AAC (Advanced Audio Coding) is a lossy compression format that discards part of the audio information in order to keep file sizes small. Like MP3, it is used to create lightweight audio files, and you will often see it on Apple devices, in video, and in streaming.
AAC is the name of a codec
AAC is a way of compressing and decoding audio data, in other words a codec. AAC data is sometimes handled as .aac, and sometimes placed inside an MPEG-4 container to become .m4a.
How it saves space
Rather than storing the original PCM data as it is, AAC reduces the amount of information while limiting the audible impact. For that reason you cannot return it exactly to the original data the way you can with WAV. Keep the pre-conversion WAV and your project files.
Bitrate and intended use
With AAC too, the bitrate, sample rate, and channel count all affect file size and how the audio sounds. Check the purpose and the recipient’s requirements first: previewing music, handing files to a video editor, loading audio onto a device. If the destination re-encodes your file, follow that service’s recommended settings.
Differences from MP3, WAV, and FLAC
MP3 and AAC are both lossy formats, and both are options when you want a light file. WAV is generally uncompressed data for production and editing, while FLAC is lossless compression that returns to the original when decoded. Do not compare formats built for different purposes on sound quality alone. When exporting to M4A, check the AAC settings inside.
Do not convert repeatedly
Converting AAC to MP3, or MP3 back to AAC, stacks one lossy compression on top of another. As a rule, create each format you need separately from the original WAV. When you want to export a WAV as M4A (AAC), you can work through the settings with the WAV to M4A converter.
