Binaural hearing for the hearing-impaired

Listening to someone in a noisy restaurant is easier with two ears (binaural) than with just one, for three reasons. When speech and noise come from spatially separate sources, differences in both their volume and arrival time at the two ears help the listener to separate them. (For more on interaural differences, see the article by Bill Hartmann, Physics Today, November 1999, page 24 .) But even when speech and noise are coincident in space, the third reason, an effect called binaural summation, centrally integrates the signals from both ears, and the signal—the speech—is enhanced over the surrounding noise. Many profoundly deaf people have regained some hearing through using cochlear implants either in both ears (bilateral) or in just one with a hearing aid in the other (bimodal). But which of the three speech-recognition advantages accrue most with which device configuration has been indeterminate. Now Kostas Kokkinakis and Natalie Pak at the University of Kansas report that both bilateral and bimodal users reaped significant speech-recognition benefit—roughly 7 dB in the signal-to-noise ratio compared with a suitable control—from the sound-level effect and very little, if any, benefit from arrival-time differences. The surprise came with summation: On average, the duo saw a 7-dB improvement in bimodal users, while bilateral users gained a meager 2 dB. The researchers say the low-frequency cues delivered to one ear by the hearing aid complement the higher frequencies delivered to the other ear by the cochlear implant. (K. Kokkinakis, N. Pak, J. Acoust. Soc. Am. 135, EL47, 2014, doi:10.1121/1.4831955 .)