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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
