by Arthur Boothroyd
It’s easy enough to restore 20/20 eyesight with glasses or contacts. But even state-of-the-art digital hearing aids can’t perfectly restore hearing for people whose inner ears have been damaged by noise exposure, medications or just the wear and tear of aging. Part of the problem is that this kind of sensorineural hearing loss — the result of permanent damage to the sensory cells of the inner ear — does more than just make sounds quieter. It can jumble the sounds, too, in ways that garble speech.
First up: Many people, especially those with age-related hearing loss, lose the ability to detect highfrequency sounds. Because consonants are typically higherpitched than vowels, the loss of high frequencies can make it difficult to tell consonant sounds apart. As a result, speech sounds muddy — in some cases, almost beyond comprehension. Some people — also, or instead — have trouble distinguishing between different pitches. This happens because of damage to the delicate hair cells in the inner ear and to the nerves connected to them, which are responsible for separating out different frequencies in sound.
“The frequencies in sound are like the colors in the spectrum,” explains Boothroyd. “Imagine that the red bleeds over into the yellow and the yellow bleeds over into the green, and so on. That’s the sort of thing that we believe happens in the cochlea.” Interestingly, many people with hearing loss report that, as sounds get louder, they abruptly go from being inaudible to painfully loud. This rapid increase in perceived loudness is known as “recruitment” because it is thought to be caused by normal hair cells suddenly being recruited to take over for nearby damaged cells.
In these cases, he says, the sound cuts in and out because only sounds above a certain volume can be heard.
Hearing aids can fix some of these issues. For example, the problem of recruitment can be solved by using “amplitude compression,” in which the volume of louder sounds is decreased before being passed on to the ear. In the newer digital hearing aids, amplitude compression can even be tailored to specific frequencies.