Why am I getting an additional linear line in my spectrogram?

My spectrogram is weird. Why am I getting an additional linear line on top of mine which is two times the frequency of my original main line?

There is an additional line, with two times the frequency as the main graph. Why does this happen?

Answer 1

[Star Strider]

It is the second harmonic of your fundamental frequency. Harmonics can be visible with certain nonlinear processes such as sampling.

[Lisha Raghani]

Thanks :) ! Could you elaborate please? Also, would something similar happen to low frequencies (<20Hz) as well?

[Star Strider]

My pleasure.

It’s difficult to give a short explanation. The essence is that the sampling process is periodic by definition, and so any sampled signal will be ‘heterodyned’ with it, something similar to multiplying two sinusoidal signals of different frequencies. Signal processing textbooks discuss this, and the Wikipedia article on Undersampling goes into some detail.

This is my best guess as to what is going on with your signal.