Detection of high-frequency spectral notches as a function of level

High-frequency spectral notches are important cues for sound localization. Our ability to detect them must depend on their representation as auditory nerve (AN) rate profiles. Because of the low threshold and the narrow dynamic range of most AN fibers, these rate profiles deteriorate at high levels. The system may compensate by using onset rate profiles whose dynamic range is wider, or by using low-spontaneous-rate fibers, whose threshold is higher. To test these hypotheses, the threshold notch depth necessary to discriminate between a flat spectrum broadband noise and a similar noise with a spectral notch centered at 8 kHz was measured at levels from 32 to 100 dB SPL. The importance of the onset rate-profile representation of the notch was estimated by varying the stimulus duration and its rise time. For a large proportion of listeners, threshold notch depth varied nonmonotonically with level, increasing for levels up to 70-80 dB SPL and decreasing thereafter. The nonmonotonic aspect of the function was independent of notch bandwidth and stimulus duration. Thresholds were independent of stimulus rise time but increased for the shorter noise bursts. Results are discussed in terms of the ability of the AN to convey spectral notch information at different levels.     

A population study of auditory-nerve fibers in unanesthetized decerebrate cats: response to pure tones

Population responses of single auditory-nerve (AN) fibers to 1-kHz pure tones were investigated in unanesthetized decerebrate cats. Driven rate (spikes/s), Fourier component amplitude (spikes/s), and d'e = (driven rate)/(standard deviation) were examined. These results extend the presently available information. First, the AN population responses, previously observed only in anesthetized animals, are now observed in unanesthetized animals. The anesthetized and unanesthetized animal responses are qualitatively similar. Second, a detectability measure d'e was applied, à la signal detection theory, to populations of AN fibers. A previously unknown behavior of the low-spontaneous-rate (SR) (less than 15 spikes/s) fibers was observed: the d'e increased to large values at moderate and high stimulus levels, giving rise to a large, sharply tuned peak in the d'e-place profile, even when the driven rate was saturated. This behavior was absent in the high-SR (greater than 15 spikes/s) fibers. This observation suggests that the d'e of the low-SR fibers is the best code for stimulus intensity among the different response measures examined, and that the d'e-place profile may be a more precise spatial code for stimulus frequency than the commonly considered rate-place code.     

The influence of systematic primary-tone level variation L2-L1 on the acoustic distortion product emission 2f1-f2 in normal human ears

The purpose of the present study was to determine the effect of primary-tone level variation, L2--L1, on the amplitude of distortion-product otoacoustic emissions (DPOAEs). The DPOAE at the frequency 2f1--f2 (f2 greater than f1) was measured in 20 ears of ten normally hearing subjects. Acoustic distortion products were generated by primaries f1 and f2 with geometric mean frequencies of 1, 2, and 4 kHz. The f2/f1 ratios were 1.25 (1 kHz), 1.23 (2 kHz), and 1.21 (4 kHz). The primary-tone level L1 was kept constant at either 65 or 75 dB SPL while the second primary-tone level L2 was varied between 20 and 90 dB SPL in 5-dB steps. The level differences L2--L1 generating maximal DPOAE amplitudes depended on L1 and on the geometric mean frequency of f1 and f2. There were large interindividual differences. Overall, the L2--L1 evoking maximal mean DPOAE amplitudes was --10 dB for geometric mean frequencies of 1 and 2 kHz with both L1 = 65 dB SPL and L1 = 75 dB SPL. For 4 kHz, L2-L1 was --5 dB with L1 = 65 dB SPL and 0 dB with L1 = 75 dB SPL. The mean slopes of the DPOAE growth functions in the initial linearly increasing portions were steeper at higher stimulus frequencies, increasing from 0.52 at 1 kHz to 0.72 at 4 kHz for L1 = 65 dB SPL and from 0.48 at 1 kHz to 0.72 at 4 kHz for L1 = 75 dB SPL.     

Frequency selectivity of single cochlear-nerve fibers based on the temporal response pattern to two-tone signals

The physiological basis of auditory frequency selectivity was investigated by recording the temporal response patterns of single cochlear-nerve fibers in the cat. The characteristic frequency and sharpness of tuning was determined for low-frequency cochlear-nerve fibers with two-tone signals whose frequency components were of equal amplitude and starting phase. The measures were compared with those obtained with sinusoidal signals. The two-tone characteristic frequency (2TCF) is defined as the arithmetic-center frequency at which the fiber is synchronized to both signal frequencies in equal measure. The 2TCF closely corresponds to the characteristic frequency as determined by the frequency threshold curve. Moreover, the 2TCF changes relatively little (2%-12%) over a 60-dB intensity range. The 2TCF generally shifts upward with increasing intensity for cochlear-nerve fibers tuned to frequencies below 1 kHz and shifts downward as a function of intensity for units with characteristic frequencies (CF's) above 1 kHz. The shifts in the 2TCF are considerably smaller than those observed with sinusoidal signals. Filter functions were derived from the synchronization pattern to the two-tone signal by varying the frequency of one of the components over the fiber's response area while maintaining the other component at the 2TCF. The frequency selectivity of the two-tone filter function was determined by dividing the vector strength to the variable frequency signal by the vector strength to the CF tone. The filter function was measured 10 dB down from the peak (2T Q 10 dB) and compared with the Q 10 dB of the frequency threshold curve. The correlation between the two measures of frequency selectivity was 0.72. The 2T Q 10 dB does change as a function of intensity. The magnitude and direction of the change is dependent on the sharpness of tuning at low and moderate sound-pressure levels (SPL's). The selectivity of the more sharply tuned fibers (2T Q 10 dB greater than 3) diminishes at intensities above 60 dB SPL. However, the broadening of selectivity is relatively small in comparison to discharge rate-based measures of selectivity. The selectivity of the more broadly tuned units remains unchanged or improves slightly at similar intensity levels. The present data indicate that the frequency selectivity and tuning of low-frequency cochlear-nerve fibers are relatively stable over a 60-dB range of SPL's when measured in terms of their temporal discharge properties.     

Toneburst-evoked auditory brainstem response in a leopard seal, Hydrurga leptonyx

Toneburst-evoked auditory brainstem responses (ABRs) were recorded in a captive subadult male leopard seal. Three frequencies from 1 to 4 kHz were tested at sound levels from 68 to 122 dB peak equivalent sound pressure level (peSPL). Results illustrate brainstem activity within the 1-4 kHz range, with better hearing sensitivity at 4 kHz. As is seen in human ABR, only wave V is reliably identified at the lower stimulus intensities. Wave V is present down to levels of 82 dB peSPL in the right ear and 92 dB peSPL in the left ear at 4 kHz. Further investigations testing a wider frequency range on seals of various sex and age classes are required to conclusively report on the hearing range and sensitivity in this species.     

Conditioning-induced protection from impulse noise in female and male chinchillas

Sound conditioning (pre-exposure to a moderate-level acoustic stimulus) can induce resistance to hearing loss from a subsequent traumatic exposure. Most sound conditioning experiments have utilized long-duration tones and noise at levels below 110 dB SPL as traumatic stimuli. It is important to know if sound conditioning can also provide protection from brief, high-level stimuli such as impulses produced by gunfire, and whether there are differences between females and males in the response of the ear to noise. In the present study, chinchillas were exposed to 95 dB SPL octave band noise centered at 0.5 kHz for 6 h/day for 5 days. After 5 days of recovery, they were exposed to simulated M16 rifle fire at a level of 150 dB peak SPL. Animals that were sound conditioned showed less hearing loss and smaller hair cell lesions than controls. Females showed significantly less hearing loss than males at low frequencies, but more hearing loss at 16 kHz. Cochleograms showed slightly less hair cell loss in females than in males. The results show that significant protection from impulse noise can be achieved with a 5-day conditioning regimen, and that there are consistent differences between female and male chinchillas in the response of the cochlea to impulse noise.     

Level dependence of distortion-product otoacoustic emissions measured at high frequencies in humans

Given that high-frequency hearing is most vulnerable to cochlear pathology, it is important to characterize distortion-product otoacoustic emissions (DPOAEs) measured with higher-frequency stimuli in order to utilize these measures in clinical applications. The purpose of this study was to explore the dependence of DPOAE amplitude on the levels of the evoking stimuli at frequencies greater than 8 kHz, and make comparisons with those data that have been extensively measured with lower-frequency stimuli. To accomplish this, DPOAE amplitudes were measured at six different f2 frequencies (2, 5, 10, 12, 14, and 16 kHz), with a frequency ratio (f2/f1) of 1.2, at five fixed levels (30 to 70 dB SPL) of one primary (either f1 or f2), while the other primary was varied in level (30 to 70 dB SPL). Generally, the level separation between the two primary tones (L1 > L2) generating the largest DPOAE amplitude (referred to as the "optimal level separation") decreased as the level of the fixed primary increased. Additionally, the optimal level separation was frequency dependent, especially at the lower fixed primary tone levels ( < or = 50 dB SPL). In agreement with previous studies, the DPOAE level exhibited greater dependence on L1 than on L2.     

Speech coding in the auditory nerve: III. Voiceless fricative consonants

Responses of auditory-nerve fibers in anesthetized cats were recorded for synthetic voiceless fricative consonants. The four stimuli (/x/, /s/, /s/, and /f/) were presented at two levels corresponding to speech in which the levels of the vowels would be approximately 60 and 75 dB SPL, respectively. Discharge patterns were characterized in terms of PST histograms and their power spectra. For both stimulus levels, frequency regions in which the stimuli had considerable energy corresponded well with characteristic-frequency (CF) regions in which average discharge rates were the highest. At the higher level, the profiles of discharge rate against CF were more distinctive for the stimulus onset than for the central portion. Power spectra of PST histograms had large response components near fiber characteristic frequencies for CFs up to 3-4 kHz, as well as low-frequency components for all fibers. The relative amplitudes of these components varied for the different stimuli. In general, the formant frequencies of the fricatives did not correspond with the largest response components, except for formants below about 3 kHz. Processing schemes based on fine time patterns of discharge that were effective for vowel stimuli generally failed to extract the formant frequencies of fricatives.     

Overshoot in normal-hearing and hearing-impaired subjects.

Overshoot was measured in both ears of four subjects with normal hearing and in five subjects with permanent, sensorineural hearing loss (two with a unilateral loss). The masker was a 400-ms broadband noise presented at a spectrum level of 20, 30, or 40 dB SPL. The signal was a 10-ms sinusoid presented 1 or 195 ms after the onset of the masker. Signal frequency was 1.0 or 4.0 kHz, which placed the signal in a region of normal (1.0 kHz) or impaired (4.0 kHz) absolute sensitivity for the impaired ears. For the normal-hearing subjects, the effects of signal frequency and masker level were similar to those published previously. In particular, overshoot was larger at 4.0 than at 1.0 kHz, and overshoot at 4.0 kHz tended to decrease with increasing masker level. At 4.0 kHz, overshoot values were significantly larger in the normal ears: Maximum values ranged from about 7-26 dB in the normal ears, but were always less than 5 dB in the impaired ears. The smaller overshoot values resulted from the fact that thresholds in the short-delay condition were considerably better in the hearing-impaired subjects than in the normal-hearing subjects. At 1.0 kHz, overshoot values for the two groups of subjects more or less overlapped. The results suggest that permanent, sensorineural hearing loss disrupts the mechanisms responsible for a large overshoot effect.     

Basilar membrane mechanics at the base of the chinchilla cochlea. I. Input-output functions, tuning curves, and response phases

Basilar membrane (BM) velocity was measured at a site 3.5 mm from the basal end of the chinchilla cochlea using the M?ssbauer technique. The threshold of the compound action potential recorded at the round window in response to tone bursts was used as an indicator of the physiological state of the cochlea. The BM input-output functions display a compressive nonlinearity for frequencies around the characteristic frequency (CF, 8 to 8.75 kHz), but are linear for frequencies below 7 and above 10.5 kHz. In preparations with little surgical damage, isovelocity tuning curves at 0.1 mm/s are sharply tuned, have Q10's of about 6, minima as low as 13 dB SPL, tip-to-tail ratios (at 1 kHz) of 56 to 76 dB, and high-frequency slopes of about 300 dB/oct. These mechanical responses are as sharply tuned as frequency-threshold curves of chinchilla auditory nerve fibers with corresponding CF. There is a progressive loss of sensitivity of the mechanical response with time for the frequencies around CF, but not for frequencies on the tail of the tuning curve. In some experiments the nonlinearity was maintained for several hours, in spite of a considerable loss of sensitivity of the BM response. High-frequency plateaus were observed in both isovelocity tuning curves and phase-frequency curves.     

Gender differences in children's singing voices: acoustic analyses and results of a listening test

Ultrasonic coded transmitters (UCTs) producing frequencies of 69-83 kHz are used increasingly to track fish and invertebrates in coastal and estuarine waters. To address concerns that they might be audible to marine mammals, acoustic properties of UCTs were measured off Mission Beach, San Diego, and at the U.S. Navy TRANSDEC facility. A regression model fitted to VEMCO UCT data yielded an estimated source level of 147 dB re 1 μPa SPL @ 1 m and spreading constant of 14.0. Based on TRANSDEC measurements, five VEMCO 69 kHz UCTs had source levels ranging from 146 to 149 dB re 1 μPa SPL @ 1 m. Five Sonotronics UCTs (69 kHz and 83 kHz) had source levels ranging from 129 to 137 dB re 1 μPa SPL @ 1 m. Transmitter directionality ranged from 3.9 to 18.2 dB. Based on propagation models and published data on marine mammal auditory psychophysics, harbor seals potentially could detect the VEMCO 69 kHz UCTs at ranges between 19 and >200 m, while odontocetes potentially could detect them at much greater ranges. California sea lions were not expected to detect any of the tested UCTs at useful ranges.     

Responses of "lower-spontaneous-rate" auditory-nerve fibers to speech syllables presented in noise. I: General characteristics.

Responses of auditory-nerve fibers in anesthetized cats to nine different spoken stop- and nasal-consonant/vowel syllables presented at 70 dB SPL in various levels of speech-shaped noise [signal-to-noise (S/N) ratios of 30, 20, 10, and 0 dB] are reported. The temporal aspects of speech encoding were analyzed using spectrograms. The responses of the "lower-spontaneous-rate" fibers (less than 20/s) were found to be more limited than those of the high-spontaneous-rate fibers. The lower-spontaneous-rate fibers did not encode noise-only portions of the stimulus at the lowest noise level (S/N = 30 dB) and only responded to the consonant if there was a formant or major spectral peak near its characteristic frequency. The fibers' responses at the higher noise levels were compared to those obtained at the lowest noise level using the covariance as a quantitative measure of signal degradation. The lower-spontaneous-rate fibers were found to preserve more of their initial temporal encoding than high-spontaneous-rate fibers of the same characteristic frequency. The auditory-nerve fibers' responses were also analyzed for rate-place encoding of the stimuli. The results are similar to those found for temporal encoding.     

Level discrimination of frozen and random noise

This paper examines how the difference limen for level, delta L, is affected by stimulus bandwidth and variability. The delta L's were measured in three normal listeners using an adaptive two-interval, forced-choice procedure. The 30-ms stimuli were a 3-kHz tone and nine noise bands with half-power bandwidths ranging from 50 Hz-12 kHz. Except for the 12-kHz bandwidth, which was a low-pass noise, the noise bands were centered at 3 kHz. The delta L's were measured for both frozen and random noises presented at 30, 60, or 90 dB SPL overall. For frozen noises, the same sample of noise was presented throughout a block of 50 trials; for the random noises, different samples of noise were used in each interval of the trials. Results show that the delta L's are higher for random than for frozen noises at narrow bandwidths, but not at wide bandwidths. The delta L's for frozen narrow-band noises decrease with increasing level and are similar to those for the pure tone, whereas the delta L's for wideband noises are only slightly smaller at 90 than at 30 dB SPL. An unexpected finding is that the delta L's are larger at 60 than at 30 dB SPL for both frozen and random noises with bandwidths greater than one critical band. The effect of bandwidth varies with level: The delta L's decrease with increasing bandwidth at low levels, but are nearly independent of bandwidth at 90 dB SPL. The interaction of bandwidth and level is consistent with the multiband excitation-pattern model, but the nonmonotonic behavior of delta L as a function of level suggests modifications to the model.     

Behavioral vocal response thresholds to mating calls in the bullfrog, Rana catesbeiana

Male bullfrogs will vocalize in response to playbacks of the mating (advertisement) calls of conspecifics. This behavior was studied in response to playbacks of bullfrog mating calls presented at six different sound intensity levels. The lowest sound intensity level tested (50 dB SPL) was insufficient to evoke calling from any of the animals. Calling was evoked by playback levels of 60 dB SPL and higher. The data suggest that behavioral evoked calling thresholds lie between 50-60 dB SPL for these animals. Playback intensity levels of 80 dB SPL were more effective in evoking responses than were intensity levels up to 20 dB higher or lower. This was true both in terms of the total number of evoked responses and the trial number at which responding ceased. Moreover, significantly less habituation of evoked calling occurred at levels of 80 dB SPL than at higher or lower levels. The data suggest that a sound pressure level of 80 dB represents a behaviorally preferred intensity level for evoked calling in the bullfrog. Field recordings of bullfrog choruses show that the intensity produced by an individual calling male reaches a level of 80 dB SPL at a distance of 1 m. This intensity level is identical to that producing maximal evoked calling in the laboratory.     

High-frequency audiometric assessment of a young adult population

The hearing thresholds of 37 young adults (18-26 years) were measured at 13 frequencies (8, 9,10,...,20 kHz) using a newly developed high-frequency audiometer. All subjects were screened at 15 dB HL at the low audiometric frequencies, had tympanometry within normal limits, and had no history of significant hearing problems. The audiometer delivers sound from a driver unit to the ear canal through a lossy tube and earpiece providing a source impedance essentially equal to the characteristic impedance of the tube. A small microphone located within the earpiece is used to measure the response of the ear canal when an impulse is applied at the driver unit. From this response, a gain function is calculated relating the equivalent sound-pressure level of the source to the SPL at the medial end of the ear canal. For the subjects tested, this gain function showed a gradual increase from 2 to 12 dB over the frequency range. The standard deviation of the gain function was about 2.5 dB across subjects in the lower frequency region (8-14 kHz) and about 4 dB at the higher frequencies. Cross modes and poor fit of the earpiece to the ear canal prevented accurate calibration for some subjects at the highest frequencies. The average SPL at threshold was 23 dB at 8 kHz, 30 dB at 12 kHz, and 87 dB at 18 kHz. Despite the homogeneous nature of the sample, the younger subjects in the sample had reliably better thresholds than the older subjects. Repeated measurements of threshold over an interval as long as 1 month showed a standard deviation of 2.5 dB at the lower frequencies (8-14 kHz) and 4.5 dB at the higher frequencies.     

Noise level, inner hair cell damage, audiometric features, and equal-energy hypothesis

Rabbits were exposed to 2- to 7-kHz noise either for a short duration at a high sound-pressure level (15 or 30 min at 115 dB SPL), or a long duration at a low level (512 h at 85 dB SPL). The high-level exposure produced a hearing loss in the frequency range 2-6 kHz, whereas the low-level exposure gave maximum hearing loss at 12-20 kHz. The 115-dB exposure caused significantly more damage to inner hair cells than the 85-dB exposure. The implications of the present results for evaluating audiograms, equal-energy hypothesis, risk criteria, and subjective auditory features are pointed out.     

Intensity-time tradeoff for constant hearing loss with high sound levels

The tradeoff relation between exposure intensity and duration for constant hearing loss was investigated in two series of experiments using Mongolian gerbils. The gerbils were exposed to a 1/3 octave band of noise at 2.5 kHz. In the first series animals were exposed to 120 dB SPL for 1 h, to 126 dB SPL for 15 min, and to 126 dB SPL for 3.75 min. In the second series, shorter durations were used: 120 dB SPL for 15 min, 126 dB SPL for 3.75 min, and 126 dB SPL for 56 s. The hearing thresholds were determined behaviorally immediately before exposure and 6 weeks after exposure. The results suggest that the intensity-time tradeoff for the investigated intensity interval is between 1.5 and 3 dB per halving of the duration.     

Psychometric functions for level discrimination

To determine the form of psychometric functions for 2I,2AFC level discrimination (commonly called intensity discrimination), ten increment levels were presented in random order within blocks of 100 trials. Stimuli were chosen to encompass a wide range of conditions and difference limens: eight 10-ms tones had frequencies of 0.25, 1, 8, or 14 kHz and levels of 30, 60, or 90 dB SPL; two 500-ms stimuli also were tested: a 1-kHz tone at 90 dB SPL and broadband noise at 63 dB SPL. For each condition, at least 20 blocks were presented in mixed order. Results for five normal listeners show that the sensitivity, d', is nearly proportional to delta L (= 20 log [(p + delta p)/p], where p is sound pressure) over the entire range of difference limens. When d' is plotted against Weber fractions for sound pressure, delta p/p, or intensity, delta I/I, exponents of the best-fitting power functions decrease with increasing difference limens and are less than unity for large difference limens. The approximately proportional relation between d' and delta L agrees with modern multichannel models of level discrimination and with psychometric functions derived for single auditory-nerve fibers. The results also support the notion that the difference limen, expressed as delta LDL and plotted on a logarithmic scale, is an appropriate representation of performance in level-discrimination experiments.     

Acoustics of ear canal measurement of eardrum SPL in simulators

The effect of standing waves on the ear canal measurement of eardrum sound pressure level (SPL) was determined by both calculation and measurement. Transmission line calculations of the standing wave were made using the dimensions of the ANSI S3.25-1979 ear simulator and three different eardrum impedances. Standing wave curves have been obtained for the standard eardrum impedance at 1-kHz intervals in the range of 1-8 kHz. The changes in standing wave position due to each of the three eardrum impedances and their effects on ear canal measurements of SPL were computed for each of the eardrum impedances. Ear canal SPL measurements conducted on simulators modified to correspond to the eardrum impedances used in the calculations were compared to the computed values. Differences between eardrum SPLs and those measured at different locations in the ear canal approached a standing wave ratio (SWR) of 10-12 dB as the position of the measuring probe approached the standing wave minimum at each frequency. These maximum differences compared favorably with data developed by other investigators from real ears. Differences due to the eardrum impedance were found to be significant only in the frequency region of 2-5 kHz. Calibration of probes in a standard or modified ANSI simulator at the same distance from the eardrum as in the real ear reduces the eardrum SPL measurement errors to those resulting from differences in eardrum impedance.