noise cancellation circuits

[Roger Dodger old codger]

I was watching some men and women shoot skeet this weekend with their loud loud shot guns and noticed that they had on ear muffs. But a friend informed me they would amplify normal talking but when the gun was shot it would muff the sound. I tried his and wow , no loud booom!
Where can I find schematics and info on these circuits?

Thanks
Roger dodger

[philba]

normal noise cancellation is a simple concept, use a microphone to pick up the sound, invert the signal, amplify it and feed it into a speaker. Superposition principle causes most of the original sound energy to be cancelled. I believe some delay is necessary to compensate for the propagation time from the mic to the speaker. I have several pairs of headsets and they work really well. The aviation NC headsets are amazingly good. google knows of a number of circuits.

However, to avoid cancelling normal conversation, they must have some sort of a trigger that turns the circuit on when the external sound exceeds some level.

[sofaspud]

I've no way of knowing if this is the same circuitry used for
the shooting range earphones, but you might be able to get
the desired effect with this circuit:
http://www.electronics-lab.com/projects/audio/023/
I'd just recently ran across this schematic after a bout with
a burst eardrum (and meningitis to boot!). I haven't put the
circuit together yet but it looks interesting.

[JPKNHTP]

-JPKNHTP

[sofaspud]

My understanding was that the request wasn't so much
for a noise-canceling circuit as for a constant-amplitude
amplifier - gain for low level inputs (voice), attenuation
for high level inputs (shotgun).

[Chris Smith]

The easiest way to cancel a out side offending signal, aside from the obvious filtration circuits, is to expose both sides of the microphone diaphragm to the loud and offending external sound.

When a signal comes in from both sides of the microphone, it cancels it self out with out much help needed from any filtering.

Helicopter mics going all the way back to the Viet Nam Era did this simple trick, as well as most modern commercial pilot head sets.

After that, active filtering, noise limiting, etc.,... works even better because all of the out side bi-directional noise is self canceling, yet the single directional sound is not.

[philba]

I believe this is a classic noise cancellation application with the twist that it is only active when the noise is above a certain level. The ones I looked at appear to also amplify lower level signals as well. for example - http://www.websoft-solutions.net/huntin ... tkrdm2.htm

The above circuit is kind of the inverse of the range headsets - it brings the low level sounds up and doesn't touch the high level ones.