Measurement and modeling of binaural loudness summation for hearing-impaired listeners.

The summation of loudness across ears is often studied by measuring the level difference required for equal loudness (LDEL) of monaural and diotic sounds. Typically, the LDEL is ∼5-6 dB, consistent with the idea that a diotic sound is ∼1.5 times as loud as the same sound presented monaurally at the same level, as predicted by the loudness model of Moore and Glasberg [J. Acoust. Soc. Am. 121, 1604-1612 (2007)]. One might expect that the LDEL would be <5-6 dB for hearing-impaired listeners, because loudness recruitment leads to a more rapid change of loudness for a given change in level. However, previous data sometimes showed similar LDEL values for normal-hearing and hearing-impaired listeners. Here, the LDEL was measured for hearing-impaired listeners using narrowband and broadband noises centered at 500 Hz, where audiometric thresholds were near-normal, and at 3000 or 4000 Hz, where audiometric thresholds were elevated. The mean LDEL was 5.6 dB at 500 Hz and 4.2 dB at the higher center frequencies. The results were predicted reasonably well by an extension of the loudness model of Moore and Glasberg.

Assessing the possible role of frequency-shift detectors in the ability to hear out partials in complex tones.

The possible role of frequency-shift detectors (FSDs) was assessed for a task measuring the ability to hear out individual "inner" partials in a chord with seven partials uniformly spaced on the ERBN-number (Cam) scale. In each of the two intervals in a trial, a pure-tone probe was followed by a chord. In one randomly selected interval, the frequency of the probe was the same as that of a partial in the chord. In the other interval, the probe was mistuned upwards or downwards from the "target" partial. The task was to indicate the interval in which the probe coincided with the target. In the "symmetric" condition, the frequency of the mistuned probe was midway in Cams between that of two partials in the chord. This should have led to approximately symmetric activation of the up-FSDs and down-FSDs, such that differential activation provided a minimal cue. In the "asymmetric" condition, the mistuned probe was much closer in frequency to one partial in the chord than to the next closest partial. This should have led to differential activation of the up-FSDs and down-FSDs, providing a strong discrimination cue. Performance was predicted to be better in the asymmetric than in the symmetric condition. The results were consistent with this prediction except when the probe was mistuned above the sixth (second highest) partial in the chord. To explain this, it is argued that activation of FSDs depends both on the size of the frequency shift between successive components and on the pitch strength of each component.

A version of the TEN Test for use with ER-3A insert earphones.

OBJECTIVE: The aim of this study was to develop a version of the threshold-equalizing noise (TEN) test for the diagnosis of dead regions for use with Etymotic ER-3A insert earphones. The use of such earphones is helpful when testing clients with asymmetric hearing loss or clients whose ear canals tend to collapse under the pressure of supra-aural headphones. It can also be useful when ambient noise levels are problematic. DESIGN: The spectral shape of the noise required to give equal masked thresholds at all frequencies, when expressed in dB HL, was derived by empirical measurements of the electrical output of audiometers using ER-3A earphones. To reduce the loudness of the noise and to minimize distortion generated in the audiometer or earphone, the noise was band-limited between 354 and 6500 Hz. In addition, the noise was synthesized using a method that leads to a low crest factor (ratio of peak to root mean square value). This further reduced audiometer/earphone distortion, and allowed higher levels per ERBN; ERBN is the equivalent rectangular bandwidth of the auditory filter at 1 kHz, as determined using young normally hearing subjects. The test tone frequencies were limited to the range 500 to 4000 Hz. Subjects with normal or near-normal hearing were tested using a noise level of 60 dB HL/ERBN, to assess whether the noise did lead to equal masked thresholds in dB HL for all audiometric frequencies from 500 to 4000 Hz. Thresholds in the TEN were measured using manual audiometry with a 2 dB final step size. RESULTS: The mean-masked thresholds varied by 1.3 dB across frequency when expressed in dB HL, and were close to the noise level per ERBN. CONCLUSION: This version of the TEN test can be used with ER-3A insert earphones.

The effect of hearing loss on the resolution of partials and fundamental frequency discrimination.

The relationship between the ability to hear out partials in complex tones, discrimination of the fundamental frequency (F0) of complex tones, and frequency selectivity was examined for subjects with mild-to-moderate cochlear hearing loss. The ability to hear out partials was measured using a two-interval task. Each interval included a sinusoid followed by a complex tone; one complex contained a partial with the same frequency as the sinusoid, whereas in the other complex that partial was missing. Subjects had to indicate the interval in which the partial was present in the complex. The components in the complex were uniformly spaced on the ERB(N)-number scale. Performance was generally good for the two "edge" partials, but poorer for the inner partials. Performance for the latter improved with increasing spacing. F0 discrimination was measured for a bandpass-filtered complex tone containing low harmonics. The equivalent rectangular bandwidth (ERB) of the auditory filter was estimated using the notched-noise method for center frequencies of 0.5, 1, and 2 kHz. Significant correlations were found between the ability to hear out inner partials, F0 discrimination, and the ERB. The results support the idea that F0 discrimination of tones with low harmonics depends on the ability to resolve the harmonics.

A new model for calculating auditory excitation patterns and loudness for cases of cochlear hearing loss.

A model for calculating auditory excitation patterns and loudness for steady sounds for normal hearing is extended to deal with cochlear hearing loss. The filters used in the model have a double ROEX-shape, the gain of the narrow active filter being controlled by the output of the broad passive filter. It is assumed that the hearing loss at each audiometric frequency can be partitioned into a loss due to dysfunction of outer hair cells (OHCs) and a loss due to dysfunction of inner hair cells (IHCs). OHC loss is modeled by decreasing the maximum gain of the active filter, which results in increased absolute threshold, reduced compressive nonlinearity and reduced frequency selectivity. IHC loss is modeled by a level-dependent attenuation of excitation level, which results in elevated absolute threshold. The magnitude of OHC loss and IHC loss can be derived from measures of loudness recruitment and the measured absolute threshold, using an iterative procedure. The model accurately fits loudness recruitment data obtained using subjects with unilateral or highly asymmetric cochlear hearing loss who were required to make loudness matches between tones presented alternately to the two ears. With the same parameters, the model predicted loudness matches between narrowband and broadband sound reasonably well, reflecting loudness summation. The model can also predict when a dead region is present.

The loudness of sounds whose spectra differ at the two ears.

Moore and Glasberg [(2007). J. Acoust. Soc. Am. 121, 1604-1612] developed a model for predicting the loudness of dichotic sounds. The model gave accurate predictions of data in the literature, except for an experiment of Zwicker and Zwicker [(1991). J. Acoust. Soc. Am. 89, 756-764], in which sounds with non-overlapping spectra were presented to the two ears. The input signal was noise with the same intensity in each critical band (bark). This noise was filtered into 24 bands each 1 bark wide. The bands were then grouped into wider composite bands (consisting of 1, 2, 4, or 12 successive sub-bands) and each composite band was presented either to one ear or the other. Loudness estimates obtained using a scaling procedure decreased somewhat as the number of composite bands increased (and their width decreased), but the predictions of the model showed the opposite pattern. This experiment was similar to that of Zwicker and Zwicker, except that the widths of the bands were based on the ERB(N)-number scale, and a loudness-matching procedure was used. The pattern of the results was consistent with the predictions of the model, showing an increase in loudness as the number of composite bands increased and their spacing decreased.

The origin of binaural interaction in the modulation domain.

The purpose of these experiments was to assess whether the detection of diotic 5 Hz "probe" modulation of a 4000 Hz sinusoidal carrier was influenced by binaural interaction of "masker" modulators presented separately to each ear and applied to the same carrier. A 50 Hz masker modulator was applied to one ear and the masker modulator applied to the other ear had a frequency of 55 or 27.5 Hz. The starting phase of the masker modulators was fixed, and the starting phase of the probe modulator was varied. For both pairs of masker modulators, the threshold for detecting the probe modulation varied slightly but significantly with probe starting phase. Further experiments measuring probe detectability as a function of probe modulation depth did not provide clear evidence to support the idea that the internal representations of the masker modulators interacted binaurally to produce a weak distortion component in the internal representation of the modulation at a 5 Hz frequency. Also, the obtained phase effects were not correctly predicted using a model based on short-term loudness fluctuations.

Perceptual learning of fundamental frequency discrimination: effects of fundamental frequency, harmonic number, and component phase.

Thresholds (F0DLs) were measured for discrimination of the fundamental frequency (F0) of a group of harmonics (group B) embedded in harmonics with a fixed F0. Miyazono and Moore [(2009). Acoust. Sci. & Tech. 30, 383386] found a large training effect for tones with high harmonics in group B, when the harmonics were added in cosine phase. It is shown here that this effect was due to use of a cue related to pitch pulse asynchrony (PPA). When PPA cues were disrupted by introducing a temporal offset between the envelope peaks of the harmonics in group B and the remaining harmonics, F0DLs increased markedly. Perceptual learning was examined using a training stimulus with cosine-phase harmonics, F0 = 50 Hz, and high harmonics in group B, under conditions where PPA was not useful. Learning occurred, and it transferred to other cosine-phase tones, but not to random-phase tones. A similar experiment with F0 = 100 Hz showed a learning effect which transferred to a cosine-phase tone with mainly high unresolved harmonics, but not to cosine-phase tones with low harmonics, and not to random-phase tones. The learning found here appears to be specific to tones for which F0 discrimination is based on distinct peaks in the temporal envelope.

Effects of pulsing of the target tone on the audibility of partials in inharmonic complex tones.

The audibility of partials was measured for complex tones with partials uniformly spaced on an ERB(N)-number scale. On each trial, subjects heard a sinusoidal "probe" followed by a complex tone. The probe was mistuned downwards or upwards (at random) by 3% or 4.5% from the frequency of one randomly selected partial in the complex (the "target"). The subject indicated whether the target was higher or lower in frequency than the probe. The probe and the target were pulsed on and off and the ramp times and inter-pulse intervals were systematically varied. Performance was better for longer ramp times and longer inter-pulse intervals. In a second experiment, the ability to detect which of two complex tones contained a pulsed partial was measured. The pattern of results was similar to that for experiment 1. A model of auditory processing including an adaptation stage was able to account for the general pattern of the results of experiment 2. The results suggest that the improvement in ability to hear out a partial in a complex tone produced by pulsing that partial is partly mediated by a release from adaptation produced by the pulsing, and does not result solely from reduction of perceptual confusion.

Effects of the build-up and resetting of auditory stream segregation on temporal discrimination.

The tendency to hear a tone sequence as 2 or more streams (segregated) builds up, but a sudden change in properties can reset the percept to 1 stream (integrated). This effect has not hitherto been explored using an objective measure of streaming. Stimuli comprised a 2.0-s fixed-frequency inducer followed by a 0.6-s test sequence of alternating pure tones (3 low [L]-high [H] cycles). Listeners compared intervals for which the test sequence was either isochronous or the H tones were slightly delayed. Resetting of segregation should make identifying the anisochronous interval easier. The HL frequency separation was varied (0-12 semitones), and properties of the inducer and test sequence were set to the same or different values. Inducer properties manipulated were frequency, number of onsets (several short bursts vs. one continuous tone), tone:silence ratio (short vs. extended bursts), level, and lateralization. All differences between the inducer and the L tones reduced temporal discrimination thresholds toward those for the no-inducer case, including properties shown previously not to affect segregation greatly. Overall, it is concluded that abrupt changes in a sequence cause resetting and improve subsequent temporal discrimination.

Effects of level and frequency on the audibility of partials in inharmonic complex tones.

The effect of level and frequency on the audibility of partials was measured for complex tones with partials uniformly spaced on an equivalent rectangular bandwidth (ERB(N)) number scale. On each trial, subjects heard a sinusoidal "probe" followed by a complex tone. The probe was mistuned downwards or upwards (at random) by 4.5% from the frequency of one randomly selected partial in the complex. The subject indicated whether the probe was higher or lower in frequency than the nearest partial in the complex. The frequencies were roved from trial to trial, keeping frequency ratios fixed. In experiment 1, the level per partial, L, was 40 or 70 dB SPL and the mean frequency of the central partial, f(c), was 1201 Hz. Scores for the highest and lowest partials in the complexes were generally high for all spacings. Scores for the inner partials were close to chance at 0.75-ERB(N) spacing, and improved as the spacing was increased up to 2 ERB(N). For intermediate spacings, performance was better for the lower level used. In experiment 2, L was 70 dB SPL and f(c) was 3544 Hz. Performance worsened markedly for partial frequencies above 3544 Hz, consistent with a role of phase locking.

Comparison of two adaptive procedures for fitting a multi-channel compression hearing aid.

We compared two adaptive procedures for fitting a multi-channel compression hearing aid. "Camadapt" uses judgements of the loudness of speech stimuli and the tonal quality of music stimuli. "Eartuner" uses judgements of the loudness and clarity of speech stimuli with differing spectral characteristics. Sixteen new users of hearing aids were fitted unilaterally, using each procedure. The fittings were assigned to Programs 1 and 2 in the aid, in a counter-balanced order. Subjects kept a diary of their experiences with each program in everyday life. Following 2-4 weeks of experience, they filled in the APHAB and other questionnaires and were re-fitted using both procedures. Camadapt generally led to higher low-level gains and lower high-level gains than Eartuner. Gains recommended by the procedures did not change following experience. Eight subjects preferred the Camadapt fitting and eight preferred the Eartuner fitting. Most subjects gave high overall satisfaction ratings for both procedures. Test-retest reliability was better for Eartuner than for Camadapt. Preference for the Camadapt fitting was associated with slightly better speech communication with Camadapt, while preference for the Eartuner fitting was associated with fewer problems with aversion for that procedure.

Auditory processing efficiency and temporal resolution in children and adults.

Children have higher auditory backward masking (BM) thresholds than adults. One explanation for this is poor temporal resolution, resulting in difficulty separating brief or rapidly presented sounds. This implies that the auditory temporal window is broader in children than in adults. Alternatively, elevated BM thresholds in children may indicate poor processing efficiency. In this case, children would need a higher signal-to-masker ratio than adults to detect the presence of a signal. This would result in poor performance on a number of psychoacoustic tasks but would be particularly marked in BM due to the compressive nonlinearity of the basilar membrane. The objective of the present study was to examine the competing hypotheses of "temporal resolution" and "efficiency" by measuring BM as a function of signal-to-masker interval in children and adults. The children had significantly higher thresholds than the adults at each of the intervals. Subsequent modeling and analyses showed that the data for both children and adults were best fitted using the same, fixed temporal window. Therefore, the differences in BM threshold between adults and children were not due to differences in temporal resolution but to reduced detection efficiency in the children.

Sequential streaming and effective level differences due to phase-spectrum manipulations.

Roberts et al. [J. Acoust. Soc. Am. 112, 2074-2085 (2002)] demonstrated that sequential stream segregation occurs with stimuli that differ only in phase spectrum. We investigated if this was partly due to differences in effective excitation level. Stimuli were harmonic complexes with a 100 Hz fundamental, 1250-2500 Hz passband, and cosine, alternating, or random component phase. In experiment 1, the complex tones were used as forward maskers of 20-ms probe tones at 1000, 1250, 1650, 2050, 2500, and 3000 Hz. While there was no significant difference in the masking produced by the cosine- and alternating-phase stimuli, the random-phase stimulus produced significantly greater masking, equivalent to a difference in overall effective excitation level of 12.6 dB. Experiments 2 and 3 used the asynchrony detection and subjective streaming tasks of Roberts et al. Successive stimuli had identical phase, but differed in level by 0, 1, 3, 5, 10, or 15 dB. Stream segregation increased once the level difference reached 5 dB. While some of the stream segregation observed by Roberts et al. may have been due to a difference in effective excitation level, this does not account for the stream segregation between cosine- and alternating-phase stimuli.

Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane at 1 and 2 kHz.

Physiological data suggest that the peak of the travelling wave on the basilar membrane evoked by a high-frequency sinusoid moves towards the base with increasing level. Previously, we used a forward-masking technique to provide evidence for a similar effect in humans at 4 and 6.5 kHz. In the present study, we used a similar technique to determine whether level-dependent shifts occur for mid-range frequencies. The signal was a brief 1-kHz or 2-kHz tone presented at 10 dB SL (approximately 30 dB SPL). For three fixed masker levels (75, 85 and 95 dB SPL), we measured the duration of the gap between the masker and signal required to give 79.4% correct detection of the signal (called the 'gap threshold') as a function of masker frequency; the longer the gap threshold, the more effective is the masker. The gap-threshold patterns nearly always showed a single peak close to the signal frequency. The gap-threshold patterns spread markedly towards lower frequencies with increasing masker level, but the frequency at the peak did not change systematically with level. We conclude that, for mid-range frequencies, the peak of the travelling wave does not shift significantly with increasing level over the range 30-95 dB SPL, but the envelope of the travelling wave becomes more shallow on its basal side.

Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane.

Physiological data suggest that the travelling wave on the basilar membrane evoked by a sinusoid of fixed frequency moves towards the base with increasing level. We describe two psychoacoustic experiments that attempted to provide evidence for and quantify the extent of such a shift in humans. In experiment 1, masking patterns were measured in forward masking using a fixed 6-kHz tone presented at 65 or 85 dB sound pressure level. The threshold for detecting a brief sinusoidal signal was measured as a function of signal frequency for several time delays of the signal relative to the end of the masker. A background noise was included to reduce 'off-frequency listening'. As the signal delay was increased, the signal level at the peaks of the masking patterns decreased and the signal frequency at the peak of the patterns moved progressively towards higher frequencies. The pattern of results was consistent with the idea of a basalward shift of the travelling wave with increasing level. The estimated shift corresponds to about 0.25 octaves for a 40-dB change in level. Experiment 2 also used forward masking. The signal was a 4-kHz tone presented at 10 dB sensation level. For three fixed masker levels (65, 85 and 95 dB), we measured the duration of the gap between the masker and signal required to give 79.4% correct detection of the signal (called the 'gap threshold') as a function of masker frequency; the longer the gap threshold, the more effective is the masker. The gap threshold patterns sometimes showed two peaks. One occurred just below the signal frequency and the frequency at the peak was hardly affected by masker level. The second peak fell at a lower frequency, and this frequency tended to decrease with increasing masker level. The gap threshold patterns tended to spread markedly towards lower frequencies with increasing masker level. The shift with level provides further evidence for a basalward spread of the travelling wave with increasing level.