ScienceWise - Summer 2012

Under pressure

Article Illustration
portable music devices may cause significant hearing damage in some individuals

Can your ears defend themselves from your MP3 player?

If you close your eyes tightly, you can hear the fluttering sound of the muscles in the middle ear at work. These tiny muscles hidden behind your ear drums, spring into action whenever loud sound strikes your ears, protecting the inner ear from hearing damage. Contracting these muscles, as you do when you hear that fluttering, reduces the sound level a thousand-fold, about the same sound reduction you get by wearing good quality ear plugs. However, the commonly accepted understanding of how this intricate system achieves this is now being reconsidered.

“Hearing loss used to be called blacksmith’s disease. In the modern age, it might come to be called MP3 disease,” says Dr Andrew Bell from the Research School of Biology, referring to the ubiquitous MP3 player, and the associated risk of hearing damage.

Dr Bell is an advocate for the pressure theory of hearing. This theory proposes that the bones and muscles in the middle ear work together as a tiny hydraulic pump, raising the pressure of fluid in our inner ear (the cochlea), like pushing a cork into a bottle of water. “It’s this pressure,” he explains, “which softens the impact of loud sound on the delicate sensing cells inside the cochlea.”

“The curious thing is that the pressure theory was first put forward more than 150 years ago and then discarded,” says Dr Bell.

“Current theory says that when the muscles contract in response to loud sound they stiffen up and reduce sound transmission by about ten-fold,” he says, “somewhat like putting your finger on a guitar string.”

But Dr Bell’s research has found some serious problems with this sound transmission theory - it can’t explain how power to the cochlea appears to be reduced a thousand-fold by contracting your middle ear muscles.

“The pressure theory explains this well, and all the evidence over the last century or more can be fitted neatly into it – once you see that the sensing cells in the cochlea are tiny pressure gauges which react instantly to pressure.”
Dr Bell’s work helps us understand why some people have ‘tough’ ears that seem resistant to noise-induced hearing loss while others are very susceptible. It’s all to do with the effectiveness of the hydraulics.

“The beauty of the pressure theory is that tiny muscles can work quickly to push on the inner ear fluids and ramp up the pressure, which can persist – like pressure in a garden hose – for many minutes. Some people seem to have leaky hoses, giving short protection, while others are nearly watertight, with long-lasting effects.”

The pressure theory has the potential to lead to better measures to protect people from damagingly loud MP3 player levels.
“Sound levels above 90 decibels, such as loud street noise, are potentially damaging,” says Dr Bell, “and this is exactly the same sound level where the middle ear muscles kick in. Perhaps if we could find a way of making the action of the middle ear muscles ‘pump’ more effectively, like they seem to do with tough ears, we could provide better protection against noise.”

So next time you are listening to your favourite MP3, spare a thought for your middle ear working hard to protect your hearing - it is under a lot of pressure.

 

(Story by Casey Hamilton)

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