Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.


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!!
share|improve this question
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. – Kyle 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

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.

share|improve this answer
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. – Kyle Sep 7 '13 at 5:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .