What Is The Maximum Frequency Shown By ComputeSpectrum?

Ben Stucki has updated his blog with part two of his findings relating to SoundMixer.computeSpectrum().

Quantifying Flash Audio Visualizations

Absolutely amazing work! So i am again testing and experimenting…

My first question regarding his post… Is 11025Hz the maximum visible frequency available from a byteArray snapshot?

I used Sound Forge 10 to create a .wav file @[44100, 16bit, Stereo] and then used the ‘Insert’ menu to add Synthesis->Simple and Silence. My demo file creates test tones @[-12db, Sine] that last for 3 seconds followed by silence for 3 seconds. The sequence is as follows:

[22,050] silence [21,050] silence [20,050] silence [19,050] silence [18,050] silence [17,050] silence [16,050] silence [15,050] silence [14,050] silence [13,050] silence [12,050] silence [11,050] silence [11,025] silence.

The last part of the file is a sweep from 11,025Hz to 0.01Hz that lasts for 10 seconds.

I saved the .wav and then saved again as .mp3 @[128kbps, 44100, 16bit, Stereo] – immediately i noticed the frequencies that were displayed in the .wav (from 22,050 to 16,050) were displayed differently in the .mp3. Frequencies from 15.050Hz down looked identical to the .wav – so i guess the top end had been compressed… is that right?

I made a quick test so as to see visually what is coming back Test Tones Demo This uses the same code as all previous recent sound examples.

It takes a while until you see or hear anything but eventually things start happening around 16,050Hz. Is it overtones??? You tell me… It’s easy to check what frequency is heard, i counted each audible or visual response up until the sweep and then counted back. When i get more time i’ll annotate the audio file so it’s even easier to understand.

This is what i found:

  • 22,050 – 17,050 / not audible / no visual
  • 16,050 / audible / no visual
  • 15,050 / audible / no visual
  • 14,050 / audible / no visual
  • 13,050 / audible / displayed
  • 12,050 / audible / displayed
  • 11,050 / audible / displayed
  • 11,025 / audible / no visual
  • SWEEP / audible / displayed

NOTE: You can adjust my demo by using these keys:

[0-5] changes sample rate 44100, 22050, 11025 etc
[6] 128 bands per channel
[7] 256 bands per channel
[9] FFT ON
[D] toggles swf performance display

NOTE: Still getting that end of file error from FMS – hit F5 and try again… it will work 🙂


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