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      07-29-2013, 04:11 PM   #106
MCaro
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Quote:
Originally Posted by bradleyland View Post
Yeah, this looks like the kind of app that performs analysis through the mic, which doesn't work so well for this application. When you do Fast Fourier Transforms, you're breaking a signal down in to its constituent components. Every time you reproduce the sound, you lose fidelity. This is especially true when you're playing the sound over a speaker, then re-recording it. You're suffering acoustic loss at the speaker and the mic. Add to this the fact that the mic in the iPad isn't a reference grade component, and your result is going to include all kinds of errors.

How are you selecting the frequency that you are?

The RTA app you're using takes an audio sample, performs Fast Fourier Transforms, then creates a histogram of the amplitude and frequencies derived from the source material.

The tools we're using allow us to import the actual source file (audio extracted digitally from the video), so there is no reproduction error, and we can operate on the full range of frequencies present in the source audio. Granted, there's some loss in the compression that occurs when the video is rendered, but from there, it's digital reproduction, so the operations are lossless.

Download Audacity on your computer, then get the source files from the end of my write-up:

http://upload.bradlanders.com/bimmer...ound-analysis/
Another viewpoint: Aside from what has already been stated, consider the size of the speaker on your iPad and its frequency response. Then also consider what part of the frequency spectrum the mic is tuned for. That should provide you with some intuition as to why you're seeing peaks at higher frequencies.


Quote:
Originally Posted by bradleyland View Post
The download link at the end of my write-up was broken, but I fixed it now, so you can download the full Audacity source files if you'd like to explore a bit.
Thanks, I grabbed the audio file from your write up to create the PSD in MATLAB (see above).

Quote:
Originally Posted by swamp2 View Post
You CANNOT average into 1/3rd octave or any other similar averaging techniques with this method. You need full spectral data. If you don't understand this, you probably should not be blindly throwing software at this problem...
I agree. Using software without a fundamental understanding of the material (in this case, signal processing) is a bit like trying to run before learning to walk -- don't do it.
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