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 the 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.
-earl
Yup.
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.
Barry
Like this:
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):
David Clark Bose Telex Sennheiser Lightspeed Flightcomm Sigtronics
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.
-- Jay
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.
Montyhp
I have a pair of the Bose units and use them (thus far) exclusively for air travel etc. They serve both functions, and I will probably give 'em a try in the shop.
All the best,
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 mathematically impossible.
Maybe not mathematically impossible, but you would need some pretty big drivers. Have seen the whole wall of a room used as a speaker.
montyhp
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 at all".
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, either.
Leaving *only* "put the canceller right next to the listener". i.e. in a headset.
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 cancell>>>>>
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