Free online AU to AAC converter

Legacy Unix recordings and sound assets in .au format may not open in phones, media libraries, or delivery systems that expect AAC. Encoding them as AAC reduces the size of uncompressed AU audio and provides broader playback support.

AU and AAC formats

AU, also called Sun/NeXT audio, is a container format associated with Sun Microsystems, NeXT systems, and older Unix software. AU files usually use .au or .snd extensions and can hold 8-bit μ-law audio, PCM, ADPCM, and other codec variants; they are not necessarily uncompressed.

AU is still found in Unix archives, legacy applications, Java sound resources, and old workstation recordings. Support in current consumer players is inconsistent, especially for μ-law audio and unusual sample rates.

AAC (Advanced Audio Coding) is a lossy audio codec that generally delivers better quality than MP3 at comparable bit rates, particularly at lower rates. It is supported by Apple and Android devices, many media players, and common audio workflows.

AAC can be stored as a raw ADTS stream, normally using the .aac extension, or in an MP4 audio container, normally using .m4a. Use .m4a for ordinary playback, metadata, and broad library compatibility; use .aac only when the receiving program specifically requires raw AAC or an .aac filename.

Convert an AU file to AAC

For a one-off conversion, upload the AU file to the free converter on 101convert.com, select AAC as the output format, start the conversion, and download the result. Keep the original AU file because AAC output is a lossy delivery copy.

For local conversion, FFmpeg can decode most AU variants:

ffmpeg -i input.au -c:a aac -b:a 192k output.m4a

For speech or telephone-quality recordings, use a lower bit rate:

ffmpeg -i input.au -c:a aac -b:a 64k output.m4a

To produce an ADTS AAC stream for software that explicitly requires .aac, use:

ffmpeg -i input.au -c:a aac -b:a 192k -f adts output.aac

Audacity is suitable when the recording also needs trimming, normalization, or editing. Open the AU file with File → Open, then choose File → Export Audio and select an M4A/AAC export option. AAC export availability may require Audacity's optional FFmpeg library, depending on the installed version and platform.

Quality and compatibility limits

AAC encoding discards audio information. If the AU source already uses a lossy codec such as μ-law, AAC creates an additional lossy generation; it cannot improve the original recording.

Do not raise the sample rate just for conversion. An 8 kHz μ-law voice recording should normally remain at 8 kHz unless the destination system requires another rate, because upsampling adds no detail and can increase file size.

64–96 kb/s AAC is usually sufficient for speech, while 128–192 kb/s AAC is appropriate for music and complex audio. Do not rename an .m4a file to .aac: the container remains MP4, and software requiring ADTS AAC may reject it.