Auditory intensity discrimination in the chinchilla

A positive reinforcement conditioning procedure was used to train chinchillas to respond to intensity differences between successively occurring tone bursts. Intensity difference limens were measured at 0.5, 1, 4, and 8 kHz at five intensities ranging from 10- to 55-dB sensation level. The intensity difference limen decreased from approximately 8 dB near threshold to approximately 3.5 dB at the highest level. The intensity difference limens for the chinchilla were considerably larger than those for humans as well as several other mammals; however, the results were similar to those obtained for the parakeet. The present results from intensity discrimination appeared to be related to previous data for the discrimination of amplitude modulated noise.     

Spectral-peak selection in spectral-shape discrimination by normal-hearing and hearing-impaired listeners

Spectral-shape discrimination thresholds were measured in the presence and absence of noise to determine whether normal-hearing and hearing-impaired listeners rely primarily on spectral peaks in the excitation pattern when discriminating between stimuli with different spectral shapes. Standard stimuli were the sum of 2, 4, 6, 8, 10, 20, or 30 equal-amplitude tones with frequencies fixed between 200 and 4000 Hz. Signal stimuli were generated by increasing and decreasing the levels of every other standard component. The function relating the spectral-shape discrimination threshold to the number of components (N) showed an initial decrease in threshold with increasing N and then an increase in threshold when the number of components reached 10 and 6, for normal-hearing and hearing-impaired listeners, respectively. The presence of a 50-dB SPL/Hz noise led to a 1.7 dB increase in threshold for normal-hearing listeners and a 3.5 dB increase for hearing-impaired listeners. Multichannel modeling and the relatively small influence of noise suggest that both normal-hearing and hearing-impaired listeners rely on the peaks in the excitation pattern for spectral-shape discrimination. The greater influence of noise in the data from hearing-impaired listeners is attributed to a poorer representation of spectral peaks.     

Intensity discrimination of pulsed tones by the goldfish (Carassius auratus)

Intensity discrimination thresholds for 500-ms pure-tone bursts were measured as a function of frequency in the goldfish (Carassius auratus) using classical respiratory conditioning. At 55-dB sensation level (SL), thresholds range from 1.44-2.2 dB between 100 and 1600 Hz. There is not important effect of frequency on intensity discrimination. Thresholds at 35-dB SL average 0.7 dB higher than at 55-dB SL. This is a small difference in the context of the threshold variability. In intensity discrimination acuity, the goldfish is quantitatively similar to other vertebrates, including birds and mammals.     

Impact noise and the equal energy hypothesis

The equal energy hypothesis (EEH) was evaluated over a limited range of conditions by exposing four groups of chinchillas to impact noise (200-ms B duration) presented at a fixed rate of four impacts per second. The intensity of the impacts (107-125 dB peak SPL) and the duration (120-1.87 h) of the four exposure conditions were counterbalanced so that the four groups received the same total energy. The traumatic power of the exposures was assessed by measuring the threshold shift of the auditory evoked response and the amount of hair cell loss. Exposure between 107 and 119 dB were consistent with the EEH in that they produced roughly the same amount of permanent threshold shift (less than 20 dB) and hair cell loss (less than 20%). However, the 125-dB exposure produced substantially more threshold shift and hair cell loss than the three lower intensities. Thus, the EEH may be applicable only at lower impact intensities; above a "critical intensity" the amount of damage increases significantly.     

On the relation between the growth of loudness and the discrimination of intensity for pure tones

The intensity jnd is often assumed to depend on the slope of the loudness function. One way to test this assumption is to measure the jnd for a sound that falls on distinctly different loudness functions. Two such functions were generated by presenting a 1000-Hz tone in narrow-band noise (925-1080 Hz) set at 70 dB SPL and in wideband noise (75-9600 Hz) set at 80 dB SPL. Over a range from near threshold to about 75 dB SPL, the loudness function for the tone is much steeper in the narrow-band noise than in the wideband noise. At 72 dB SPL, where the two loudness curves cross, the tone's jnd was measured in each noise by a block up-down two-interval forced-choice procedure. Despite the differences in slope (and in sensation level), the jnd (delta I/I) is nearly the same in the two noises, 0.22 in narrow-band noise and 0.20 in wideband noise. The mean value of 0.21 is close to the value of 0.25 interpolated from Jesteadt et al. [J. Acoust. Soc. Am. 61, 169-176 (1977)] for a 1000-Hz tone that had the same loudness in quiet as did our 72-dB tone in noise, but lay on a loudness function with a much lower slope. These and other data demonstrate that intensity discrimination for pure tones is unrelated to the slope of the loudness function.     

Effects of masker-spectral variability and masker fringes in children and adults

This study examined the degree to which masker-spectral variability contributes to children's susceptibility to informational masking. Listeners were younger children (5-7 years), older children (8-10 years), and adults (19-34 years). Masked thresholds were measured using a 2IFC, adaptive procedure for a 300-ms, 1000-Hz signal presented simultaneously with (1) broadband noise, (2) a random-frequency ten-tone complex, or (3) a fixed-frequency ten-tone complex. Maskers were presented at an overall level of 60 dB SPL. Thresholds were similar across age for the noise condition. Thresholds for most children were higher than for most adults, however, for both ten-tone conditions. The average difference in threshold between random and fixed ten-tone conditions was comparable across age, suggesting a similar effect of reducing masker-spectral variability in children and adults. Children appear more likely to be susceptible to informational masking than adults, however, both with and in the absence of masker-spectral variability. The addition of a masker fringe (delayed onset of signal relative to masker) provided a release from masking for fixed and random ten-tone conditions in all age groups, suggesting at least part of the masking observed for both ten-tone maskers was informational.     

Binaural forward and backward masking: evidence for sluggishness in binaural detection

The threshold of a short interaurally phase-inverted probe tone (20 ms, 500 Hz, S pi) was obtained in the presence of a 750-ms noise masker that was switched after 375 ms from interaurally phase-inverted (N pi) to interaurally in-phase (No). As the delay between probe-tone offset and noise phase transition is increased, the threshold decays from the N pi S pi threshold (masking level difference = 0 dB) to the No S pi threshold (masking level difference = 15 dB). The decay in this "binaural" situation is substantially slower than in a comparable "monaural" situation, where the interaural phase of the masker is held constant (N pi), but the level of the masker is reduced by 15 dB. The prolonged decay provides evidence for additional binaural sluggishness associated with "binaural forward masking." In a second experiment, "binaural backward masking" is studied by time reversing the maskers described above. Again, the situation where the phase is switched from No to N pi exhibits a slower transition than the situation with constant interaural phase (N pi) and a 15-dB increase in the level of the masker. The data for the binaural situations are compatible with the results of a related experiment, previously reported by Grantham and Wightman [J. Acoust. Soc. Am. 65, 1509-1517 (1979)] and are well fit by a model that incorporates a double-sided exponential temporal integration window.     

Hearing loss from interrupted, intermittent, and time varying Gaussian noise exposures: the applicability of the equal energy hypothesis

Eight groups of chinchillas (N=74) were exposed to various equivalent energy [100 or 106 dB(A) sound pressure level (SPL)] noise exposure paradigms. Six groups received an interrupted, intermittent, time varying (IITV) Gaussian noise exposure that lasted 8 h/d, 5 d/week for 3 weeks. The exposures modeled an idealized workweek. At each level, three different temporal patterns of Gaussian IITV noise were used. The 100 dB(A) IITV exposure had a dB range of 90-108 dB SPL while the range of the 106 dB(A) IITV exposure was 80-115 dB SPL. Two reference groups were exposed to a uniform 100 or 106 dB(A) SPL noise, 24 h/d for 5 days. Each reference group and the three corresponding IITV groups comprised a set of equivalent energy exposures. Evoked potentials were used to estimate hearing thresholds and surface preparation histology quantified sensory cell populations. All six groups exposed to the IITV noise showed threshold toughening effects of up to 40 dB. All IITV exposures produced hearing and sensory cell loss that was similar to their respective equivalent energy reference group. These results indicate that for Gaussian noise the equal energy hypothesis for noise-induced hearing loss is an acceptable unifying principle.     

Detection of a temporal gap in low-frequency narrow-band signals by normal-hearing and hearing-impaired listeners

Temporal processing ability in the hearing impaired was investigated in a 2IFC gap-detection paradigm. The stimuli were digitally constructed 50-Hz-wide bands of noise centered at 250, 500, and 1000 Hz. On each trial, two 400-ms noise samples were paired, shaped at onset and offset, filtered, and presented in the quiet with and without a temporal gap. A modified up-down procedure with trial-by-trial feedback was used to establish threshold of detection of the gap. Approximately 4 h of practice preceded data collection; final estimate of threshold was the average of six listening blocks. There were 10 listeners, 19-25 years old. Five had normal hearing; five had a moderate congenital sensorineural hearing loss with relatively flat audiometric configuration. Near threshold (5 dB SL), all listeners performed similarly. At 15 and 25 dB SL, the normal-hearing group performed better than the hearing-impaired group. At 78 dB SPL, equal to the average intensity of the 5-dB SL condition for the hearing impaired, the normal-hearing group continued to improve and demonstrated a frequency effect not seen in the other conditions. Substantial individual differences were found in both groups, though intralistener variability was as small as expected for these narrow-bandwidth signals.     

Loudness recalibration as a function of level

Recent research on loudness has focused on contextual effects on loudness, both assimilation and recalibration. The current experiments examined loudness recalibration [Marks, J. Exp. Psychol. 20, 382-396 (1994)]. In the first experiment, an adaptive tracking procedure was used to measure loudness recalibration as a function of standard- and recalibration-tone level. The standard-tone frequencies were 500 and 2500 Hz and the levels were 80-, 70-, 60-, and 40-dB SPL, and threshold. Seventeen dB of loudness recalibration was obtained (combined over both frequencies) in the 60-dB SPL condition. This amount of loudness recalibration, while substantial, is still less than that obtained by Marks (22 dB), using the method of paired comparisons. The second experiment sought to duplicate Marks' earlier experiment [Marks, J. Exp. Psychol. 20, 382-396 (1994), experiment 2]. The results of this experiment (21 dB) were almost identical to those obtained by Marks. The results of experiment 1 indicate that loudness recalibration is maximum when the recalibration tone is moderately louder than the subsequent standard tones. Relatively little loudness recalibration is exhibited when the standard-tone level equals the recalibration-tone level. In addition, there is no loudness recalibration at threshold. The tracking procedure also identified that the onset of loudness recalibration is very rapid. The difference between the maximum loudness recalibration obtained at each frequency (11 dB at 500 Hz, 6 dB at 2500 Hz) suggests that loudness recalibration is dependent upon the frequency of the standard tone.     

Speech recognition in noise by individuals with mild hearing impairments

The study was designed to test the validity of the American Academy of Ophthalmology and Otolaryngology's (AAOO) 26-dB average hearing threshold level at 500, 1000, and 2000 Hz as a predictor of hearing handicap. To investigate this criterion the performance of a normal-hearing group was compared with that of two groups, categorized according to the AAOO [Trans. Am. Acad. Ophthal. Otolaryng. 63, 236-238 (1959)] guidelines as having no handicap. The latter groups, however, had significant hearing losses in the frequencies above 2000 Hz. Mean hearing threshold levels for 3000, 4000, and 6000 Hz were 54 dB for group II and 63 dB for group III. Two kinds of speech stimuli were presented at an A-weighted sound level of 60 dB in quiet and in three different levels of noise. The resulting speech recognition scores were significantly lower for the hearing-impaired groups than for the normal-hearing group on both kinds of speech materials and in all three noise conditions. Mean scores for group III were significantly lower than those of the normal-hearing group, even in the quiet condition. Speech recognition scores showed significantly better correlation with hearing levels for frequency combinations including frequencies above 2000 Hz than for the 500-, 1000-, and 2000-Hz combination. On the basis of these results the author recommends that the 26-dB fence should be somewhat lower, and that frequencies above 2000 Hz should be included in any scheme for evaluating hearing handicap.     

Comparison of different forms of compression using wearable digital hearing aids

Four different compression algorithms were implemented in wearable digital hearing aids: (1) The slow-acting dual-front-end automatic gain control (AGC) system [B. C. J. Moore, B. R. Glasberg, and M. A. Stone, Br. J. Audiol. 25, 171-182 (1991)], combined with appropriate frequency response equalization, with a compression threshold of 63 dB sound pressure level (SPL) and with a compression ratio of 30 (DUAL-HI); (2) The dual-front-end AGC system combined with appropriate frequency response equalization, with a compression threshold of 55 dB SPL and with a compression ratio of 3 (DUAL-LO). This was intended to give some impression of the levels of sounds in the environment; (3) Fast-acting full dynamic range compression in four channels (FULL-4). The compression was designed to minimize envelope distortion due to overshoots and undershoots; (4) A combination of (2) and (3) above, where each applied less compression than when used alone (DUAL-4). Initial fitting was partly based on the concept of giving a flat specific-loudness pattern for a 65-dB SPL speech-shaped noise input, and this was followed by fine tuning using an adaptive procedure with speech stimuli. Eight subjects with moderate to severe cochlear hearing loss were tested in a counter-balanced design. Subjects had at least 2 weeks experience with each system in everyday life before evaluation using the Abbreviated Profile of Hearing Aid Benefit (APHAB) test and measures of speech intelligibility in quiet (AB word lists at 50 and 80 dB SPL) and noise (adoptive sentence lists in speech-shaped noise, or that same noise amplitude modulated with the envelope of speech from a single talker). The APHAB scores did not indicate clear differences between the four systems. Scores for the AB words in quiet were high for all four systems at both 50 and 80 dB SPL. The speech-to-noise ratios required for 50% intelligibility were low (indicating good performance) and similar for all the systems, but there was a slight trend for better performance in modulated noise with the DUAL-4 system than with the other systems. A subsequent trial where three subjects directly compared each of the four systems in their everyday lives indicated a slight preference for the DUAL-LO system. Overall, the results suggest that it is not necessary to compress fast modulations of the input signal.     

Evaluation of array-processing algorithms for a headband hearing aid

Several array-processing algorithms were implemented and evaluated with experienced hearing-aid users. The array consisted of four directional microphones mounted broadside on a headband worn on the top of the listener's head. The algorithms included two adaptive array-processing algorithms, one fixed array-processing algorithm, and a reference condition consisting of binaural directional microphones. The algorithms were evaluated under conditions with both one and three independent noise sources. Performance metrics included quantitative speech reception thresholds and qualitative subject preference ratings for ease-of-listening measured using a paired-comparison procedure. On average, the fixed algorithm improved speech reception thresholds by 2 dB, while the adaptive algorithms provided 7-9-dB improvement over the reference condition. Subjects judging ease-of-listening generally preferred all array-processing algorithms over the reference condition. The results suggest that these adaptive algorithms should be evaluated further in more realistic acoustic environments.     

Monaural and binaural auditory frequency resolution measured using bandlimited noise and notched-noise masking

Several studies using bandlimited masking noise have indicated that NOSO frequency resolution is better than that for NOS pi. The present study examined NOSO and NOS pi frequency resolution with two different masking methods: bandlimited noise and notched noise. Noise spectrum levels of 10, 30, and 50 dB/Hz were used. Thresholds were determined for a 500-Hz signal, using a three-alternative forced-choice adaptive procedure, as a function of masker bandwidth and notchwidth. For NOSO presentation, 3-dB down points were comparable for the notched-noise and bandlimiting methods. For NOS pi presentation, 3-dB down points were generally greater for the bandlimiting method than the notched noise method. Furthermore, for NOS pi presentation, the 3-dB down estimate increased as noise level increased for the bandlimiting method, but stayed constant for the notched-noise method. It is suggested that the two masking methods measured different aspects of binaural processing.     

Binaural speech intelligibility in noise for hearing-impaired listeners

The effect of head-induced interaural time delay (ITD) and interaural level differences (ILD) on binaural speech intelligibility in noise was studied for listeners with symmetrical and asymmetrical sensorineural hearing losses. The material, recorded with a KEMAR manikin in an anechoic room, consisted of speech, presented from the front (0 degree), and noise, presented at azimuths of 0 degree, 30 degrees, and 90 degrees. Derived noise signals, containing either only ITD or only ILD, were generated using a computer. For both groups of subjects, speech-reception thresholds (SRT) for sentences in noise were determined as a function of: (1) noise azimuth, (2) binaural cue, and (3) an interaural difference in overall presentation level, simulating the effect of a monaural hearing acid. Comparison of the mean results with corresponding data obtained previously from normal-hearing listeners shows that the hearing impaired have a 2.5 dB higher SRT in noise when both speech and noise are presented from the front, and 2.6-5.1 dB less binaural gain when the noise azimuth is changed from 0 degree to 90 degrees. The gain due to ILD varies among the hearing-impaired listeners between 0 dB and normal values of 7 dB or more. It depends on the high-frequency hearing loss at the side presented with the most favorable signal-to-noise (S/N) ratio. The gain due to ITD is nearly normal for the symmetrically impaired (4.2 dB, compared with 4.7 dB for the normal hearing), but only 2.5 dB in the case of asymmetrical impairment. When ITD is introduced in noise already containing ILD, the resulting gain is 2-2.5 dB for all groups. The only marked effect of the interaural difference in overall presentation level is a reduction of the gain due to ILD when the level at the ear with the better S/N ratio is decreased. This implies that an optimal monaural hearing aid (with a moderate gain) will hardly interfere with unmasking through ITD, while it may increase the gain due to ILD by preventing or diminishing threshold effects.     

Monosyllabic word recognition at higher-than-normal speech and noise levels

The effects of intensity on monosyllabic word recognition were studied in adults with normal hearing and mild-to-moderate sensorineural hearing loss. The stimuli were bandlimited NU#6 word lists presented in quiet and talker-spectrum-matched noise. Speech levels ranged from 64 to 99 dB SPL and S/N ratios from 28 to -4 dB. In quiet, the performance of normal-hearing subjects remained essentially constant in noise, at a fixed S/N ratio, it decreased as a linear function of speech level. Hearing-impaired subjects performed like normal-hearing subjects tested in noise when the data were corrected for the effects of audibility loss. From these and other results, it was concluded that: (1) speech intelligibility in noise decreases when speech levels exceed 69 dB SPL and the S/N ratio remains constant; (2) the effects of speech and noise level are synergistic; (3) the deterioration in intelligibility can be modeled as a relative increase in the effective masking level; (4) normal-hearing and hearing-impaired subjects are affected similarly by increased signal level when differences in speech audibility are considered; (5) the negative effects of increasing speech and noise levels on speech recognition are similar for all adult subjects, at least up to 80 years; and (6) the effective dynamic range of speech may be larger than the commonly assumed value of 30 dB.     

Hazard from an intense midrange impulse

It has been hypothesized that the ear would become increasingly susceptible to impulses (gunfire) as the spectral peak of the impulse approached the frequency region where the ear was tuned best (about 4 kHz for the cat ear) [G. R. Price, J. Acoust. Soc. Am. Suppl. 1 62, S95 (1977)]. This prediction was counter to the predictions of the world's damage-risk criteria for impulse noise. It has been supported by experiments using exposures to 100-Hz and 800- to 1000-Hz impulses; but no test had been run at the point of predicted maximum susceptibility. In the present experiment, three groups of cats were exposed to 50 impulses produced by a primer explosion (spectral peak at 4 kHz) at peak levels of 135, 140, or 145 dB. Auditory thresholds were electrophysiologically measured from the vertex to 2-, 4-, 8-, and 16-kHz tone pips and losses were determined 30 min after exposure and more than 2 months post-exposure. Losses were greatest at 4 kHz, began to develop at 134-dB peak pressure, and the immediate losses grew at a rate of about 7 dB for every dB increase in peak pressure. About half of the loss measured immediately became permanent. The energy required to begin producing a permanent threshold shift was only about 0.07 J/m2, far lower than that required with continuous noises at lower sound pressures. The data were interpreted as supporting the original hypothesis of greater susceptibility in the midrange.     

Minimum spectral contrast for vowel identification by normal-hearing and hearing-impaired listeners

To determine the minimum difference in amplitude between spectral peaks and troughs sufficient for vowel identification by normal-hearing and hearing-impaired listeners, four vowel-like complex sounds were created by summing the first 30 harmonics of a 100-Hz tone. The amplitudes of all harmonics were equal, except for two consecutive harmonics located at each of three "formant" locations. The amplitudes of these harmonics were equal and ranged from 1-8 dB more than the remaining components. Normal-hearing listeners achieved greater than 75% accuracy when peak-to-trough differences were 1-2 dB. Normal-hearing listeners who were tested in a noise background sufficient to raise their thresholds to the level of a flat, moderate hearing loss needed a 4-dB difference for identification. Listeners with a moderate, flat hearing loss required a 6- to 7-dB difference for identification. The results suggest, for normal-hearing listeners, that the peak-to-trough amplitude difference required for identification of this set of vowels is very near the threshold for detection of a change in the amplitude spectrum of a complex signal. Hearing-impaired listeners may have difficulty using closely spaced formants for vowel identification due to abnormal smoothing of the internal representation of the spectrum by broadened auditory filters.     

A comparison of threshold estimation methods in children 6-11 years of age

Estimating detection threshold for auditory stimuli in children can be problematic because of lapses in attention and the time limits usually imposed by scheduling restrictions or fatigue. Data reported here were collected to compare the stability of threshold estimation procedures in testing children ages 6 to 11 in a three-alternative, forced-choice paradigm. Stimuli consisted of a 1-kHz tonal signal and a Gaussian noise masker, bandpass filtered between 500-2,000 Hz and presented at 25-dB spectrum level. The signal was either presented for 400 ms in the presence of a continuous masker (simultaneous masking) or for 10 ms just prior to a 400-ms masker (backward masking). For each masking paradigm the 79% correct threshold was assessed via each of three procedures: 3-down, 1-up adaptive staircase (Levitt), maximum likelihood estimation (MLE), and method of constant stimuli. Percent correct was measured at the end of the study for a signal 10 dB above the previously determined threshold in order to estimate the most appropriate psychometric function asymptote for fitting data collected via the method of constant stimuli. Both the MLE and Levitt procedures produced equally stable threshold estimates for both conditions and age groups. This was the case despite considerable variability in backward-masking thresholds.     

Detection of intensity decrements by the chinchilla.

Three monaural chinchillas were trained to detect intensity decrements in broadband noise (20 kHz) using a shock-avoidance conditioning procedure. The intensity decrements were presented at one of nine different durations between 2 and 35 ms at noise levels of 25, 45, and 65 dB SPL. At each intensity-duration combination, the level of the decrement was varied to obtain a decrement threshold. The minimal detectable decrement decreased from approximately 20 dB at the shortest duration to an asymptote of roughly 4 dB at approximately 30 ms. The data were modeled by a low-pass filter with an 11-ms time constant. The decrement detection function of the chinchilla is similar to that of humans. However, long-duration decrement thresholds are larger in the chinchilla, as would be predicted from the large intensity difference limen of the chinchilla. In general, there was little change in the decrement function across background intensities except that 2-ms decrements were not detected at the 25-dB SPL background intensity.