Ear Plugs and Internal Noise?

11 May 2008

Question

I’ve noticed that if I have earplugs or ear bud style headphones in my ears that chewing food sounds incredibly loud, but when my ears are unplugged it’s much more quiet. Why is that?

Answer

Trevor Cox, Professor of Acoustic Engineering, University of Salford: Why is it that when you listen to yourself when you've got earplugs you sound very different and all the sounds of munching and crunching sound very different if you eat something? When I'm speaking I'm not just hearing the sound coming out of my mouth, going round and down the side of my head and into my ear canal. I'm also hearing my bones vibrate. It's called bone conduction. When my vocal fold are opening and closing, when the air in my mouth is resonating the sound is also passing through my head to my ears and being picked up as sound. When you block up the outside path by sticking in things like earplugs all you can hear is the bone conduction. Actually you've got something also happening called the occlusion effect. Because you've got an ear plug in you've got a little resonant cavity in the ear canal so you actually get a little boost towards the top of speech around 2000Hz. That also means that certain sounds are amplified as well. Daniel Rowan, Audiologist, Institute of Sound and Vibration Research, University of Southampton: It's probably related to a well-known phenomenon called the occlusion effect. All sounds generated within the body such as mastication or talking are transmitted all around the body or the body tissues, particularly the bone. That sort of sound escapes, if you will, into the air-filled cavities of the body including the ear canals. The sound in the ear canals usually follows the line of least resistance and comes out of the ear canals rather than going into the ear drum and into the inner ear. When you plug your ear canal up with your finger then that sound goes into the eardrum, into the cochlear making things sound louder. In acoustical terms the amount of sound that gets into your inner ear can be as much as 1000 times. In terms of how loud we perceive it, it can be 5 times as loud or even more. This is a particular problem with people who wear hearing aids because having a hearing aid in the ear canal can make the sound of their own voice uncomfortable. That's one problem that occurs through it.

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