Sound neutralizer?

**PiRsq** · Sep28-03, 09:13 PM

Is there a way to build a device that neutralizes the sound frequencies so you or someone else cannot hear anything ie: when glass breaks on a cement floor?

**Chi Meson** · Sep28-03, 09:35 PM

Sort of. You can cancel a sound wave by producing an exact replica of the wave except that it is "out of phase." That is, you make a wave that has a crest exactly where the original wave has a trough, and a trough where the original has a crest.

Bose makes a very nice sound cancellation headphone set. It looks like those old fashioned "70s" headphones, but the sound is far superior.

To cancel all the sound, though, you need a cancellation device right at the source of the original noise. Otherwise you can only cancel at specific locations, usually right where the device is located.

I know of one such device used on destroyers and other Navy craft where the engine drone is constantly above 80 dB. The device is located in every bunk, just above the sailor's head. Sound is canceled within a semi-circle of about 2 foot radius, usually where the sailor's ears are. THis way, they don't have to sleep with earplugs. BUt each and every bunk needs one.

It would be possible (and I'm sure it's being developed) to make a device that would cancel sound in a third location (1st being the origin, 2nd the location of the device), but since you can't push the speed of sound, it would require that you physically place a pick-up microphone in one spot, put the cancelling sound speakers in another spot, in order to cancel sound in the third spot. Very cumbersome, and not controllable by twisting an electronic gaget. I would be neat if I'm proved wrong!

**russ_watters** · Sep29-03, 01:42 AM

Originally posted by Chi Meson
To cancel all the sound, though, you need a cancellation device right at the source of the original noise. Otherwise you can only cancel at specific locations, usually right where the device is located.

I know of one such device used on destroyers and other Navy craft where the engine drone is constantly above 80 dB. The device is located in every bunk, just above the sailor's head. Sound is canceled within a semi-circle of about 2 foot radius, usually where the sailor's ears are. THis way, they don't have to sleep with earplugs. BUt each and every bunk needs one.

These two paragraphs contradict each other - the second one is right. All you need is for the mic and speaker to be far enough apart that the signal can be calculated and generated in time to cancel the sound. Thats how headphones work - they are self-contained.

I never heard of sound cancellation in bunks and I was in the Navy (though not on an aircraft carrier). I wouldn't think it would work very well since the frequency is so wide and the lower end low, you'd need big speakers. Plus metal ships transmit sound VERY well making you pretty much surrounded by speakers.

The other example you had - a triangle between the source, target, and cancellation device wouldn't be that hard to do as long as the longest leg of the triangle is between the target and source and the geometry is known.

**Chi Meson** · Sep29-03, 02:55 PM

Yep that was a typo. It was supposed to be an "either or" paragraph; that is, you can EITHER cancel all the sound if you put the device at the source, OR you can cancel the sound at specific individual points.

And the device used on ships is real. I've seen it, and it works. It was specifically designed for smaller ships (maybe not destroyers)and maybe it was the Coast Guard not the Navy. where the engine makes a constant drone around 50-60 Hz and 80--90dB. THe sailors did suffer hearing dropouts in that range of frequency. A friend of mine was on the engineering team that built them. At the time he worked for BBN.

The speaker is large (I think 8") and there were two pick-up microphones about 1 foot to either side of the speaker. The sound cancellation occurred in a zone just inside the mics.

**Integral** · Sep29-03, 04:39 PM

The device is located in every bunk, just above the sailor's head. Sound is canceled within a semi-circle of about 2 foot radius, usually where the sailor's ears are. This way, they don't have to sleep with earplugs. But each and every bunk needs one

I bunked on the O2 level of the USS JFK, 2 decks up, on the flight deck was the spot where the flyboys tied down the jets and tested engines. Just imagine the teenager next door reving his hotrod, only give him afterburners. Just below, my bunk, on the hanger bay level was the metal shop where they worked on things like aircraft skin patches, high speed band saws would be my guess, what ever was in there was noisy.

The next contributing factor, we worked 12 on 12 off, (I was a ships company ET)I had the night shift.

I sure wish they would have had such noise cancellation devices in 1972!

**Artman** · Sep29-03, 05:18 PM

Phase cancellation is also used to "clean" electrical waves altered by such things as variable frequency drives. By cancelling the "bad" waves from the power system motors as large as chiller compressors can be started at only a few amps draw, and several times an hour, without overheating the electrical circuit.