The perception of amplitude modulated signals

Researcher(s): J. Koopman, PhD., C.S.G.M. Bos. MSc.
Supervisor(s): prof. W.A. Dreschler, PhD.

Background:
Everyday signals are often not of constant level, but fluctuate in time. Hearing impaired listeners often have considerable difficulty with understanding speech, when speech is presented in fluctuating background noise (e.g. restaurant, bar). The reasons for this finding are still unclear. Based on the outcome of previous studies, the project focused on the phenomenon that hearing impaired subjects experience a prolonged masking effect after modulated signals.

Aim:
The project is meant to gain insight in the perception of modulated sounds.

Method:
For different kind of measurements were conducted with hearing impaired and normal hearing subjects.

The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone.
The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone, when preceded by a 1 kHz continuous signal.
The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone, when preceded by an 8 Hz modulated signal with a carrier frequency of 1 kHz.
Speech intelligibility of the VU98 sentences was measured in a continuous and a fluctuating background noise, both noises had spectral content equal to the speech material.

Results:
The results of experiment 1 have been used to present the signals at a comparable sensation level. The normal hearing subjects were also tested at SPL-levels comparable to the level at which the average hearing-impaired subject was tested. Hearing impaired subjects turn out to have a lower sensitivity for modulations if the target is preceded by an amplitude modulated signal than if it is preceded by a continuous signal (the sensitivity for amplitude modulations decreased when a modulated signal was presented previous to the target).
Also the differences between the measurements with a modulated and a continuous signal preceding the target signal were determined. These differences show a high correlation (r = 0.60) with the difference in signal-to-noise ratio for a continuous background noise and a fluctuating background noise (p<0.001). This can be regarded as an indication that at least part of the inability of the hearing impaired to listen into the gaps in the noise can be explained by an inability to detect modulations, because of modulating signals elsewhere.

Future research:
This study has been completed with a PhD thesis by dr. J. Koopman, titled “The perception of sinusoidally amplitude modulated signals and its relevance to listening in noise”. The promotion took place on march 16 2004.


Home Onderwijs Wetenschap Research (EN)
General Otology Prevention of hearing loss Auditory profile Sensorineural hearing loss and loudness perception Predicting the speech intelligibility under fluctuating background conditions Auditory filter measurements Auditory profile: spectral and temporal resolution (HEARCOM, WP2) Consonant confusions for filtered and consonant deteriorated nonsense VCV syllables (HEARCOM, WP2) Auditory localisation (HEARCOM, WP2) Dead Regions The perception of amplitude modulated signals Rehabilitation Hearing at work Publications Presentations	The perception of amplitude modulated signals Researcher(s): J. Koopman, PhD., C.S.G.M. Bos. MSc. Supervisor(s): prof. W.A. Dreschler, PhD. Background: Everyday signals are often not of constant level, but fluctuate in time. Hearing impaired listeners often have considerable difficulty with understanding speech, when speech is presented in fluctuating background noise (e.g. restaurant, bar). The reasons for this finding are still unclear. Based on the outcome of previous studies, the project focused on the phenomenon that hearing impaired subjects experience a prolonged masking effect after modulated signals. Aim: The project is meant to gain insight in the perception of modulated sounds. Method: For different kind of measurements were conducted with hearing impaired and normal hearing subjects. The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone. The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone, when preceded by a 1 kHz continuous signal. The sensitivity for an 8 Hz sinusoidal amplitude modulation (fluctuation) was measured for a 1 kHz tone, when preceded by an 8 Hz modulated signal with a carrier frequency of 1 kHz. Speech intelligibility of the VU98 sentences was measured in a continuous and a fluctuating background noise, both noises had spectral content equal to the speech material. Results: The results of experiment 1 have been used to present the signals at a comparable sensation level. The normal hearing subjects were also tested at SPL-levels comparable to the level at which the average hearing-impaired subject was tested. Hearing impaired subjects turn out to have a lower sensitivity for modulations if the target is preceded by an amplitude modulated signal than if it is preceded by a continuous signal (the sensitivity for amplitude modulations decreased when a modulated signal was presented previous to the target). Also the differences between the measurements with a modulated and a continuous signal preceding the target signal were determined. These differences show a high correlation (r = 0.60) with the difference in signal-to-noise ratio for a continuous background noise and a fluctuating background noise (p<0.001). This can be regarded as an indication that at least part of the inability of the hearing impaired to listen into the gaps in the noise can be explained by an inability to detect modulations, because of modulating signals elsewhere. Future research: This study has been completed with a PhD thesis by dr. J. Koopman, titled “The perception of sinusoidally amplitude modulated signals and its relevance to listening in noise”. The promotion took place on march 16 2004.