Free online AAC to SF2 converter

What the AAC format is

AAC, or Advanced Audio Coding, is a lossy audio codec used for music, podcasts, video soundtracks, and mobile playback. Apple Music and iTunes commonly create AAC streams inside .m4a or .mp4 containers; standalone AAC files may use the .aac extension. Phones, media players, video editors, and streaming tools also frequently produce or accept it.

AAC stores compressed audio rather than instrument definitions. It contains no SoundFont zones, MIDI key assignments, velocity layers, loop points, envelopes, or synthesizer settings.

What the SF2 format is

SF2, also called SoundFont 2, is a RIFF-based format for sampled instruments. An SF2 bank can contain PCM samples, instruments, presets, key and velocity ranges, root-note assignments, loop points, tuning, filters, and amplitude envelopes.

SF2 is used by MIDI software, hardware samplers, virtual synthesizers, game-audio tools, and operating-system MIDI renderers. A single decoded AAC recording can become one sample in an SF2 instrument, but the conversion does not automatically create a professionally sampled piano, drum kit, or multisampled instrument.

How to actually convert AAC to SF2

The reliable desktop workflow uses FFmpeg to decode the AAC and Polyphone to construct the SoundFont.

1. Install FFmpeg and Polyphone, then place the source file in an accessible folder.
2. Decode the AAC to uncompressed PCM WAV. For a mono sample at 44.1 kHz and 16-bit depth, run:

ffmpeg -i input.aac -ar 44100 -ac 1 -sample_fmt s16 sample.wav

Use the real filename if the audio is in an MP4 or M4A container, for example ffmpeg -i recording.m4a sample.wav. Keep stereo with -ac 2 when the target sampler requires stereo or the source depends on stereo imaging.

3. Open Polyphone and create a new SoundFont.
4. In the Samples section, import sample.wav. Polyphone can read standard PCM WAV files.
5. Create an instrument and add the imported sample to it.
6. Set the sample's root key to the pitch at which it was recorded. Assign a key range if the sample should respond only to part of the MIDI keyboard.
7. For a multisampled instrument, decode additional AAC recordings at different pitches or dynamic levels and assign each sample to its own key and velocity range.
8. Set loop start and end points if the sound must sustain. Add an amplitude envelope, tuning correction, filter, and release time as needed.
9. Create a preset that points to the instrument, enter the bank and program settings required by the target device, and save the result as instrument.sf2.

For a browser-based attempt, the built-in converter on 101convert.com can convert the file for free without installing software. A desktop editor is still needed when the result requires key mapping, looping, velocity layers, or envelope adjustments.

Quality, compatibility, and limitations

Decoding AAC cannot restore information removed by lossy compression. Exporting the decoded audio as WAV avoids another lossy encode, but it does not improve the original AAC quality. Use the source's native sample rate when possible; resampling to 44.1 or 48 kHz is usually compatible with samplers.

A complete song is usually a poor SF2 source. When mapped across a keyboard, the song will play at different pitches and durations, and the SF2 format does not preserve album metadata, playlists, or the original compression. SF2 is appropriate for a short note, vocal hit, drum sound, phrase, or other sound intended to be triggered by MIDI.

AAC encoder delay and silence at the beginning or end can make an instrument feel late. Trim unused silence before importing, and inspect loop boundaries at zero crossings or during a steady portion of the waveform to reduce clicks. Some older hardware supports only basic SF2 features, so use PCM samples, conventional key ranges, and simple envelopes when compatibility is more important than advanced modulation.