Convert an audio file

Which format to choose

MP3 is the universal standard: light and compatible with any device. M4A (AAC) gives slightly better quality at the same size, ideal on Apple. WAV is lossless (identical to the original) but much heavier: useful for editing or archiving, not for everyday listening.

A note on quality

Converting from a compressed format (e.g. M4A) to MP3 does not «recover» quality already lost: at best it keeps it. For the best result always start from the highest-quality file you have.

What really happens in a conversion

A lossy compressed format, like MP3 and AAC, does not contain all the original sound: it throws away what the ear cannot hear because it is masked by louder sounds. That work cannot be undone, so converting an MP3 to WAV recovers nothing: it produces a bigger file with exactly the same content.

Worse still: converting from one compressed format to another does the same job twice over: the second pass discards things the first had already left on the edge. The result is audible above all on sustained sounds, like cymbals and high voices, where a watery timbre appears.

The three rules that avoid trouble

Always start from the highest-quality file you have, not from the most convenient copy. Convert once only, at the end of the work and not in the middle. And if you need to edit the audio, take it to WAV at the start, work there and compress only at the end: that is the sequence that loses least.

There is one conversion that takes nothing away, and it is the one to a lossless format like FLAC: it compresses like a zip, so the file weighs half a WAV and the samples are identical. It is the right choice for archiving, not for sending a file to someone who only has to listen to it.

Which format for which use

The MP3 is the format that opens everywhere, car stereos and old devices included: when the file has to work on something you do not know, it is the safe choice. M4A, that is AAC, sounds better for the same size, and is the native format of the Apple world and of nearly all video. WAV is the uncompressed base you work from. FLAC is the lossless archive.

The bitrate is the knob that sets the compromise: below 128 kilobits per second an MP3 sounds compressed in headphones, around 192 the difference is hard for most people to catch, above 320 there is nothing left to gain. For speech, instead, far lower values are enough, which is why podcasts weigh so little.

The things lost along the way

The tags, that is title, artist, album and cover art: many conversions carry them over, but not all, and ending up with a folder of «track 01» files is the typical surprise. It is worth checking them before deleting the originals.

And the channels: a stereo file converted to mono halves the size and is perfect for speech, but on a music recording it removes all the depth and there is no way back. In the opposite direction, turning mono into stereo adds nothing: it duplicates the same signal on two channels and doubles the size.

Nearby tools

To trim there is Audio trimmer, to join several files Merge audio and for speed Change audio speed. For the audio inside a clip Extract audio from video, and to measure how loud it is Video loudness.