Manatee

How to make an audio file smaller to email from a Mac

· 6 min read

An audio file’s size comes down to two things: its bitrate and its length. You cannot shorten a recording without editing it, so to make an audio file smaller on a Mac you lower the bitrate, which in practice means converting an uncompressed WAV or AIFF into M4A or MP3. A long voice recording that is far too big to attach usually ends up around a tenth of its size, and for speech the difference is hard to hear.

Why the file is so big in the first place

Uncompressed audio stores every sample exactly as it was recorded. CD-quality stereo (44.1 kHz, 16-bit) runs at about 1,411 kilobits per second, which works out to roughly 10 MB for every minute. Many recorders and audio interfaces default to 48 kHz at 24-bit, which is closer to 17 MB a minute. An hour-long interview in either format runs to hundreds of megabytes, several times over any email limit.

Compressed formats discard detail the ear is unlikely to miss and describe the rest more efficiently. An M4A at 128 kbps uses just under 1 MB per minute; at 64 kbps, about half that.

To see what you are starting with, open the file in QuickTime Player and choose Window, Show Movie Inspector. It lists the format, the data rate and the data size. Finder’s Get Info shows the size and, under More Info, the duration.

Work out the bitrate you need

The arithmetic is simple enough to do before you convert. For a size in MB, multiply the bitrate in kbps by the length in minutes and divide by 133.

A 45-minute lecture at 64 kbps comes to about 22 MB. The same lecture at 128 kbps is about 43 MB. If you are aiming at a 25 MB attachment limit, run it backward: 25 times 133, divided by 45 minutes, gives about 74 kbps, so 64 kbps is the setting that fits with room to spare.

Then pick a format and a rate:

  • M4A (AAC) gives the best quality for the size and plays on Apple devices, PCs, most phones and modern browsers. It is the sensible default.
  • MP3 plays on essentially everything, including old car stereos, and is slightly less efficient at the same bitrate.
  • 64 kbps is usually plenty for a single voice: lectures, interviews, voice notes, dictation.
  • 128 kbps suits general listening and mixed speech and music.
  • 192 to 256 kbps is for music you care about.

A voice recorded on one microphone often has identical left and right channels. Converting it to mono lets the whole bitrate go to one channel, which sounds better at a low rate than two identical copies would.

If you need the file to stay lossless, FLAC is smaller than WAV with nothing thrown away, but the saving is far less dramatic, and a long recording will usually still be too big to email. Lossless vs lossy explains the trade in more detail.

Option 1: QuickTime Player, for a quick M4A

QuickTime Player is already on your Mac and opens WAV, AIFF, MP3 and M4A files.

  1. Open the audio file in QuickTime Player.
  2. Choose File, Export As, Audio Only.
  3. Give the file a name, pick a folder, and click Save.

The result is an M4A with AAC audio, written as a new file next to wherever you saved it. QuickTime does not ask about bitrate, so you get whatever quality it chooses. Check the new file’s size in Finder. If it is small enough, you are done; if not, use the Terminal route below, which lets you set the rate.

Option 2: afconvert in Terminal, for an exact size

afconvert is Apple’s audio converter, and it ships with macOS. Open Terminal, type cd and a space, drag the folder holding your recording onto the window, and press Return. Then run:

afconvert -f m4af -d aac -b 64000 "interview.wav" "interview.m4a"

-f m4af asks for an M4A file, -d aac for AAC audio, and -b 64000 for 64 kbps (the number is in bits per second). To fold a stereo recording down to mono at the same time, add -c 1 --mix:

afconvert -f m4af -d aac -b 64000 -c 1 --mix "interview.wav" "interview.m4a"

For a whole folder of WAV files:

for f in *.wav; do afconvert -f m4af -d aac -b 64000 "$f" "${f%.wav}.m4a"; done

afconvert writes new files and leaves the originals alone. It reads the formats Apple’s own frameworks understand, which covers WAV, AIFF, CAF, MP3, M4A and FLAC, but not formats such as WMA, OGG or APE.

Option 3: Manatee, for any format and a batch

If the recordings are in a mix of formats, or in one that QuickTime and afconvert will not open, Manatee converts them without Terminal:

  1. Open Manatee and choose Convert, or drag the files onto its menu bar icon.
  2. Drop in the audio files. Several at once is fine; each becomes a job in the list with its own progress.
  3. Choose M4A for the best size, or MP3 if the recipient needs the widest compatibility.

Manatee accepts WAV, AIFF, FLAC, CAF, WMA, OGG, Opus, APE and many other audio types. It writes each converted file alongside the original and never modifies the source, and nothing is uploaded: the conversion runs on your Mac. One honest note: Manatee’s Shrink mode, where you type a target size in MB, works on video and PDF, not audio. For audio, converting to a compressed format is how the size comes down. Manatee is free for 24 hours with every feature, then $9 once.

If the file is still too big

Some recordings are simply long. A three-hour meeting at 64 kbps is still close to 90 MB. You have three honest choices.

  • Send it in parts. QuickTime Player can trim: choose Edit, Trim, drag the yellow handles to keep the first half, click Trim, then export. Reopen the original and repeat for the second half.
  • Lower the rate further. For speech only, 48 kbps mono is often still intelligible, though it starts to sound thin. Listen to a minute before you send.
  • Send a link instead. Mail on the Mac can send large attachments through Mail Drop, and iCloud Drive can share a file as a link. The recipient downloads it rather than receiving it in the message.

For the limits each email service sets, see getting a file under a 25 MB attachment limit and email attachment size limits by provider.

Questions

Will the person I send it to hear the difference? For speech at 64 kbps, rarely. For music, a 128 kbps M4A sounds fine to most listeners on most equipment. Keep the original WAV either way, because a compressed copy cannot be turned back into the full-quality recording.

Should I just zip the WAV instead? Zipping saves a little on uncompressed audio, but far less than converting it, and the recipient has to unzip it before they can listen. Why zipping photos and videos barely makes them smaller covers the reason.

Can I make an MP3 smaller? Yes, by converting it again at a lower bitrate, but each lossy pass removes a bit more detail. If you still have the WAV or the original recording, start from that instead.