I have some active ear muffs that I use for shooting. Does anyone make
a product like this for the shop? I would like to hook a microphone
above the router or shaper and feed it into a device that would blast out
sound wave 180 degrees out of phase and cancel the noise. Wouldn't have
to wear hearing protection etc.
Seems like the technology should be out there.
Are the active muffs you wear for shooting truly phase cancelling, or
passive devices with a mini amp built in?
I use an ANR intercomm headset for flying and they work wonderfully
for steady noise. I don't think they'd do much beyond their passive
rating for a gunshot. The price of the ANR units encourages me to use
passive protection the shop. <G>
Interesting thread. ANR is good up to about 1000 hertz. Would probably
best for induction motors with lower pitched sounds. In broadband
environments, often a combination of ANR and passive is required.
You're talking about ANR (Active Noise Reduction). They've become very
popular in the past few years for small plane pilots. They're kind of
pricy, in the $300-700 range, but those are for units with microphones
and connection to an intercom system built in. You should be able to
find them as nose reduction only units for less. Brands to look at
include (more or less in order of quality):
Or just google for "anr headset".
One of the problems is that ANR tends to work best on low frequencies.
In my experience, the worst shop noises are the high frequency
screaching you get from things like routers and miter saws.
We have some high-noise environments at work (heavily cooled rooms, etc.)
and we use the Bose active noise reduction headphones. They work very well
and don't require any external microphones. They are, as has been said,
quite expensive and I don't know how they'd fare in an environment with
chemical and particulate aerosols. If you have good ventilation and dust
collection systems, I'd say go for it. In the long run I think the expense
is worth preserving your hearing into old age.
Are you asking about some type of headphones that would cancel the
router noise just for you or a device that would broadcast a
cancelling signal for the entire room?
The headphones are available today. The latter may well be
NOT "impossible", merely "*extremely* difficult".
The speed of sound, in air, is roughly 1,000 ft/sec.
Thus, a 1 kilohertz sound has a wavelength of roughly 1 foot,.
a 400 Hz sound has a wavelength of roughly 2-1/2 feet,
a 250 hz sound has a wavelength of roughly 4 ft,
and a 100 Hz sound has a wavelength of roughly 10 ft.
One more data-point -- a 4 kilohertz sound has a wavelength of
roughly 3 inches.
For noise-cancelling to _work_ the sounds from the original source,
and the 'canceller' must arrive `180 degrees' out of phase.
If the _difference_ in the distances between (a) the original source
to the listener, and (b) the 'canceller' to the listener, changes by
'half a wavelength', then, instead of cancelling each other, they *add*,
and the noise is *twice*as*loud*.:
That's a change of 6 inches for 1 kilohertz, 1-1/2 inches for 4 kHz,
15 inches for 400 Hz
At _half_ those changes in distance, it's like the canceller "wasn't there
Now, consider how far apart a person's two ears are. Active noise cancellation
that is right for the right ear, may be 'all wrong' for the left ear.
One has to consider the location of three things -- the noise source,
the 'canceller' source, and the 'listener'.
If the noise source, and the 'canceller' source are *very* close together,
the difference in 'path length' from the listener to the two sources is
relatively _constant_, regardless of the listener's position.
Alternatively, if the 'canceller' source is very close to the *listener*,
one can regard it as being _in_a_straight_line_ between the noise source
and the listener, *regardless* of where the noise source is. And a simple
'180 degree inversion' of the sound will cancel _at_the_listener_.
"Theoretically", neither of the above are necessary. With technology to
continuously measure the distance from the noise source to the listener, _and_
from the canceller source to the listener, a canceller source could be located
well away from either the noise source, or the listener. *BUT*, this
methodology is capable of providing *very*small* zones of cancellation,
measured in inches, at most. Thus, "practical", it is *NOT*.
In general, locating the canceller at/near the noise source isn't practical,
Leaving *only* "put the canceller right next to the listener". i.e. in a
Here's the bottom-line problem. The "anti" sound will radiate
uniformly in all directions from a point source. The target sound will
also do so, but from a different (and probably moving) point source.
The two spheres will intersect in a complicated way that cannot be
canceled by a single wave form.
Consider the wave form a short distance on either side of the target
source and on a line between it and the anti source. These wave forms
are mirror images of each other. How are you going to generate a wave
that will know to mirror itself as it crosses the target source?
The noise cancelling headphones work, I believe, because they onle
have to generate an anti wave for a wave coming down a narrow channel
and only in one direction.
On Fri, 09 Jan 2004 01:51:07 +0000, firstname.lastname@example.org
(Robert Bonomi) wrote:
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