Thomas McGahee wrote:

> The two microphones are separated by a distance that can be
> anywhere from five or six inches to several feet.

  In fact, the separation in "communications" noise-cancelling
microphones, as others have alluded, is one to three *centimetres*.

> The further away the source of noise is, the greater the
> separation you can have between microphones.

  Generally true, but as others have pointed out, if the primary
microphone becomes obscured by your face, and the cancelling mic does
not, noise cancellation stops working.  Increasing the distance does
*not* improve cancellation.  What *is* important is that one mike is
*much* closer to you than the other.

  So, if you plan to experiment with this, either get a standard noise-
cancelling assembly to use "close-up", or set up your two mikes a few
inches apart, make sure that you and the two mikes are in line, and both
mikes are in a line perpendicular to the noise source:

                        M2 X
    Window
  (Street               M1 X
     noise)                _
                           U (You)

--
  Cheers,
        Paul B.