Free online MP3 to AU converter

Older Unix and Java applications, Sun workstations, telephony systems, and some archival workflows may accept AU files but reject compressed MP3 audio. Converting locally with FFmpeg or using the free converter on 101convert.com creates the required AU file without changing the original MP3.

What the MP3 format is

MP3 is a lossy MPEG audio format that reduces file size by discarding audio information judged less audible. It is widely used for music, podcasts, voice recordings, downloads, portable playback, and sharing because most modern operating systems, devices, and media applications support it.

What the AU format is

AU, also called Sun Audio, is a container format originally associated with Sun Microsystems workstations. It stores audio together with a header describing the encoding, sample rate, channel count, and data length. AU can contain uncompressed PCM as well as μ-law, A-law, and ADPCM audio, so the .au extension alone does not identify the codec.

PCM-based AU is useful when a legacy application requires a Sun audio header or an uncompressed source. Traditional speech and telephony software often expects 8 kHz, mono, 8-bit μ-law AU. Uncompressed PCM avoids adding another lossy compression stage, but it produces much larger files than MP3.

Convert with FFmpeg

1. Install FFmpeg from your operating system’s package manager or the official FFmpeg distribution.
2. Open a terminal in the folder containing the MP3 file.
3. For general uncompressed AU output, run ffmpeg -i input.mp3 -c:a pcm_s16be output.au.
4. For software that specifically requires traditional 8 kHz mono μ-law, run ffmpeg -i input.mp3 -ar 8000 -ac 1 -c:a pcm_mulaw output.au.
5. Replace input.mp3 and output.au with the actual filenames. Put filenames containing spaces in quotation marks, for example ffmpeg -i "My recording.mp3" -c:a pcm_s16be "My recording.au".

The first command decodes the MP3 to 16-bit big-endian PCM and keeps its source sample rate and channel count. The second command resamples to 8 kHz, mixes to mono, and encodes μ-law; use it only when the receiving software documents that profile.

Convert without installing software

On 101convert.com, select the MP3-to-AU converter, upload the MP3, start conversion, and download the AU file. If the converter exposes output settings, match the receiving application’s required codec, sample rate, bit depth, and channel count. Do not upload confidential recordings unless the service’s handling and retention terms are suitable.

Quality and compatibility limits

Decoding MP3 into PCM AU cannot restore details removed during the original MP3 encoding. The PCM output is only a new container and sample representation; it is not a higher-quality version of the source. μ-law also reduces fidelity compared with linear PCM and is intended mainly for speech and telephone audio.

File size depends on the AU encoding. Stereo 16-bit PCM at 44.1 kHz requires about 10.6 MB per minute, while 8-bit μ-law mono at 8 kHz requires about 480 KB per minute, excluding the small AU header. Test the result in the target application because AU support does not guarantee support for every codec, byte order, sample rate, or channel layout stored in an AU file.