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How do you compress audio silence?

I'm using Audacity and have been exporting variable compression mp3s. But a 1minute audio track is about the same file size regardless of how much silence there is.

And it's complete silence (0% audio).

Is there another audio compression format I have to use, or a config option, I'm missing. My audio tracks are 75% silent (because it's instructional audio), and I need to optimize them for the web.

UPDATE

Apparently, OGG might, but it doesn't look well supported in Safari and IE.

Here are my results.

Original: audio clip of 34s of dialogue.
Silenced: audio clip of 34s with 20s silences (14s of dialogue).

  • Avg compression @ 128bits: 490KB vs 497KB
  • Standard preset: 567KB vs 527KB
  • Variable compression (145-185kb): 472KB vs 436KB
  • Ogg (5 quality): 398KB vs 176KB <-- Success!!
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I just tried doing this with Adobe Audition, and by making a bunch of the middle silent I could shrink a file from 11MB to 3.6MB. So MP3 format should work. It might be a problem in Audacity or the settings you have? –  Alan Shutko Sep 6 '13 at 0:19
    
@AlanShutko, thanks for the heads up. I tried a variable of export options and posted my results above. No luck, except for Ogg. If you have any insights about your export options, let me know. One thing I noticed is that my mp3 encoder doesn't allow a wider variable compression range than 40kb. –  Emile Sep 6 '13 at 0:46
    
Are you using CBR or VBR (or even ABR)? If you use CBR, the size will not change regardless of the content, right? Have you tried VBR/ABR? –  jaychris Sep 6 '13 at 1:20
    
The problem is that "silence" probably isn't digital silence, it's low-amplitude white noise. Most encoders are going to attempt to encode it and use much more space then necessary. Pre-processing your audio to turn that into digital silence and using a VBR encoding should get an encoder to use the minimum possible space. –  afrazier Sep 6 '13 at 1:42
    
@afrazier, that's a good point. When I did my test in Audition, I created the file by setting a range to all-bit-zero. –  Alan Shutko Sep 7 '13 at 1:51
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1 Answer

I'd posit you can get away with VBR (Variable bitrate) AND a lower overall bitrate specification. I believe if you specify the same bitrate on 2 audio streams and one compresses better then the other, you would land up with better quality at the same size, so simply invert that logic.

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I don't know how you'd get a VBR range larger than 40kbps. There's just no range that would produce great audio and optimize for silence in that range. –  Emile Sep 7 '13 at 5:08
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