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I'm trying to create two tones at different frequencies. So I have one tone at 15000 Hz and one at 15400 Hz using Audacity. The problem i'm running into is there is a third tone created at a much lower frequency I can tell exists from both my ears and a spectrogram.

How can I get rid of the lower tone? Where does it come from? I only have two tracks

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I have tried the following:

  • Launch Audacity.
  • Generate a 15000 Hz tone in the track created by default.
  • Add a new track.
  • Generate a 15400 Hz tone in the new track.

A lower frequency tone appears during playback.

The reason is that both tracks have high levels, so their sum exceeds 1.0; and Audacity applies clipping or limiting. This non-linear operation is what creates the overtone.

Simply decrease the level of both tracks to -6dB (or use an amplitude of 0.5 during signal generation) to make the overtone disappear.

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As far as hearing goes, there are non-linearities in the human cochlea that can produce perceived difference tones at certain volume levels and frequencies.

Quantization noise in small enough digital signals might also create enough non-linear mixing to show up in FFT results. Noise shaping in the quantization could help reduce this possibility.

Any other non-linearities in the system are also potential difference/sum intermodulation/beat tone sources.

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  • $\begingroup$ With the non linear mixing I still get sounds down to 3000 Hz, far from the sum/difference would this be able to be defeated from the sound? $\endgroup$ – user2610470 Jul 28 '13 at 9:35
  • $\begingroup$ NON-linear mixing?! If you want to avoid extraneous frequencies you must mix linearly. linear mixing is achieved by taking the sample-by-sample average of the tracks in question. Any other process will produce distortion. $\endgroup$ – Bjorn Roche Jul 28 '13 at 13:39
  • $\begingroup$ The 1st difference tone from a nonlinear mix of 15kHz and 15.4kHz would be at 400 Hz, which is below 3 kHz. $\endgroup$ – hotpaw2 Jul 28 '13 at 13:57
  • $\begingroup$ I thought the difference mean't I should have a tone at 14600 Hz and 15800 Hz. Where does the one you mention originate from? $\endgroup$ – user2610470 Jul 28 '13 at 14:47
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    $\begingroup$ Look up "intermodulation" and "Tartini tone" on Wikipedia. $\endgroup$ – hotpaw2 Jul 28 '13 at 16:05

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