Recording 3kHz digital sound, plot shows 6,9,12 etc kHz

I am trying to record a 3khz tone generated by smartphone. When I plot the spectrum analysis, there are peaks at 3,6,9,12 khz. Can anyone please explain to me why this could happen? I kinda know musical instruments produce sounds in many wavelengths multiples of fundamental frequency, but don’t know anything about digital instrument.


That tells you the 3KHz tone is not a pure sine-wave.
upper waveform is pure sine-wave , lower is distorted.jpg