Speech-shaped Masker Research Articles

Predicting speech reception thresholds of cochlear implant users using a modified envelope based measure

An envelope-correlation based measure (ECM), proposed in Yousefian and Loizou (2012), was applied to predict the speech reception thresholds (SRTs) of nine pre- and post-lingual cochlear implant (CI) users in the presence of speech-shaped masker and a female competing talker. A principal component analysis showed that CI users’ aided pure-tone thresholds were almost orthogonal to the predictions made by ECM, indicating that the ECM does not take aided hearing loss into account. A modified ECM is proposed which includes pure-tone thresholds into the predictions. The proposed model adjusts the ECM predictions based on the pure tone threshold values, and is able to predict CI users’ SRTs with higher correlation (r=0.92) compared to the original ECM (r=0.59). The results reported in this study indicate that information available on MAPs (e.g. T-levels, number of active electrodes) may not be sufficient to predict CI users’ SRTs in noise. In additions, results show that lack of audibility (i.e. elevated pure-tone thresholds) can dramatically limit speech intelligibility in noise in (even experienced) CI users.

The Development of Frequency Weighting for Speech in Children with a History of Otitis Media with Effusion

To determine the effect of chronic (recurrent) otitis media with effusion (OME) on frequency weighting in the perception of speech in noise. It was hypothesized that children with a history of OME weight speech information in the mid frequency region higher than control children. This is a matched cohort study looking at differences in frequency weighting in 12 children with a history of OME 1 to 2 wks after placement of tympanostomy tubes compared with 21 control children. Children were tested on their ability to identify key words in sentences presented in speech-shaped noise. The frequency content of the sentences was manipulated to determine the relative importance of frequencies in the regions of 1, 2, and 4 kHz. The frequency bands selected were 798 to 1212 Hz (low band), 1575 to 2425 Hz (mid band), and 3000 to 5000 Hz (high band). Initial testing involved adaptive runs where a speech-shaped masker was held at a constant level and the level of the speech with all three bands present varied. Once a level corresponding to 85% to 90% correct was identified, novel sentences were then presented at this signal-to-noise ratio in fixed block runs, with all bands present, or with one of the three bands omitted. The children in the OME group achieved 85% to 90% correct at a lower signal-to-noise ratio than controls in the adaptive testing, where all three speech bands were present. Fixed block testing indicated that children with OME history gave more weight to speech frequencies in the region of 2000 Hz compared with the age-matched control group. The results are consistent with an interpretation that the development of frequency weighting in the perception of speech can be affected by a history of OME.

Speech Recognition in Noise: Estimating Effects of Compressive Nonlinearities in the Basilar-Membrane Response

This experiment was designed to estimate effects of cochlear nonlinearities on tonal and speech masking for individuals with normal hearing who have a range of quiet thresholds. Physiological and psychophysical evidence indicates that for signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower stimulus levels and compressed at medium to higher stimulus levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, the hypothesis tested was that masker effectiveness would change as a function of stimulus level consistent with the underlying basilar membrane response. Specifically, with a fixed-level speech signal and a speech-shaped masker that ranges from low to higher levels, the resulting response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels. This would result in relatively less effective masking at higher masker levels. It was further hypothesized that the transition from linear to compressed responses to both tones and maskers would occur at higher levels for listeners with higher quiet thresholds than for listeners with lower quiet thresholds. Tonal thresholds and speech recognition in noise were measured as a function of masker level. A 10-msec, 2.0-kHz tone was presented in a lower frequency masker ranging from 40 to 85 dB SPL. Moderate-level speech was presented in interrupted noise at six levels ranging from 47 to 77 dB SPL. To minimize differences in speech audibility that could arise during the "off" periods of the interrupted noise, a low-level steady-state "threshold-matching noise" was also present during measurement of speech recognition. Subjects were 30 adults with normal hearing with a 20-dB range of average quiet thresholds. Tonal breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were significantly correlated with quiet thresholds, whereas slopes measured above the breakpoints were not. Speech recognition in noise was consistent with the hypothesis that the response of the basilar membrane to the masker was linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. That is, at lower masker levels, as masker level increased, mean observed speech scores declined as predicted using the articulation index, an audibility-based model. With further increases in masker level, mean scores declined less than predicted. Moreover, for subjects with higher quiet thresholds, masker effectiveness remained constant for a wider range of masker levels than for subjects with lower quiet thresholds, consistent with the hypothesis that the transition from linear to compressed responses occurred at higher levels. Finally, significant negative correlations were obtained between individual subjects' tonal and speech measures. Results from tonal and speech tasks were consistent with basilar membrane nonlinearities and consistent with changes in nonlinearities with minor threshold elevations, providing support for their role in the understanding of speech in noise with increases in noise level.

Spectral and threshold effects on recognition of speech at higher-than-normal levels

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.

Recognition of filtered words in noise at higher-than-normal levels: Decreases in scores with and without increases in masking

To examine spectral effects on declines in speech recognition in noise at high levels, word recognition for 18 young adults with normal hearing was assessed for low-pass-filtered speech and speech-shaped maskers or high-pass-filtered speech and speech-shaped maskers at three speech levels (70, 77, and 84 dB SPL) for each of three signal-to-noise ratios (+8, +3, and -2 dB). An additional low-level noise produced equivalent masked thresholds for all subjects. Pure-tone thresholds were measured in quiet and in all maskers. If word recognition was determined entirely by signal-to-noise ratio, and was independent of signal levels and the spectral content of speech and maskers, scores should remain constant with increasing level for both low- and high-frequency speech and maskers. Recognition of low-frequency speech in low-frequency maskers and high-frequency speech in high-frequency maskers decreased significantly with increasing speech level when signal-to-noise ratio was held constant. For low-frequency speech and speech-shaped maskers, the decline was attributed to nonlinear growth of masking which reduced the "effective" signal-to-noise ratio at high levels, similar to previous results for broadband speech and speech-shaped maskers. Masking growth and reduced "effective" signal-to-noise ratio accounted for some but not all the decline in recognition of high-frequency speech in high-frequency maskers.

Word recognition in noise at higher-than-normal levels: Decreases in scores and increases in masking

Under certain conditions, speech recognition in noise decreases above conversational levels when signal-to-noise ratio is held constant. The current study was undertaken to determine if nonlinear growth of masking and the subsequent reduction in "effective" signal-to-noise ratio accounts for this decline. Nine young adults with normal hearing listened to monosyllabic words at three levels in each of three levels of a masker shaped to match the speech spectrum. An additional low-level noise equated audibility by producing equivalent masked thresholds for all subjects. If word recognition was determined entirely by signal-to-noise ratio and was independent of overall speech and masker levels, scores at a given signal-to-noise ratio should remain constant with increasing level. Masked pure-tone thresholds measured in the speech-shaped maskers increased linearly with increasing masker level at lower frequencies but nonlinearly at higher frequencies, consistent with nonlinear growth of upward spread of masking that followed the peaks in the spectrum of the speech-shaped masker. Word recognition declined significantly with increasing level when signal-to-noise ratio was held constant which was attributed to nonlinear growth of masking and reduced "effective" signal-to-noise ratio at high speech-shaped masker levels, as indicated by audibility estimates based on the Articulation Index.

Spectral contributions to the benefit from spatial separation of speech and noise.

Speech recognition in noise improves when speech and noise sources are separated in space. This benefit has two components whose effects are strongest in different frequency regions: (1) interaural level differences (e.g., head shadow), which are largest at higher frequencies, and (2) interaural time differences, which have their greatest contribution at lower frequencies. Binaural interactions enhance the separation of signals from noise through the use of these interaural differences. Here, the benefit attributable to spatial separation was measured as a function of the low- and high-pass cutoff frequency of speech and noise. Listeners were younger adults with normal hearing, older adults with normal hearing, and older adults with hearing loss. Binaural thresholds for narrowband noises were measured in quiet and in a speech-shaped masker as a function of masker low-pass cutoff frequency. Speech levels corresponding to 50% correct recognition of sentences from the Hearing in Noise Test (HINT) were measured in a 65-dB SPL speech-shaped noise. Thresholds for narrowband noises and for speech were measured with two loudspeaker configurations: (1) signals and speech-shaped noise at 0 degrees azimuth (in front of the listener) and (2) signals at 0 degrees azimuth and speech-shaped noise at 90 degrees azimuth (at the listener's side). The criterion measure was spatial separation benefit, or the difference in thresholds for the two conditions. Benefit of spatial separation for unfiltered speech averaged 6.1 dB for younger listeners with normal hearing, 4.9 dB for older listeners with normal hearing, and 2.7 dB for older listeners with hearing loss. Benefit was differentially affected by low-pass and high-pass filtering, suggesting a trade-off of the contributions of higher frequency interaural level differences and lower frequency interaural timing cues. As expected, older listeners with hearing loss benefited little from the improved signal-to-noise ratios in the higher frequencies resulting from head shadow, but showed some benefit from lower frequency cues. Spatial benefit for older listeners with normal hearing was reduced relative to benefit for younger listeners. This result may be related to older listeners' elevated thresholds at frequencies above 6.0 kHz.