Quote:
Originally Posted by Mangler
I took a 1/3 octave snapshot of the highest rev point at the 3:06 point in the video. There are two peaks at 300 and 500hz but all the energy is around 1200hz and they are not harmonics. How are you deriving the 370hz for the calculations? When I am back to a PC ill post a screenshot. Don't know why we can't just upload pics from mobile. So stupid.
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A few questions:
1) What tool are you using to perform the FFT?
2) What windowing function and size are you using?
3) What do you mean by a 1/3 octave snapshot? Do you mean a bandpass filter; if so, what specific frequency range?
Below are answers to my own questions
1) Swamp and I are both using Audacity's "Plot Spectrum" feature, so it'd be worth running it through another tool. The math should be the same, regardless of the tool though.
2) I've tried Hanning (preferred), Hamming, and Rectangular at 512 window size. All produced highest amplitude peaks at 320 Hz - 330 Hz
3) I applied a lowpass filter, but it's completely unnecessary for the FFT analysis because of the windowing function. It just eliminates noise, where "noise" is defined as anything that falls outside what could reasonably be assumed to be the primary cylinder firing impulse.
In general, I don't see much amplitude at all in the 1200hz range when using rectangular/hamming/hanning windows at a window size of 512. How are you extracting audio from the video? I used a tool called AudioHijack to capture system audio internally, but a better method would be to download the video data, then extract the audio stream.