Detection in noise by spectro-temporal pattern analysis

Detectability of a 400-ms, 1000-Hz pure-tone signal was examined in bandlimited noise where different spectral regions were given similar waveform envelope characteristics. As expected, in random noise the threshold increased as the noise bandwidth was increased up to a critical bandwidth, but remained constant for further increases in bandwidth. In the noise with envelope coherence however, threshold decreased when the noise bandwidth was made wider than the critical bandwidth. The improvement in detectability was attributed to a process by which energy outside the critical band is used to help differentiate signal from masking noise, provided that the waveform envelope characteristics of the noise inside and outside the critical band are similar. With flanking coherent noise bands either lower or higher in frequency than a noise band centered on the signal, it was next determined that the frequency relation and remoteness of the coherent noise did not particularly influence the magnitude of the unmasking effect. An interpretation in terms of nonsimultaneous masking was reconciled with some aspects of the data, and with an interpretation in terms of across-frequency temporal pattern analysis. This paradigm, in which detection is based upon across-frequency temporal envelope coherence, was termed "comodulation masking release." Comodulation offers a controlled way to investigate some of the mechanisms which permit signals to be detected at adverse signal-to-noise ratios.

Factors influencing the masking level difference in cochlear hearing-impaired and normal-hearing listeners

The masking level difference (MLD) at 500 Hz was examined in wide-band (960 Hz) and narrow-band (50 Hz) noise for normal-hearing subjects and subjects with symmetrical mild-to-moderate cochlear hearing loss. Monaural tasks of intensity discrimination, temporal resolution, and frequency resolution were performed in order to examine relationships between monaural dysfunction and MLD performance. Interaural time discrimination for a 500-Hz pure tone also was examined. The performance of the hearing-impaired subjects was poorer than that of the normal-hearing subjects for MLD, interaural delta t, and most monaural tasks. However, no significant relationships were found between monaural and MLD performance when effects of threshold were taken into account. MLDs were more reduced in wide-band noise than in narrow-band noise for the hearing-impaired subjects (when contrasted with normal-hearing subjects). MLD performance was correlated with interaural time discrimination, and it is suggested that one reason for poor MLD performance with hearing impairment may be poor temporal coding of stimulus-fine structure.

Temporal integration, frequency resolution, and off-frequency listening in normal-hearing and cochlear-impaired listeners

Temporal integration for a 1000-Hz signal was determined for normal-hearing and cochlear hearing-impaired listeners in quiet and in masking noise of variable bandwidth. Critical ratio and 3-dB critical band measures of frequency resolution were derived from the masking data. Temporal integration for the normal-hearing listeners was markedly reduced in narrow-band noise, when contrasted with temporal integration in quiet or in wideband noise. The effect of noise bandwidth on temporal integration was smaller for the hearing-impaired group. Hearing-impaired subjects showed both reduced temporal integration and reduced frequency resolution for the 200-ms signal. However, a direct relation between temporal integration and frequency resolution was not indicated. Frequency resolution for the normal-hearing listeners did not differ from that of the hearing-impaired listeners for the 20-ms signal. It was suggested that some of the frequency resolution and temporal integration differences between normal-hearing and hearing-impaired listeners could be accounted for by off-frequency listening.

The effect of random intensity fluctuation on monaural and binaural detection

Two experiments were performed to determine the effects of random intensity fluctuation on NoSo and NoS pi performance. Noise was used as both signal and masker, and stimuli were bands of noise from either 0-2.0 or 2.0-4.0kHz. Signal and masker were either coherent (from the same source) or noncoherent (from independent sources). In the first experiment, noise fluctuation was achieved by modulating a wide band of noise. In the second experiment, fluctuation was achieved by narrowing the noise bandwidth. Results from both experiments indicated that NoSo performance was adversely affected by fluctuation and by noncoherent relation between signal and masker. NoS pi detection was not adversely affected by fluctuation at low frequency, and was affected less adversely than was NoSo detection at high frequency. This difference between NoSo and NoS pi performance is an important consideration when making inferences about monaural and binaural processing when the stimuli are fluctuating rather than temporally steady.

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.

Monaural and binaural intensity discrimination in normal and cochlear-impaired listeners

Monaural and binaural intensity difference limens for 75-dB SPL pure tones were determined for 6 normal subjects and for 6 subjects with cochlear hearing loss. The magnitude of binaural masking level difference (BMLD) was also determined. Both normal and hearing-impaired subjects showed a 0.45-dB binaural advantage for intensity discrimination. In contrast, the BMLD was appreciably reduced with hearing impairment. Results are discussed in terms of specific effects of hearing loss on binaural hearing.

Validation of a screening oto- admittance instrument

A within-subject comparison between an American Electromedics 85R screening tympanometer and a Grason-Stadler 1723 clinical otoadmittance meter indicated equivalent results for middle-ear pressure. Tympanometer admittance readings needed to be rescaled to concur with the oto-admittance meter. Screening by acoustic reflex response using the single-intensity stimulus of the tympanometer was problematic and gave many misleading results.

Masking of pure tones by broad-band noise in cochlear-impaired listeners

We measured masked thresholds for pulsed pure tone (5, 1.0, 2.0, and 4.0 kHz) in the presence of different levels of broad-band noise (nominally 0, 20, 40, and 60 dB/Hz). Several of the 16 cochlear-impaired listeners displayed masked thresholds that were considerably higher than those obtained from 10 normal listeners. At the 60 dB/hz noise level the correlation coefficients between thresholds in noise and thresholds in quiet were r = 36, .44, .63 and .64 for signal frequencies of .5, 1.0, 2.0, and 4.0 kHz, respectively. The growth of masking, as masker level was increased, was linear for the normal listeners but was disproportionate and nonlinear in some cochlear-impaired listeners. In these data and data from others studies, it is clear that thresholds in noise cannot be predicted from thresholds in quiet. Masked thresholds are related to other measures of frequency resolution and to speech intelligibility in noise, but it is argued that psychoacoustic tuning curves provide more direct measures of the auditory-filter characteristics.