[Elecraft] Noise-cancelling headphones mystery

[Jim Brown]

On 12/9/2018 8:40 AM, CUTTER DAVID via Elecraft wrote:
> Noise cancelling that I've come across is limited to low/medium frequencies.

Yes. Others have correctly identified the reason, but I'll go into a bit 
of detail for clarity. Noise cancellation works by amplifying a tiny mic 
on the outside of an earphone and feeding it to the headphones so that 
the acoustic output is out of polarity with the noise that is 
acoustically coupled to the ear. For deep cancellation, the sound from 
the mic and the sound that reaches the ear directly must be of precisely 
the same amplitude AND precisely the same phase (but out of polarity).

This is particularly difficult to achieve, because 1) the headphones 
themselves attenuate the direct coupling of the noise to the ear, and 
that attenuation is not flat, and it varies depending on subtle 
differences in how the earphones fit/seal to the listener's head; 2) 
Phase is a variable, not a constant, and increases in direct proportion 
both to frequency and the distance that sound travels. 3) The small 
distance between the noise-sensing mic and the ear results in phase 
shift that increases with frequency. 4) The earphones themselves have 
non-flat phase response. 5) Directivity of both the microphone and the 
ear covered by the headphone is another variable.

The above leads to these conclusions. 1) Low frequency noise is far 
easier to cancel than higher frequency noise. Noise produced by fans in 
the KPA1500 has significant higher frequency spectra. 2) Extreme care is 
needed in the design of noise cancelling headphones. I would expect 
products of serious audio companies like Sennheiser and Bose to be 
better performers. 3) I would expect to see new designs using DSP to 
introduce delay to reduce the time difference resulting from mic 
placement and equalization for non-flat amplitude and phase response of 
the total system.

K9MA says that Radio Sport headphones effectively kill the sound. I have 
no problem believing that -- they're designed for use in high noise 
environments. But don't buy a pair until you've tried them on -- 
headphone comfort is a VERY individual thing that depends on the shape 
of our heads. Some contesting friends like them, so I've tried them on 
at several hamfests, and found them an instrument of torture. I couldn't 
imagine wearing them for even a few hours.

73, Jim K9YC