I recently downloaded a bunch of raw audio streams from the web as MP3s and I realize now that they're huge! 60 MB a song… I would like to compress these, but the other question similar to mine links to a software for Windows, and I have a Mac.

The streams are an hour, so I guess that might be one reason they're so large.

First, I wasn't looking for a program to buy or download to do this, but rather wanting to know if I could use Flash, GarageBand, AfterEffects or any other program to import the stream, lower the quality a bit, and export it.

Secondly, there would be no video inside because it's not coming from a video.

  • 1) If you already saved them as MP3 then they already are extremely compressed. 2) This feels as a shopping question for a best program to do this. – Hennes Oct 21 '12 at 5:32
  • 1
    MP3s are not that big. You probably downloaded the video too,embedded inside, or the file is corrupt. MP3 128kbps files are about 5 Megabytes for a 5 minute song. if I knew of a program to split audio and video besides the outdated VirtualDub, I'd suggest that. – cloneman Oct 21 '12 at 5:53

iTunes allows you to easily compress these files. Just drag them to your music library, then open Preferences » Import Settings….

Select the MP3 encoder and then click Setting » Custom…:

Select a custom bitrate here. Now, your one-hour files with 60 MB seem to have a reasonable size anyway – you'd expect 1 MB per minute of music for MP3s. However, if you want to re-encode them to become smaller, select something less than 128 kBit/s here. Note that this will reduce your quality, and it might lead to enormous quality loss. Recompressing something that's already compressed generally isn't a good idea.

Once you've set all that, go to your file in the library, right click and select Create MP3 version.

You can now right-click the new version and select Show in Finder to reveal the newly created file.


Since you don't want to download anything, I hope you have LAME installed already. Here's how you can decode and encode your MP3 file a lot easier than with iTunes.

lame --decode soundfile1.mp3 soundfile1.wav
rm soundfile1.mp3
lame soundfile1.wav

Example session:

% ls -l 01.\ Drive\ My\ Car.mp3 
-rwxrwxr-x 1 janus plugdev 6064941 jan 26  2012 01. Drive My Car.mp3
% lame --decode 01.\ Drive\ My\ Car.mp3 
input:  01. Drive My Car.mp3  (44.1 kHz, 2 channels, MPEG-1 Layer III)
output: 01. Drive My Car.wav  (16 bit, Microsoft WAVE)
skipping initial 1105 samples (encoder+decoder delay)
skipping final 743 samples (encoder padding-decoder delay)
Frame#  5754/5754   168 kbps   MS
% rm 01.\ Drive\ My\ Car.mp3 
% lame 01.\ Drive\ My\ Car.wav 
LAME 3.99.3 32bits (http://lame.sf.net)
CPU features: MMX (ASM used), SSE (ASM used), SSE2
Using polyphase lowpass filter, transition band: 16538 Hz - 17071 Hz
Encoding 01. Drive My Car.wav to 01. Drive My Car.mp3
Encoding as 44.1 kHz j-stereo MPEG-1 Layer III (11x) 128 kbps qval=3
    Frame          |  CPU time/estim | REAL time/estim | play/CPU |    ETA 
  5754/5754  (100%)|    0:09/    0:09|    0:09/    0:09|   16.338x|    0:00 
   kbps        LR    MS  %     long switch short %
  128.0       89.6  10.4        91.6   4.7   3.7
Writing LAME Tag...done
ReplayGain: -3.7dB
% ls -l 01.\ Drive\ My\ Car.mp3 
-rw-rw-r-- 1 janus plugdev 2405354 okt 21 19:28 01. Drive My Car.mp3

As you can see, the default settings gives you a file with less than half the size of the original, using the original file.

If you don't have LAME, but you do have internet: get homebrew and then brew install lame && brew link lame

  • LAME doesn't come by default on OS X, so it would be great to explain how to get the binary in the first place. – slhck Oct 21 '12 at 17:43
  • @slhck: added :) – Janus Troelsen Oct 21 '12 at 18:03
  • Nice, thanks! Note that brew does not accept sudo anymore, at least since a couple of months. IIRC the linking step is not necessary, but I might be wrong. – slhck Oct 21 '12 at 18:11

Average quality mp3s are typically 1mb/minute. Thats stereo 128kbps. Yes, it will vary slightly depending on the encoder used. However, the size you indicate is pretty much on target for the length of the recording. Of course without knowing the actual details of the file, it makes it difficult to be exact. Consider that if it was uncompressed, the same file could be 600mb or larger.

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