OK, I think what you are talking about is
combination tones. When you say "you get tones of 100 Hz and 900 Hz", you should really say "you
hear tones of 100 Hz and 900 Hz". There is actually no energy at 100 Hz and 900 Hz in the sound signal. The reason you hear those tones even though they are not in the signal is explained in the link I provide above:
Sum and difference tones are thought to be caused sometimes by the non-linearity of the inner ear. This causes intermodulation distortion of the various frequencies which enter the ear. They are combined linearly, generating relatively faint components with frequencies equal to the sums and differences of whole multiples of the original frequencies.
We perceive those tones, but they do not exist - physically speaking - in the resulting sum signal: a spectral analysis would only show energy at 400 Hz and 500 Hz.
I do not have the QC20 headphones to run experiments, but you could, if you had a mic, sample the output of the headphones and run a Fourier transform to get the spectral distribution graph to see if there is any unexpected tone in the signal. Audacity is a free piece of software that would let you do that.
Good luck!
"There are two types of combination tones:
sum tones whose
frequencies are found by adding the frequencies of the real tones, and
difference tones whose frequencies are the difference between the frequencies of the real tones"
https://en.wikipedia.org/wiki/Combination_tone
Yeah, these tones. I can definitely hear them. The QC20 from Bose is that kind of headseat that has both ANR, as well as sound input (in this case, from a 3.5mm stereo jack). I think it even has a microphone built in, so you can use it with a smartphone (if ... it has a 3.5mm jack).
When the audio in is given a 16000Hz signal, there is (apparently) nothing to be heard, or not clearly anyway. But when I play a tone of lower or higher than 16000Hz, then I hear what is at least similar to a difference tone. It's used in radiotelephony as well: An example is NDB transmitters on LF and MF (or whatever bands they are). If they don't have a modulated signal, but just a carrier wave, you can hear them with a BFO (Beat Frequency Oscillator). If you are tuning an NDB beacon at - say - 360 KHz, then a BFO option will add in for instance 2KHz above that (or below, depending on which brand). You can't send 360KHz straight to a speaker, but when you mix 360KHz and 362KHz (as in this example), you get a difference tone of 2KHz. THIS in turn, you can hear in a speaker. If the carrier wave of said beacon is cycled on and off, the BFO function will make that audible, so you can code in a morse identification.
THAT is what it sounds like, when I add 16050Hz to my Bose QC20, I get a tone out - which is NOT the difference, but something like the difference x3. But same principle as BFO.
Yes, I have Audacity. I didn't use it for any spectrum analysis, I didn't even look for it. I will see if I can record it simply by making a video, using the microphone in my iPhone.
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