How do you achieve amplification and directionality without localization drawbacks?
Localization ability and auditory representation of space are pieced together by the auditory system from a multitude of cues provided by the ear. Differences in timing and level of sounds arriving at each ear are key acoustic cues used in localization. Hearing loss has an obvious impact on localization and spatial hearing by reducing the audibility of such cues. Hearing aids, by virtue of microphone placement, microphone mode, and between-ear differences, can potentially disrupt them even while increasing their audibility. The Directional Mix is a technology that can meet the challenge of achieving amplification - and the benefits of directionality - without compromising localization. The Directional Mix applies advanced directional processing to high frequency sounds, while an omnidirectional response is maintained in the low frequencies. In this way, improved signal-to-noise ratio relative to omnidirectional processing is provided, while important localization cues are preserved.
Some hearing aids attempt to preserve localization cues - specifically, differences in levels of high frequency sounds - by using wireless communication to coordinate the gains applied by bilaterally fit devices. The rationale for this processing is to prevent compression from applying different gains at the two ears, thereby changing the naturally occurring level differences. Although intuitively appealing, evidence of this processing as beneficial has not been forthcoming. For example, MacPherson (2010) reported a study finding that the binaural processing algorithm in two manufacturers' hearing aids had no significant influence on the performance of hearing impaired participants performing a localization task. An explanation for this could be that the difference in gains applied to signals at each ear may not actually cause significant localization issues. In addition, researchers at the National Acoustics Laboratory in Australia have investigated whether such alterations in level difference patterns are indeed problematic for localization (Kediser, Convery & Hamacher, 2011). Their finding was that gain mismatches of up to 9 dB between ears were not disruptive to localization, thus calling the necessity of binaural processing schemes for localization into question.
Jennifer Groth, M.A., is Director of Audiology Communication at GN ReSound, where she is responsible for product definition and audiology-driven support for products in the market. She has served in roles such as research audiologist and product manager through the course of her career. She holds a Master of Arts degree in speech pathology & audiology from the University of Iowa.
MacPherson, E. (2010, October). The role of interaural difference cues in horizontal, vertical and dynamic sound localization by normal-hearing and hearing-impaired. Paper presented at the Canadian Academy of Audiology Conference, Montreal, Canada.
Keidser, G., Convery, E., & Hamacher, V. (2011). The effect of gain mismatch on horizontal localization performance. The Hearing Journal, 64(2), 26-33. Retrieved from: journals.lww.com/thehearingjournal/Fulltext/2011/02000/.
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