Manatee

Lossless vs lossy: what happens to audio when you convert it

· 6 min read

Lossless formats (WAV, AIFF, FLAC, ALAC) store every sample of the original audio, so converting between them changes nothing but the file size. Lossy formats (MP3, AAC, Opus, Vorbis) permanently discard the parts of the sound the encoder judges least audible, and each further lossy conversion discards more. Convert lossless to lossless freely, lossless to lossy once for sharing, and avoid lossy to lossy when you can.

What “lossless” and “lossy” mean

Digital audio is a long list of numbers: tens of thousands of samples per second describing the air pressure at a microphone. A CD holds 44,100 samples per second, each a 16-bit number, for two channels. That works out to about 10 MB per minute, which is the size of a WAV or AIFF file at CD quality.

A lossless format compresses that list the way a ZIP file compresses a document: it finds patterns and stores them more compactly, and when you decode it you get back the exact same numbers. FLAC and Apple Lossless (ALAC) typically shrink a WAV to 50 or 60 percent of its size. Nothing is lost. You can convert WAV to FLAC to ALAC to AIFF and back a thousand times and the samples are identical at the end.

A lossy format does something different. It analyzes the sound, decides which details a listener is least likely to notice (quiet sounds masked by loud ones, frequencies above what most people hear, subtle stereo differences), and simply does not store them. What remains is encoded efficiently. An MP3 at 128 kbps is about a tenth the size of the WAV; at 320 kbps about a quarter. The missing information is gone from that file permanently.

The four conversions and what each does

Lossless to lossless (WAV to FLAC, AIFF to ALAC, FLAC to WAV): safe in both directions. The only change is file size. Do this to save space on an archive or to match what a device expects.

Lossless to lossy (WAV to MP3, FLAC to AAC): a one-time, deliberate loss. At a good bitrate (256 kbps AAC or 320 kbps MP3), most people cannot distinguish the result from the original on most equipment. This is the right way to make a shareable or portable copy. Keep the lossless original.

Lossy to lossless (MP3 to WAV, AAC to FLAC): safe but pointless for quality. The new file is large and contains exactly what the MP3 contained, no more. The detail the MP3 threw away does not come back. Do it only when a tool refuses to accept anything but WAV, and understand that the file is “WAV-shaped MP3 audio”.

Lossy to lossy (MP3 to AAC, AAC to MP3, M4A to MP3 at a lower bitrate): the one to avoid when you have a choice. The second encoder cannot see what the first one removed, so it makes its own removals on top, and artifacts compound: a slightly swishy high end, smeared cymbals, a hollow quality to voices. One generation at a high bitrate is usually fine. Three or four are audible. If a lossless original exists anywhere, convert from that instead.

Which format to choose

  • WAV or AIFF: for editing, mastering, and handing audio to someone who will process it further. Largest files. AIFF is the same thing with Apple’s container and better metadata support in Music.
  • FLAC: for archiving music you ripped or bought lossless. Half the size of WAV, supports tags and album art, and plays in Music on recent macOS versions and in most players elsewhere.
  • ALAC (in an M4A container): Apple’s lossless format; the right choice if your library lives entirely in Music and on iPhones.
  • AAC (usually in an M4A container): the lossy format Apple devices and most streaming services use. Better than MP3 at the same bitrate. 256 kbps is the standard high-quality setting.
  • MP3: the universal lossy format. Every device made since the late 1990s plays it. Choose it when compatibility matters more than efficiency, and use 192 to 320 kbps.
  • Opus: excellent lossy quality at low bitrates, used for voice messages and browser recordings. Convert to MP3 or M4A before sending to anyone who is not on a modern app.

Converting audio on a Mac

The Music app can convert between its own supported formats through File > Convert > Create MP3 Version (or AAC, AIFF, WAV, Apple Lossless, depending on the import setting in Music > Settings > Files). It only works on files already in the library and it does not handle FLAC, Opus, OGG or WMA.

Manatee converts directly from the file, with no library import, and accepts 122 input types including FLAC, OGG, Opus, WMA, APE, WavPack, AMR, CAF and the audio inside video files. Its audio targets are MP3, M4A, WAV and FLAC.

  1. Drag the audio file (or a batch) onto the Manatee icon in the menu bar, or drop it into the window.
  2. Choose Convert and pick a target: FLAC or WAV to keep a lossless source lossless, MP3 or M4A for a shareable copy.
  3. The converted file is written alongside the original. The original is never touched, so a FLAC master stays a FLAC master.

Manatee is not an audio editor. It does not trim, normalize volume, change sample rates by choice, or split tracks, and it cannot restore detail that a lossy file has already lost: converting an MP3 to FLAC in Manatee gives you a larger file with the same sound, exactly as described above. For the common tasks, see How to convert FLAC to MP3 on a Mac, How to convert WAV to MP3 on a Mac and How to convert M4A to MP3 on a Mac.

Bitrate is the knob that matters

Within a lossy format, the bitrate (kilobits per second) decides how much is kept. A rough guide for stereo music:

  • 96 to 128 kbps: fine for speech and podcasts; music sounds noticeably thin.
  • 192 kbps: good for casual listening.
  • 256 kbps AAC or 320 kbps MP3: transparent for almost everyone.

Doubling the bitrate doubles the file size. For a spoken-word recording, a lower bitrate and mono channel can cut the size by three quarters with no practical loss. For music you care about, stay at the top of the range or keep it lossless.

Questions

Is FLAC better than WAV? They contain identical audio. FLAC is smaller and carries tags; WAV is more universally accepted by editing and production software. Pick by what the destination expects.

Will converting MP3 to WAV improve the sound? No. It makes the file larger and changes nothing audible. The detail the MP3 encoder removed cannot be reconstructed.

Why does my converted file sound worse than the original? Either the bitrate chosen was low, or the source was already lossy and this was a second lossy pass. Convert from the lossless original if one exists, and use a higher bitrate.

Does converting lose the album art and tags? It depends on the target. MP3, M4A and FLAC all carry tags; WAV support for metadata is limited, so artist and title may not survive a trip through WAV.