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Author Topic: QotW - 08.05.11 - Why do ear plugs amplify internal noises?  (Read 23667 times)

Offline thedoc

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Iíve 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 itís much more quiet.  Why is that?
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« Last Edit: 03/06/2008 16:53:44 by BenV »


 

Offline thedoc

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Trevor Cox, Professor of Acoustic Engineering, University of Salford:
Why is it that when you listen to yourself when youíve got earplugs you sound very different and all the sounds of munching and crunching sound very different if you eat something?  When Iím speaking Iím not just hearing the sound coming out of my mouth, going round and down the side of my head and into my ear canal. Iím also hearing my bones vibrate. Itís 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 youíve got something also happening called the occlusion effect. Because youíve got an ear plug in youíve 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:
Itís 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. Thatís one problem that occurs through it.
« Last Edit: 03/06/2008 16:54:00 by BenV »
 

Offline techmind

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #2 on: 08/05/2008 11:11:02 »
This phenomena is well-known to audiologists and hearing-aid providers. It is called the "occlusion effect".
For more information, see for example:
http://www.hearingresearch.org/Dr.Ross/occlusion.htm

Earphones from a few years back do not seal in the ear canal and don't have this problem. The occlusion effect is quite pronounced on the newest designs of consumer in-the-ear phones which completely block the ear canal and have become popular over the past 2-3 years.

(edit to correct typo - thank Chris!)
« Last Edit: 08/05/2008 21:26:48 by techmind »
 

Offline chris

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #3 on: 08/05/2008 14:13:35 »
This phenomena is well-known to audiologists and hearing-air providers.

Hearing air? That's the sea-in-the-seashell effect isn't it?!
 

Offline turnipsock

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #4 on: 08/05/2008 21:54:13 »
I have noticed people singing in groups using this effect to adantage by sticking a finger in their ear so they can hear their own voice better. See Steeleye Span.

 

Offline rosalind dna

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #5 on: 09/05/2008 01:10:06 »
Could it be because our sinuses are in the same distance/region
of our bodies, noses, ears and forehead so that is why when
you are wearing headphones it sounds louder or because your senses
are being pressed harder together?>
 

Offline techmind

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #6 on: 09/05/2008 10:48:11 »
Quoting from the link I gave earlier...

Quote
An occlusion effect occurs when some object (like an unvented earmold) completely fills the outer portion of the ear canal. What this does is trap the bone-conducted sound vibrations of a person's own voice in the space between the tip of the earmold and the eardrum. Ordinarily, when people talk (or chew) these vibrations escape through an open ear canal and the person is unaware of their existence. But when the ear canal is blocked by an earmold, the vibrations are reflected back toward the eardrum and increases the loudness perception of their own voice. Compared to a completely open ear canal, the occlusion effect may boost the low frequency (usually below 500 Hz) sound pressure in the ear canal by 20 dB or more.

This is a real and measurable increase in sound. One way that it can be determined is with a probe-tube microphone (a device that should be used, in my judgment, in just about every hearing aid fitting). A probe-tube microphone consists of a very fine, flexible tube that is connected to various types of sound measurement equipment. The tube is inserted in the ear canal and the audiologist measures the sound levels in the ear canal while the hearing aid user utters some standard vowel (like "ee"). Then, with the tube still in place, an earmold (or hearing aid shell) is inserted in the ear canal, making sure that the tube extends a few millimeters past the earmold tip. Then, with the hearing aid turned off, the audiologist again measures the sound levels when the hearing aid user says the same vowel (at the same loudness level). The difference between the sound levels occurring when the ear is open and when it is closed with a hearing aid is a measure of the amount of the occlusion effect. As I noted above, differences of 20 dB or more are common.
 

lyner

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #7 on: 12/05/2008 16:21:28 »
The Ear is an incredibly sophisticated system for getting sounds in without suffering from interference from internally generated sounds. The cochlea, which contains the actual sound sensing cells is fed with two sound signals - not just one. The 'oval window' is vibrated by the three tiny bones (the ossicles) which link it to the ear drum. It and the Ear drum are also subject to vibration of air in the Eustacean tube - this tube joins to the other cavities inside the head (nose, mouth and sinuses). So if you blow your nose or sniff etc., you would hear a very loud noise. To avoid this, there is another window - the 'round window' which also takes sound from inside the middle ear. The Cochlea senses the DIFFERENCE between signals from the two windows - which is the sound from the drum, via the ossicles. Any other internal sounds are canceled out because both windows are moved in the same direction.
So the hearing 'amplifier' is, essential a differential amplifier with high 'common mode rejection' of sounds generated within your head. If you block your ear canal, you are preventing you eardrum from vibrating as much as it should due to internally generated sounds but the round window vibrates just as much as normal. You no longer canceling out the internal sounds and the Cochlea senses much more of them. Hence boom boom rustle rustle as gunge moves around inside your head - either by breathing or knocking your skull.
 

Offline chris

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #8 on: 12/05/2008 22:51:03 »
That's a really nice answer SC. It's also the way "balanced" audio electrical equipment works to miminise noise in lines and mic leads. In this situation the signal is applied to one of three leads, the other two being a ground and a neutral line.

The principle is that any external source of electrical noise will affect both the signal line and the neutral line equally. The equipment at the other end then merely responds to the difference between the two lines (signal vs neutral). Since the noise is in both there is no difference and it cancels itself out. This is the same as the cochlear rejecting the input from the the round window in order to be selective for ear-drum inputs.

Chris
 

lyner

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Thanks for the kind comments.
Quote
. It's also the way "balanced" audio electrical equipment works to miminise noise in lines and mic leads.
That's the way I got to it. I saw a detailed diagram of the ear and sussed out what goes on - subsequently confirmed somewhere or other.
Balanced amplification is used all over the field of measurement and communications. The performance of modern differential amplifiers is staggering!
The amplification in the ear is pretty damn smart; the gain is constantly adjusted so that it's almost unstable so that it can be as sensitive as poss. It's not surprising that, once we're past our sell by date, the system starts to go down hill. Imagine a 60 year old valve amplifier which was state of the art at the time (and probably performed very well). It could be forgiven for going a bit soggy! I have the equivalent of damp paper capacitors and crumbly carbon resistors in my ears these days.
Quick nurse, the screens!
 

Offline Alan McDougall

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #10 on: 25/06/2008 21:56:27 »
Heck,

What detailed answers I like this forum
 

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QotW - 08.05.11 - Why do ear plugs amplify internal noises?
« Reply #10 on: 25/06/2008 21:56:27 »

 

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