Comparing the information conveyed by envelope modulation for speech intelligibility, speech quality, and music quality

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Abstract

<p class="first" id="d13729710e137">This paper uses mutual information to quantify the relationship between envelope modulation fidelity and perceptual responses. Data from several previous experiments that measured speech intelligibility, speech quality, and music quality are evaluated for normal-hearing and hearing-impaired listeners. A model of the auditory periphery is used to generate envelope signals, and envelope modulation fidelity is calculated using the normalized cross-covariance of the degraded signal envelope with that of a reference signal. Two procedures are used to describe the envelope modulation: (1) modulation within each auditory frequency band and (2) spectro-temporal processing that analyzes the modulation of spectral ripple components fit to successive short-time spectra. The results indicate that low modulation rates provide the highest information for intelligibility, while high modulation rates provide the highest information for speech and music quality. The low-to-mid auditory frequencies are most important for intelligibility, while mid frequencies are most important for speech quality and high frequencies are most important for music quality. Differences between the spectral ripple components used for the spectro-temporal analysis were not significant in five of the six experimental conditions evaluated. The results indicate that different modulation-rate and auditory-frequency weights may be appropriate for indices designed to predict different types of perceptual relationships. </p>

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Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise.

M. Nilsson, S D Soli, J A Sullivan (1994)

A large set of sentence materials, chosen for their uniformity in length and representation of natural speech, has been developed for the measurement of sentence speech reception thresholds (sSRTs). The mean-squared level of each digitally recorded sentence was adjusted to equate intelligibility when presented in spectrally matched noise to normal-hearing listeners. These materials were cast into 25 phonemically balanced lists of ten sentences for adaptive measurement of sentence sSRTs. The 95% confidence interval for these measurements is +/- 2.98 dB for sSRTs in quiet and +/- 2.41 dB for sSRTs in noise, as defined by the variability of repeated measures with different lists. Average sSRTs in quiet were 23.91 dB(A). Average sSRTs in 72 dB(A) noise were 69.08 dB(A), or -2.92 dB signal/noise ratio. Low-pass filtering increased sSRTs slightly in quiet and noise as the 4- and 8-kHz octave bands were eliminated. Much larger increases in SRT occurred when the 2-kHz octave band was eliminated, and bandwidth dropped below 2.5 kHz. Reliability was not degraded substantially until bandwidth dropped below 2.5 kHz. The statistical reliability and efficiency of the test suit it to practical applications in which measures of speech intelligibility are required.

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Effects of fluctuating noise and interfering speech on the speech-reception threshold for impaired and normal hearing.

J Festen, R Plomp (1990)

The speech-reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal-hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady-state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal-hearing listeners, the SRT for sentences in modulated noise is 4-6 dB lower than for steady-state noise; for sentences masked by a competing voice, this difference is 6-8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing-impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing-impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively.