Free online ULAW to M4A converter

ULAW audio commonly comes from telephone systems, voicemail platforms, and older voice-recording software that stores G.711 μ-law data. Converting it to M4A makes the recording easier to play, edit, share, and store in applications that support AAC audio.

What the ULAW format is

ULAW usually means 8-bit G.711 μ-law audio, a companded PCM format designed for speech rather than music. It is common in PBX and VoIP systems, call recordings, voicemail exports, Unix or Sun audio tools, and telephony APIs.

A file with a .ulaw extension is often a raw audio byte stream with no header. The usual properties are 8,000 Hz, mono, and one byte per sample, but recording systems can use different sample rates or channel layouts. Because raw data does not store its own metadata, the correct sample rate and channel count must be supplied during conversion.

What the M4A format is

M4A is an MPEG-4 audio container. It most commonly contains AAC audio, although it can also contain lossless ALAC audio. AAC M4A files are supported by modern phones, media players, browsers, editors, and music libraries, and normally provide much smaller files than uncompressed PCM.

For speech recordings, AAC at 64–96 kbps is often sufficient. Use a higher bitrate when the source contains music or when preserving more of the source detail matters.

How to convert ULAW to M4A

FFmpeg is the most reliable desktop option for raw ULAW because it lets you specify missing audio parameters explicitly.

1. Install FFmpeg and open a terminal in the folder containing the recording.
2. Convert a standard 8,000 Hz mono ULAW file with:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -c:a aac -b:a 80k output.m4a

Replace 8000 with the source sample rate if the recording system used another value. Use -ac 2 only when the source actually contains two channels; duplicating mono into stereo does not improve quality.

For a higher AAC bitrate, use:

ffmpeg -f mulaw -ar 8000 -ac 1 -i input.ulaw -c:a aac -b:a 128k output.m4a

To convert several files in a shell, run the command separately for each input so that each output keeps its original filename, or use a batch script appropriate to your operating system.

If the file is a Sun AU file containing μ-law audio rather than a headerless raw stream, use the AU demuxer instead:

ffmpeg -f au -i input.au -c:a aac -b:a 80k output.m4a

Audacity can also import raw data through File → Import → Raw Data. Select μ-law encoding, enter the actual sample rate, choose mono when applicable, then export through File → Export Audio and select M4A or AAC if that encoder is installed.

You can also convert the file for free on 101convert.com without installing software. For raw ULAW files, provide the correct sample rate and channel count if the site asks for them.

Quality and compatibility considerations

ULAW is already lossy and optimized for narrowband speech; converting it to AAC cannot restore frequencies or detail removed during the original telephone encoding. Re-encoding at an unnecessarily high bitrate increases file size without recovering quality.

If the converted audio sounds too fast, too slow, or has the wrong pitch, the input sample rate was specified incorrectly. If it contains static or distorted speech, verify that the source is μ-law rather than A-law, linear PCM, or a headered AU file. Raw ULAW files generally cannot retain original recording dates, caller metadata, or other fields because those values were not present in the audio stream.