Audio spectrum FFT

[Mosaic]

Hi all:
I am familiarizing my self with profiling an audio spectrum to help identify audio feedback issues discussed in another thread.
Here is a pic of a 4Khz sig. from a function gen with the MATH (purple) dBV FFT waveform @ 2.5Khz per grid. I note the largest peak is the 4Khz sig, but there are a lot of other 'harmonics' going on down the line.

Can anyone explain how it is an AC coupled, function gen signal (50 Ohm terminated) into the Rigol CH2 input is showing up all these harmonics? Thx.

 

[ronsimpson]

Here is the harmonics of a 1khz square wave.

Here is a video:

Interesting picture below:

 

[alec_t]

Your 4kHz signal, if a perfect square-wave, would contain all the odd-number harmonics. However, it is a rectangular wave with imperfections so will contain those harmonics plus additional frequencies.

 

[KeepItSimpleStupid]

Yep, the Fourier expansion of a square wave is here: https://en.wikipedia.org/wiki/Square_wave

 

[Mosaic]

Ok, so to create the 4Khz square wave there are some odd harmonics that are deliberately added to create it and that is what the FFT is showing. The even harmonics are as a result of non ideal deficiencies in the circuit.

But this would be different from ,say, and instrument that created the audio frequency ?

Here is a 1Khz sine wave that doesn't have harmonics and it looks good.

 

[KeepItSimpleStupid]

I don't understand your question.

Mathematically, a square wave is made up of an infinite number of a sum of odd harmonics. As each odd harmonic is added, the waveform becomes more square.

The amplitude of the nth odd harmonic could be different than what the nth odd harmonic is supposed to be in the real world. Even harmonics don't belong.

Your waveform is far from square, The tops aren't flat and severe crossover distortion is present.

 

[Mosaic]

The pure sine wave trace answered the question. It doesn't have harmonics. A sound from a musical instrument should have some but not as much as shown by the square wave.

 

[KeepItSimpleStupid]

Yep. Look here https://en.wikipedia.org/wiki/Sine_wave under traveling and standing waves for string instruments.

 

[audioguru]

The musical instrument in this thread is a Caribbean steel pan drum. Its sound-producing parts resonate so it produces a sound similar to a sinewave when it is hit.
The shape of the sinewave and its tone are probably different if a sound-producing part is hit in the center or to one side.