Human listeners were seated in a rotating chair in the middle of a loudspeaker array, and a low frequency audio signal was played at an elevation. The task was to identify the elevation spatial angle under conditions when vision was allowed and denied. The hypothesis was, with vision, a much more robust observation of self-motion is available, and the report of elevation would be more accurate. The results largely agreed with the hypothesis. A similar study for robotic hearing generated similar results.

Processing of simple and complex acoustic signals in a tonotopically organized ear.

Processing of complex signals in the hearing organ remains poorly understood. This paper aims to contribute to this topic by presenting investigations on the mechanical and neuronal response of the hearing organ of the tropical bushcricket species Mecopoda elongata to simple pure tone signals as well as to the conspecific song as a complex acoustic signal. The high-frequency hearing organ of bushcrickets, the crista acustica (CA), is tonotopically tuned to frequencies between about 4 and 70 kHz. Laser Doppler vibrometer measurements revealed a strong and dominant low-frequency-induced motion of the CA when stimulated with either pure tone or complex stimuli. Consequently, the high-frequency distal area of the CA is more strongly deflected by low-frequency-induced waves than by high-frequency-induced waves. This low-frequency dominance will have strong effects on the processing of complex signals. Therefore, we additionally studied the neuronal response of the CA to native and frequency-manipulated chirps. Again, we found a dominant influence of low-frequency components within the conspecific song, indicating that the mechanical vibration pattern highly determines the neuronal response of the sensory cells. Thus, we conclude that the encoding of communication signals is modulated by ear mechanics.

FPGA-based voltage and current dual drive system for high frame rate electrical impedance tomography.

Electrical impedance tomography (EIT) is used to image the electrical property distribution of a tissue under test. An EIT system comprises complex hardware and software modules, which are typically designed for a specific application. Upgrading these modules is a time-consuming process, and requires rigorous testing to ensure proper functioning of new modules with the existing ones. To this end, we developed a modular and reconfigurable data acquisition (DAQ) system using National Instruments' (NI) hardware and software modules, which offer inherent compatibility over generations of hardware and software revisions. The system can be configured to use up to 32-channels. This EIT system can be used to interchangeably apply current or voltage signal, and measure the tissue response in a semi-parallel fashion. A novel signal averaging algorithm, and 512-point fast Fourier transform (FFT) computation block was implemented on the FPGA. FFT output bins were classified as signal or noise. Signal bins constitute a tissue's response to a pure or mixed tone signal. Signal bins' data can be used for traditional applications, as well as synchronous frequency-difference imaging. Noise bins were used to compute noise power on the FPGA. Noise power represents a metric of signal quality, and can be used to ensure proper tissue-electrode contact. Allocation of these computationally expensive tasks to the FPGA reduced the required bandwidth between PC, and the FPGA for high frame rate EIT. In 16-channel configuration, with a signal-averaging factor of 8, the DAQ frame rate at 100 kHz exceeded 110 frames s (-1), and signal-to-noise ratio exceeded 90 dB across the spectrum. Reciprocity error was found to be for frequencies up to 1 MHz. Static imaging experiments were performed on a high-conductivity inclusion placed in a saline filled tank; the inclusion was clearly localized in the reconstructions obtained for both absolute current and voltage mode data.

Binaural Masking Release in Children With Down Syndrome

Binaural hearing results in a number of listening advantages relative to monaural hearing, including enhanced hearing sensitivity and better speech understanding in adverse listening conditions. These advantages are facilitated in part by the ability to detect and use interaural cues within the central auditory system. Binaural hearing for children with Down syndrome could be impacted by multiple factors including, structural anomalies within the peripheral and central auditory system, alterations in synaptic communication, and chronic otitis media with effusion. However, binaural hearing capabilities have not been investigated in these children. This study tested the hypothesis that children with Down syndrome experience less binaural benefit than typically developing peers. Participants included children with Down syndrome aged 6 to 16 years (n = 11), typically developing children aged 3 to 12 years (n = 46), adults with Down syndrome (n = 3), and adults with no known neurological delays (n = 6). Inclusionary criteria included normal to near-normal hearing sensitivity. Two tasks were used to assess binaural ability. Masking level difference (MLD) was calculated by comparing threshold for a 500-Hz pure-tone signal in 300-Hz wide Gaussian noise for N0S0 and N0Sπ signal configurations. Binaural intelligibility level difference was calculated using simulated free-field conditions. Speech recognition threshold was measured for closed-set spondees presented from 0-degree azimuth in speech-shaped noise presented from 0-, 45- and 90-degree azimuth, respectively. The developmental ability of children with Down syndrome was estimated and information regarding history of otitis media was obtained for all child participants via parent survey. Individuals with Down syndrome had higher masked thresholds for pure-tone and speech stimuli than typically developing individuals. Children with Down syndrome had significantly smaller MLDs than typically developing children. Adults with Down syndrome and control adults had similar MLDs. Similarities in simulated spatial release from masking were observed for all groups for the experimental parameters used in this study. No association was observed for any measure of binaural ability and developmental age for children with Down syndrome. Similar group psychometric functions were observed for children with Down syndrome and typically developing children in most instances, suggesting that attentiveness and motivation contributed equally to performance for both groups on most tasks. The binaural advantages afforded to typically developing children, such as enhanced hearing sensitivity in noise, were not as robust for children with Down syndrome in this study. Children with Down syndrome experienced less binaural benefit than typically developing peers for some stimuli, suggesting that they could require more favorable signal-to-noise ratios to achieve optimal performance in some adverse listening conditions. The reduced release from masking observed for children with Down syndrome could represent a delay in ability rather than a deficit that persists into adulthood. This could have implications for the planning of interventions for individuals with Down syndrome.

Modulation masking attributes of narrowband and low-noise noise forward maskers in normal-hearing and hearing-impaired listeners

Savel and Bacon (2003) measured detection thresholds for a 4000 Hz pure-tone signal in the presence of a narrowband noise (NBN) or a low-noise noise (LNN) simultaneous masker. The authors asserted that fluctuations in the envelope of the NBN were likely responsible for its increased masking effectiveness. Because modulation detection interference (MDI) is larger for hearing-impaired (HI) than normal-hearing (NH) listeners using modulated simultaneous maskers (e.g., Lorenzi et al., 1997) and forward maskers (e.g., Koopman et al., 2008), we measured detection thresholds for NH and HI listeners for pure tones in the presence of either NBN or LNN forward maskers that were either 1-ERB wide or 1/3-ERB wide. Based on previous MDI findings (Koopman et al., 2008), we predicted larger differences in masked thresholds for the NBN and LNN conditions for HI than NH listeners. These results for detection thresholds for pure-tone signals have implications for interpreting differences in modulated forward masking for NH and HI listeners. [Work supported by grants from NIH/NIDCD.]