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From: "Phil Allison"
Subject: Re: Sound cancellation technology?
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Date: Sun, 29 Dec 2002 21:04:38 +1100
NNTP-Posting-Date: Sun, 29 Dec 2002 20:54:44 EST
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wrote in message
> In sci.electronics.design Phil Allison wrote:
> > wrote in message
> > news:email@example.com...
> >> Earplugs don't have to be passive, and could get a few more db down,
> >> especially if they don't have to cancel HF, which can be mostly killed
> >> the plugs.
> > ** You are making no sense to me.
> > Care to explain more or give a reference.
> An earplug attenuates high frequencies quite well, though is generally
> not as effective at low ones.
> If you add a small earphone in the plug so that the low ones are cancelled
> perhaps with some user adjustment, then you should be able to decrease the
> frequency at which they work to somewhat.
> But you don't have the trickier problem of trying to cancel the noise
> all the way up to 20Khz, as the plug absorbs most of that.
** I have never heard an earpiece produce low frequencies like 50 Hz -
but maybe it can be done.
Imagine a would be sleeper in a bed affected by a 50 Hz tone (like I
was). Ear pieces in both ears with the sense mic next to the top of the
ead - right ? Now, if the mouth is open the pressure in the ear canal
changes compared to closed, if the nose is blocked by the pillow the
pressure changes again. If the person rolls over in bed the mic is no
longer close to the head and the level and phase changes again.
To get even a 20 dB reduction no change more than 5% is tolerable.
I have doubts it can work - OK ?
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