Product: Noise Canceling
Silent Mic   (-2)  [vote for, against]
Nobody else can hear you

Noise-cancelling headphones and noise-cancelling mics already exist to remove ambient noise.

The same technology can be used to remove the voice of the person speaking from the surrounding area, so that the enemy can't hear a soldier talking to HQ. It could also be applied to phones to reduce office noise.
-- marklar, Jul 10 2007

Baby Volume Control Baby_20volume_20control
Won't work either [wagster, Jul 11 2007]

Skull mic
Great product name [wagster, Jul 11 2007]

[miasere] is quite correct. Noise cancelling headphones cancel sound at only one point - your eardrum. They can't cancel sound everywhere which is what you want to do.
-- wagster, Jul 10 2007

Si-lent mic, ho-ly mic...
-- Texticle, Jul 10 2007

One-two-one, two-one-two...
-- imaginality, Jul 10 2007

Wait, though - if the device could be used directionally to apparently cancel sound waves emanating from the source, but not reflected waves that result from the source wave propogating in other directions, why, ventriloquists the world over would flock to buy one.
-- egbert, Jul 11 2007

[miasere] There is only one source, the person's mouth. I was thinking that the back of the mic, about 5cm from the mouth would be close enough to remove 80% (wild guess based on what I want) which would be plenty for both the office and military applications.

[bigsleep] A keyboard? and of course a Hawkingesque voice at the other end or would everyone need to carry a monitor with them too? At some point in the future I think soldiers will basically have PDAs but I think it would be far easier to retrofit radios with a new mic than equip them with keyboards.

Ah, I just remembered, military radios have a beep button so you can transmit morse code.
-- marklar, Jul 11 2007

//There is only one source, the person's mouth//

It's just not that simple. Most sound is generated by the larynx, located in the throat. A lot of the middle and higher frequencies travel up and emanate from the mouth and nose, though quite a lot of the mids are transmitted directly from the throat and some are transmitted via the skull. The low frequencies are generated initially by the larynx but much of what you hear comes from the chest cavity vibrating in sympathy. Sibilence (s, th, sh) is generated entirely within the mouth and is the only vocal sound that behaves almost like a point source sound. So... different sounds and frequencies come from different areas of the upper body, sometimes as a point source, usually not.

//I was thinking that the back of the mic, about 5cm from the mouth would be close enough to remove 80% //

If you have a point source sound you can cancel it by around 80% so long as you have another point source sound generating the anti-phase which is separated from the original point source by only a small fraction of the highest wavelength that you wish to cancel. The highest sound we can hear is 20kHz which has a wavelength of 18mm, so your maximum separation is going to be around 5mm. If the original sound is a point source, which it isn't.

I should probably hold my hands up now and admit that my first ever post was similar to this and was impossible for the same reasons. (link)
-- wagster, Jul 11 2007

I think that a throat mike can allow voice to be transmitted without any audible speech. Not sure though.
-- david_scothern, Jul 11 2007

Well you have to make a *little* noise. Mics only pick up noise, anything else is mindreading.

Direct contact transducers will always be more efficient than ones separated by an air gap, so a throat mic would be a good place to start. Firemen often use skull mics (link) under protective gear which pick the sound directly from the skull. Problem is... if you're being really quiet you will be whispering and throat and skull mics will be listening to the wrong things. A very small lavalier mic mounted inside or at the edge of the mouth might work better.
-- wagster, Jul 11 2007

<reads [jutta]'s recently added last paragraphs... waits>

Well... how?
-- wagster, Jul 11 2007

Throat mics are pretty good, rather than speaking quietly you just talk into your sleeve, takes a bit of getting used to and you sound like a Charlie Brown parent if you get it wrong.
-- marklar, Jul 12 2007

random, halfbakery