By Nirmal Kumar Srinivasan, Ph.D.
In real-world listening situations, we always listen to speech in the presence of other sources of masking, or competing sounds. One of the major sources of masking in such situations is the speech signal that the listener is not paying attention to. The process of understanding the target speech in the presence of a masking speech involves separating the acoustic information of the target speech and tuning out masker speech.
Young, typical-hearing individuals can function in moderate amounts of speech distortions with a minimal reduction in speech understanding, while older individuals with and without hearing loss are much more susceptible to the distortions of the speech signal and often experience increased difficulties in understanding speech in noisy environments.
The process of understanding target speech in the presence of masking speech partially involves separating out and attending to the acoustic information of the target speech while ignoring the acoustic information of the masking speech. Target speech and masking speech can be segregated based on factors such as the locations of the target and maskers in the environment, differences in fundamental frequencies between the target and maskers, interaural (between the two ears) time differences, interaural level differences, or differences in contextual information between the target and maskers.
Spatial release from masking (SRM) is the reduction in target identification thresholds when the maskers are physically distanced from the target. Results from recent speech masking studies confirmed that an individual’s ability to obtain a better identification threshold when the target and maskers are spatially separated is driven by a combination of better-ear listening, binaural (using both ears) unmasking, and perceived location of the target and the maskers.
Better-ear listening is an auditory phenomenon created by the head shadow effect wherein the listener uses the interaural level differences between the two ears to better understand speech. Binaural unmasking is the increased ability of the listeners to better understand speech in the presence of noise when there is an interaural time difference between the signals reaching the two ears.
An individual’s temporal acuity is typically described using the gap detection threshold which is the smallest silent interval in the stimulus detected by the listener. While many studies have concluded that older individuals have broader difficulties in temporal and spatial processing tasks, very few studies are available in the literature that contains multiple temporal and spatial processing measurements from the same individual.
We sought to measure the temporal and spatial processing capabilities of older individuals with hearing loss and to use statistical models to identify the individual contributions of gap detection thresholds and localization acuity to SRM. Twenty-five older listeners with varying degrees of hearing loss participated in this experiment whose results were published in the International Journal of Audiology in November 2021. We found that spatially separating target and masker sources helps to better identify speech in speech-on-speech masking scenarios, and that difficulties in spatial and temporal processing co-occur in older individuals and individuals with reduced hearing acuity.
However, reduced temporal and spatial processing due to aging, independent from reduced hearing acuity, was not observed. Multiple regression models predicting SRM using gap detection thresholds and localization acuity indicated that the ability of the listeners to use interaural time differences between the signals arriving at the two ears is responsible for obtaining better identification thresholds in spatially separated conditions.
A 2016 Emerging Research Grants scientist generously funded by Royal Arch Research Assistance, Nirmal Kumar Srinivasan, Ph.D., is an assistant professor in the audiology program in the speech-language pathology and audiology department at Towson University in Maryland.
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