“Listener fatigue” prompts research and in-ear headphone redesign


STEVE MCNALLY: Ever since portable Walkman’s and then CD players, on up to today’s phones and MP3 players, in-ear headphones have moved from something only used by musicians to something almost omnipresent. Alongside that trend though, have been worrisome reports about the effects of ear buds on hearing including listener’s fatigue. Until recently, though, researchers didn’t know how the damage was being caused. Now engineers investigating audio fatigue have found not only what they think is the cause, but also a potential solution. When you listen to sounds out in the open, normal eardrum movement is in the range of nanometers, or billionths of a meter. But when sound is sealed by an ear bud, it creates a column of pressure that moves back and forth in the ear canal. This pressure bends the eardrum by micrometers, or 1,000 times further than open air listening. When you look at the eardrum using a scope you can see this dramatic movement. Here there’s specs of glitter on the eardrum. You can see them bounce when the sounds are played from the ear bud. Researcher Steve Ambrose explains. STEVE AMBROSE: Those are huge motions for the tympanic membrane and very small motions for the speaker. They’re quite distinct. These are glitter specs deposited on my eardrum to reflect the light and show the over excursions. STEVE MCNALLY: Those movements are the likely cause of fatigue. Ambrose and his team found that this dramatic ear drum movement triggers the acoustic reflex, a defense mechanism that dampens your ability to hear the sound, but it doesn’t reduce ear drum movement. And in response to the dampening, you might just turn the volume up further amplifying the eardrum movement. To counteract this, Ambrose discovered all that’s needed is a sacrificial membrane to take the beating in place of the eardrum. STEVE AMBROSE: So let’s go ahead and put this in my ear canal. And you can see these motions here. STEVE MCNALLY: That movement is the sacrificial membrane. STEVE AMBROSE: And you’ll focus on the eardrum. And we can see that this motion, which was so great that we had up here, is now gone. STEVE MCNALLY: Ambrose’s membrane, so far, takes two forms. The simplest is a retrofit that could be built into existing ear buds. The retrofit has a small circle of membrane in the stem of the ear bud. Secondly, Ambrose has also developed an entirely new kind of bud, a balloon that inflates by means of sound waves. Like the retrofit the membrane here absorbs the excess sound energy acting as a sort of second eardrum. That means that sounds are clearer even at lower volumes and hearing is protected. STEVE AMBROSE: So this is a new breakthrough, and it’s a way to have the recommended listening volumes sound more than loud enough without the audio fatigue and with much clearer fidelity. Thank you. STEVE MCNALLY: For the National Science Foundation, I’m Steve McNally.

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