The House Ear Institute Presents Summer 1998

 

Understanding Loudness Perception
Will Improve Hearing Aid Usage

Research Experiment Picture


Research subject John McManamey, performs loudness perception experiments with
John Galvin, Lendra Friesen and Fan-Gang Zeng, PhD (left to right)

In recent years, Research Scientists Fan-Gang Zeng, Ph.D. and Robert V. Shannon, Ph.D., have been investigating how the human brain ascertains the loudness of sound. "It is necessary to understand how the auditory system determines loudness, before we can attempt to artificially reproduce normal sensation of sound intensity with an auditory prosthesis," explains Dr. Zeng who directs the Auditory Perception Laboratory. Their research studies show that two mechanisms, rather than one, help the brain determine variations in loudness. Low-pitched sounds (less than 300 Hz) appear to be transformed linearly by the cochlea (hearing organ), and travel along the auditory nerve to the brainstem, where they are then processed. Sounds with frequencies higher than 300 Hz however, appear to be compressed in the cochlea and then expanded by the brain to restore the intensity information. This discovery directly impacts the future design and function of prosthetic hearing devices such as cochlear implants for persons with profound hearing impairment and hearing aids for those with a hearing loss.

A new series of experiments designed by Dr. Zeng and Chao Zhang, Ph.D. explores the perception of loudness between a steady state sound and a dynamic sound. "A pure tone is an artificial steady-state sound created under laboratory conditions that has been traditionally used to fit auditory prostheses," comments Dr. Zeng. "The pure tone does not reflect naturally ocurring sounds such as speech, music and environmental sounds that have a dynamic characteristic - this means that the intensity fluctuates up and down as the sound occurs." Tests were performed on a group of normal-hearing listeners and a group of cochlear implant listeners and the results showed that dynamic sounds produced a greater sensation of loudness over time than steady-state sounds. This louder sensation of dynamic sounds also differs if one has normal hearing or uses hearing aids or a cochlear implant. 
 


Sound Waves
Examples of a steady-state sound (pure tones) above
and dynamic sound (speech) below

This finding prompted the scientists to start questioning the efficacy of using pure-tones to determine hearing thresholds and set the level of gain in hearing aids and cochlear implants. How loudness levels in hearing aids and cochlear implants are set directly affect the acceptance and efficient usage of the device by an individual. Therefore, the next step for the scientists is to develop protocols that will use dynamic sounds to fit an auditory prosthesis so that speech and music sounds are clear and comfortable as well as audible to the hearing-impaired individual. And they have already begun to examine how pitch, phase and intensity combined to give the perception of loudness. 

"Based on our new understanding of the loudness dimension, we think we can significantly help the hearing impaired population by designing better protocols for fitting the devices. This means more natural and comfortable sounds for hearing aid and cochlear implant users," said Dr. Zeng, "and this is exactly what they have been requesting."