Convert and Compress Audio From the Terminal on macOS (afconvert)

afconvert -f m4af -d aac -b 128000 song.wav song.m4a

Read it left to right. -f m4af is the file container (M4A). -d aac is the data format (AAC). -b 128000 is the total bitrate in bits per second. Then input, then output. If you leave out the output name, afconvert makes one next to the input.

On our 30 second stereo test file (a 5,292,078 byte WAV) that command produced a 427,472 byte M4A. The encoder landed under the requested rate, at about 112 kbps by afinfo. That test signal was noise plus a tone, which is harder to compress than most music, so real recordings often land smaller.

The lossless rows (ALAC, FLAC) cut the WAV to roughly a third here, with no change to the sound. The lossy rows go much smaller. For the trade-off between rates, read audio bitrate explained.

• -f: file format. Useful values are m4af (M4A), WAVE, AIFF, caff, flac. Run afconvert -hf for the full list.
• -d: data format, optionally with a sample rate, like aac@32000 or LEI16@44100.
• -b: total bitrate in bits per second. For stereo, 256000 means about 128000 per channel.
• -c 1: number of channels. Use it for mono.
• -s: bitrate strategy. 0 is constant, 1 average, 2 constrained variable, 3 variable.
• -u vbrq 0-127: quality level when using variable bitrate.
• -q 0-127: encoder quality, separate from bitrate.

Mono helps speech a lot. A 32 kHz mono AAC at 64 kbps (afconvert -f m4af -d aac@32000 -b 64000 -c 1 in.wav out.m4a) came out at 246,989 bytes on our test, about the full 64 kbps. The stereo 64 kbps run undershot its target, so mono is not automatically smaller at the same number. Check the result with afinfo out.m4a before you batch a whole folder.

mkdir -p out
for f in *.wav; do
  afconvert -f m4af -d aac -b 128000 "$f" "out/$(basename "$f" .wav).m4a"
done

Quote every variable so filenames with spaces work. Writing into a separate out folder means a typo never overwrites an original. Two 30 second files converted in about 0.3 seconds total on our Mac. For other input types, change the glob (*.aiff, *.flac) or loop over several.

More on loops and Automator in how to batch convert audio on a Mac.

No. We tried afconvert -f MPG3 -d .mp3 in.wav out.mp3 and it stopped with an error about the output format. afconvert can read MP3, so MP3 to WAV or M4A works. Writing MP3 needs another tool. If MP3 is the target, our guides for WAV to MP3 and M4A to MP3 cover the options.

If you are comfortable in Terminal, afconvert is free, fast and enough for most one-off or scripted jobs. Smol is the wrong purchase if all you need is WAV to M4A in a shell loop.

Smol earns its $29 (once, one Mac) if you would rather drag a mixed folder in, get per-file savings, and have MP3, FLAC, OGG and Opus output as well as AAC, M4A and WAV, with bitrate from 16 to 512 kbps. It adds a Finder right-click action and watched folders, and nothing is uploaded. See pricing or the best audio converter for Mac comparison.