Acoustic recordings of brushless motors

Hello dear experts,

I have some Questions about measures with AKG C1000 S microphone of my drone brushless-motors with Propeller (2 Blades).
If I make some recordings with one motor and make a spectrogram (windows size 32768, hamming algorithm) of that. My 1. Harmonics (rotating frequency) is 74Hz. But, if see a spectrogram between 0 and 300Hz - you see a higher level on 2. Harmonics (148Hz). An if you see a spectrogram between 0 and 4000 Hz, you see a high Level on 2700 Hz.

Question 1. Why is it happens? I was expecting to see always a high level on my first harmonics.
Question 2. That is the influence of 2 blades propeller in my measures/spectrogram/FFT? - per rotation propeller make two waves = f(2 blades)=2*f(motor), right?
Question 3. Measures under 50 Hz have a very bad quality. It is depends on microphone, sound card or analyze tool?

Thank you very much in advice!
Sincerely yours,

I would expect that a lot of the lower frequency sound would come from the prop rather than directly from the motor.
It could be interesting (if possible) to record the motor without a propeller attached (but be careful to not over-rev the motor).

That just looks like mains-hum, rather than sound created by a propeller,
Like the hum produced by the (slowly) rotating turntable in a microwave oven, (also a brushless motor).

Thank you very much for answers!

Hello dear people,

I’m looking for reason, why my spectrogram from drone propeller are showing the first harmonics with higher amplitude than Fundamental Frequency?

Motor is rotating with 60 1/s
My propeller has two rotor blades - is maybe this a reason? Or maybe it’s of course of low-cut/ high pass filter of microphone/line-in?

And if I’m zooming out, I see also a high amplitude on 2,57 kHz - how it is possible - resonance of frames?
Thank you very much in advice!


I’ve merged your two topics as they appear to be on the same subject.

Because the sound pressure level is higher at 120 Hz than at 60 Hz.
There is no rule to say that harmonics must always be lower amplitude than the fundamental.

Oh yes, thank you, I forget that I already asked similar question.

Is the 60 Hz frequency in FFT a motor noise, or noise of single blades or both?

Okay, now I know for sure - thank you!

The 60Hz & harmonics thereof are via magnetostriction,
like the hum from mains electricity transformers (which have no moving parts).