Re: Directional Mic - using only aural tubes

[EMAIL PROTECTED] (Mike Halloran) wrote in message news:<[EMAIL PROTECTED]>...
> Put a little phone booth around the factory end of the tube.  A three- sided,
> absorbent lined box, as used to be sold for factory based telephones, would
> help.  Or encircle the tube end with a closed perimeter of those vinyl hanging
> strip doors.  At least place a sound absorbing wall behind the speaking person.
> 
> A noise- cancelling microphone at the office end of the tube would probably not
> work, because its cancelling mechanism (really just the back of the diaphragm) 
> basically needs a sample of the factory floor ambient noise from the factory
> end of the tube in order to cancel out the factory noise.
> 
> You might be able to do it with two tubes from the factory speaking station,
> with a noise cancellng mic at the office end, the speaking tube feeding the
> front of the mic and the other tube (isolated from the speaking person at the
> factory end) feeding the back of the mic.
> 
> Or two ordinary mics at the end of two tubes, one tube carrying noise only, the
> other tube carrying noise plus speech.  In an amplifier, Invert one signal, add
> them, and if you've got the gains balanced right, you should get speech from
> the output.  You will probably need a big knob and a pot at the office
> listening station to help the listener adjust the relative gains.
> 
> First thing I'd try: two tubes of equal length and diameter, one for speaking,
> one open to the factory noise but not within earshot of the speaker, both tubes
> connected to the run ends of a tube tee, with the branch end of the tee
> connected to the listener by a short tube running into the office.
>  
> In all three of the paragraphs immediately above, you are trying to subtract a
> real time feed of the factory noise from the speech tube carrying noise plus
> speech.  The noise feed without speech has to be very similar, in real time, to
> the background noise at the speaking station, or it doesn't work.
> 
> Don't expect miracles.
> 
> -Mike-

Thanks.  They seem like all good suggestions.  The example in the last
two paragraphs has got me intrigued.  We can't use anything electric
or with metal parts, not even small batteries.  I was hoping to
overcome this problem, by design with some sort of mechanical noise
reduction...  I wonder if this is posible ?
John