What is an audio codec?

An audio codec (coder-decoder) is the method that turns sound into a stream of data and back again for playback. The codec is the maths; the file format or container is the box the data travels in. That is why the same AAC audio can sit in an .m4a file or inside a streaming app.

Lossy and lossless

Codecs fall into two camps.

• Lossy codecs throw away detail the encoder judges you are unlikely to hear, to make files much smaller. MP3, AAC and Opus are lossy.
• Lossless codecs shrink the file but give back every bit on decode. FLAC, the Free Lossless Audio Codec, describes itself as like Zip but designed specifically for audio, so it compresses sound better.

The four you will meet

• MP3. Development started in the late 1980s at Fraunhofer IIS, which calls it the de facto audio codec in the world for two decades. Fraunhofer itself says most modern streaming and broadcast services now use the AAC family, which gives better quality at much lower bitrates.
• AAC. The default lossy codec of the streaming era, and the one Apple Music and Spotify's web player name.
• Opus. An open codec defined in IETF RFC 6716. It mixes linear prediction and MDCT coding to handle speech and music, and "supports all bitrates from 6 kbit/s to 510 kbit/s". It was built for interactive audio: calls, game chat and even live, distributed music performances.
• FLAC. Open and lossless, it shrinks audio the way Zip shrinks files, but designed for audio.

What streaming services use

• Spotify: its audio quality page lists AAC at 128 kbit/s (Free) and 256 kbit/s (Premium) in the web player, and on desktop and mobile quality steps from about 24 up to about 320 kbit/s, without naming the codec there. Premium also gets lossless, "equivalent up to 24-bit/44.1kHz FLAC" (checked 28 Oct 2026).
• Apple Music: Apple contrasts "standard AAC files" with its Lossless and Hi-Res Lossless tiers, available up to 24-bit/192 kHz.

What we think

Deliver lossless, and let the service pick the codec. Streaming services serve their own formats (Spotify's AAC and lossless tiers, Apple's AAC), so if they encode from what you deliver, a lossy master would be compressed twice. Send WAV or FLAC to your distributor and keep the MP3 or AAC files for previews and email. Codec choice matters more for live and low-latency work, where Opus's design for interactive audio is the reason to reach for it. For which file to send where, see our guide to audio file formats.