The most efficient step down from lossless
AAC delivers more quality per kilobit than MP3, which makes it an excellent target for FLAC music. At 256 kbps it is very hard to tell AAC from the FLAC original, and at 160 to 192 kbps it is still very good. Starting from lossless FLAC means the encoder sees the full signal, so you get the best quality AAC can deliver at the chosen bitrate.
Raw .aac or .m4a?
This page writes raw AAC streams with the .aac extension. Some car head units, streaming tools and embedded players ask for exactly that. However, raw AAC cannot hold tags, so the FLAC's title, artist and album information is lost, and some players seek slowly in it. For a music library, iPhone or Apple Music, the same AAC audio inside an M4A container is the better choice, because it keeps tags and supports gapless playback. In that case, use FLAC to M4A.
Bitrate and size
| AAC bitrate | Size per minute | Compared with 16-bit FLAC |
|---|---|---|
| 128 kbps | 0.96 MB | about 6 times smaller |
| 192 kbps | 1.44 MB | about 4 times smaller |
| 256 kbps | 1.92 MB | about 3 times smaller |
| 320 kbps | 2.4 MB | about 2.5 times smaller |
For 24-bit, high sample rate FLAC files, the savings are two to three times bigger still, because those sources are larger.
High-resolution sources
AAC encodes at 44.1 or 48 kHz in normal use. Hi-res FLAC at 88.2, 96 or 192 kHz is converted to a suitable rate during encoding. Content above 20 kHz is removed, which no one can hear, and the 24-bit depth of the source helps nothing at this stage. A CD-quality FLAC and a hi-res FLAC of the same master produce practically the same AAC.
Tips
- Name files with track number and title before converting, since raw AAC carries no tags.
- Keep the FLAC library as your master and convert again when you need a different format.
- If several albums differ in loudness, use normalize on the results.
For the widest device support, FLAC to MP3 remains the safest option.