[music-dsp] Noise Cancellation

[Aran Mulholland]

I am doing some research into noise cancellation. I have come across
the statement

"Cancellation is achieved by playing back sound received through the
microphone through the headphones with a delay equal to half a
wavelength of the sound received.  This results in the peaks in the
played sound wave coinciding with the troughs in the ambient sound,
cancelling it and creating a near flat line.  This is the technique
used for noise cancelling headphones; these may be either in-the-ear
earphones or headphones which cover the whole ear.  There is a
microphone on the outside which receives the sound which is processed
within the earphone itself and then replayed into the ear.  On
passenger aircraft microphones on the wings pick up the engine sound
and play it back in the cabin resulting in a huge reduction in
volume."

Now if the above statement is correct the two questions i ask are:

what kind of analysis is required to determine the wavelength of the
received audio (to time the delay) ?

will the latency of the system be directly proportional to the
frequencies that the system can cancel?


thanks

Aran.