[Ham-Mac] Noise Canceling Microphones (somewhat off topic)

[Philip]

At some time fairly close to Mon, 12 Jul 2004 10:35:13 -0700,
rumor has it that K6LMP wrote:

> I have a mobile transceiver that I use, part of the time, in a very 
> noisy environment -- a Swiss Army surplus troop carrier called a 
> Pinzgauer.  It is essentially impossible to talk on the radio while
> the vehicle is in motion, due to the very high ambient noise level.  I
> have a pair of active noise suppression headphones that make listening
> feasible, but I don't have a good solution for noise cancellation into
> the microphone.
> 
> Two alternatives seem viable.  First, there's the Astatic 636L
> handheld microphone, which sells for about $40.  Does anyone have any
> experience with this unit?

Based on my experience - and that of a former auto-transport driver ham
friend - the most effective readily available noise canceling mic is the
Telex/Turner RK56. I have two of them, one for my 2-meter radio. I can
be driving my _noisy_ pickup (probably not as loud as a Pinzgauer...) at
full highway speed with the windows rolled down while being passed by a
cement mixer - a combination that makes the ambient noise level high
enough that I can't hear _myself_ - and the folks I'm having a QSO with
can still understand me. "There's a bit of background noise on your
audio, OM." *grin*

> Second, I found a circuit diagram online for a noise canceling mike 
> setup using two RadioShack electret mikes, fed through two small and 
> simple transistor amplifiers and with outputs merged 180° out of
> phase. One mike is apparently arranged pointed toward the mouth; the
> other away from the mouth, to pick up ambient noise.  In theory, the
> anbient noise component cancels, leaving only the voice.  This was
> apparently written up in the December 2000 issue of QST, which I don't
> have.  If anyone has experience with this setup, I will order a copy
> of the article from ARRL.

That should work. It could also be done with an op-amp. One signal into
the inverting input and the other into the non-inverting input. The
challange with any such system is the frequencies that will be canceled.
The closer the two mics are to each other, the higher the maximum
frequency that can be canceled. But if you get them _too_ close there
won't be enough level difference between the voice-pickup and the
noise-pickup. Thus you may want to figure on a low-pass filter to go
with the polarity-reversal circuit.

Oh, as a side note, all noise-cancelling mics that I know of have to be
held very close to you mouth for them to work right.


--


                Philip
                73, de KB0SWA