Convert to M4A
Better sound than MP3 at the same file size.
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AAC holds up better than MP3 at the same bitrate, so 128 kbps here is roughly equivalent to 160–192 kbps of MP3.
As a rule of thumb, AAC at one step below your usual MP3 bitrate sounds about the same: 128 kbps here is comparable to a 192 kbps MP3, and 256 kbps matches what Apple Music streams.
How to use it
- Drop an MP3, WAV, FLAC, OGG or Opus file onto the page. It is decoded in your browser, not sent anywhere.
- Choose a bitrate in the export bar. AAC at 128 kbps is roughly the quality of a 192 kbps MP3, so you can usually drop a step from whatever you would have picked for MP3.
- Click Download. The first M4A of your session downloads the AAC encoder, about 31 MB, and the page shows that as a separate progress step.
- Once the encoder is loaded it stays in memory, so any further M4A exports start immediately.
AAC was built to replace MP3, and technically it did
Advanced Audio Coding was standardised in 1997 as MPEG-2 Part 7 and extended two years later in MPEG-4 Part 3, by a group that included Fraunhofer IIS, Dolby, Sony, AT&T and Nokia, several of the same people who had built MP3. The brief was to design a modern perceptual codec with no obligation to remain backward compatible with anything, and the freedom shows.
MP3 inherited a 32-band polyphase filterbank from earlier MPEG layers and bolted an MDCT on top of it, a hybrid that leaks energy between adjacent bands and wastes bits correcting for itself. AAC uses one clean MDCT with 1024-sample windows that switch to 128 samples when the signal turns percussive. It adds temporal noise shaping, which pushes quantisation noise into the loud part of a transient rather than smearing it backwards as pre-echo. It supports sample rates from 8 kHz to 96 kHz and up to 48 channels, where MP3 stops at 48 kHz and two channels in practice. The practical result is that AAC needs roughly two thirds the bitrate of MP3 for the same perceived quality.
M4A is a filename, not a format
The file this tool writes is an MPEG-4 Part 14 container, the ISO base media file format, which descends directly from Apple's QuickTime .mov structure. Everything inside is organised into nested boxes: one holds the audio samples, others hold the track description, the timing table, the edit list and the metadata.
Apple introduced the .m4a extension with iTunes 4 in 2003 purely so that users and software could tell an audio-only MP4 from one with video, and the convention stuck. The same container with .m4b tells a player the file is an audiobook and should remember its position; with .m4r it tells iOS the file is a ringtone; with .m4p it is a FairPlay-encrypted purchase. Rename one to another and, DRM aside, players generally go along with it. Which is exactly how ringtone tools work.
When to pick something else
Choose MP3 instead if the file is going to a car stereo or a dedicated player from before about 2010, or if you simply do not want to think about whether the recipient's device will cope. Choose OGG Vorbis if the audio is an asset in a Unity or Godot project, where it is the expected format. Choose Opus if the content is speech and you want the smallest possible file. Below 96 kbps it beats AAC comfortably. Choose FLAC or ALAC if the point is archival, since AAC discards information permanently no matter how high you set the bitrate.
And do not convert an existing lossy library to M4A hoping to reclaim space. The efficiency advantage applies to encoding an original recording. Applied to an MP3, it compresses the artifacts along with the music and leaves you worse off than when you started.