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Active Noise Cancellation Headphone High-Frequency Performance?

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JohnLai

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Hello, Folks,

I am looking for a pair of Active Noise Cancellation (ANC) headphones that could cancel human speaking. But almost all headphones in the market are just cancelling the low-frequency noises such as engine or fan. I do not understand why the higher-frequency parts of human voices could not be cancelled since ear hearing spectrum just reaches 20KHz. And 20KHz is rather slow in modern circuit designs, especially in IC designs running @ GHz speed.

I would suspect that if a voice signal were inverted 180 degree phase and added back to the speaker by a feedback circuit, human voices could be cancelled as well. Could somebody please explain this to me why ANC headphones can not cancel human voices.

Thanks.
 

if you take a sine wave, then add another sine wave to it and alter the amplitude and phase of the second one, you can get to the point where they cancel. If you get the amplitudes correct, then swinging the phase will do the business. To get a cancellation of 20 dB (10:1), you need a phase balance of better then 27 degrees, which is impossible with current loudspeakers and microphones at 20 KHZ.
Frank
 
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