Convert AU to M4A — Free Online Tool
Convert Sun AU audio files to M4A format, re-encoding the raw PCM audio (typically 16-bit big-endian) into AAC at 128kbps inside an MPEG-4 container. This makes legacy Unix audio files compatible with iTunes, Apple devices, and modern streaming platforms.
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FFmpeg Command
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DownloadHow It Works
AU files store audio as uncompressed PCM data (most commonly 16-bit signed big-endian, as developed by Sun Microsystems for Unix systems) with a minimal header. During this conversion, FFmpeg reads that raw PCM stream and re-encodes it using the AAC codec at 128kbps, then wraps the result in an MPEG-4 (.m4a) container. This is a full transcoding operation — not a remux — because AU's PCM audio has no overlap with M4A's supported codecs. The result is a dramatically smaller file with near-transparent audio quality for most content, fully compatible with Apple iTunes, iOS, and modern web players. The -vn flag is included as a safeguard to discard any non-audio data, keeping the output as a clean audio-only M4A file.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg command-line tool, which handles all decoding, encoding, and container operations for this AU-to-M4A conversion. |
-i input.au
|
Specifies the input Sun AU file. FFmpeg reads the AU header to detect the PCM encoding (e.g., pcm_s16be), sample rate, and channel count before decoding the raw audio data. |
-c:a aac
|
Sets the audio codec to AAC (Advanced Audio Coding), which is the standard codec for M4A files and the format natively supported by iTunes, iOS, macOS, and most modern media players. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second, which provides a good balance between file size and audio quality suitable for most music and voice content converted from PCM AU sources. |
-vn
|
Explicitly disables any video stream in the output, ensuring the M4A file contains only the AAC audio track. This is a safeguard to prevent unexpected data from appearing in the output container. |
output.m4a
|
Defines the output filename and tells FFmpeg to write an MPEG-4 audio container (.m4a), the correct format for AAC audio intended for Apple ecosystem compatibility and broad web playback support. |
Common Use Cases
- Importing old Unix workstation sound recordings or system audio samples into iTunes or an Apple Music library
- Preparing classic AU-format audio files from early internet websites or newsgroup archives for playback on modern smartphones and tablets
- Converting AU audio assets from legacy Sun Solaris or NeXT software projects into a format usable in modern DAWs and podcast production tools
- Reducing the file size of uncompressed PCM AU recordings before sharing them online or embedding them in web applications that require M4A/AAC
- Migrating a library of AU-format sound effects or voice clips into M4A so they can be used in Apple's GarageBand or Logic Pro
- Converting AU audio exported from scientific or research instruments that use Unix-native formats into a portable format compatible with standard media players
Frequently Asked Questions
Yes, but in practice it is minimal. AU files typically contain uncompressed PCM audio, which is lossless. M4A with AAC encoding is lossy, so some audio information is discarded during compression. At the default 128kbps setting, the result is transparent for most speech and general audio content, though audiophiles working with high-fidelity recordings may prefer to use a higher bitrate like 256kbps or 320kbps.
AU files store audio as raw, uncompressed PCM data, which produces very large file sizes — a one-minute stereo file at 16-bit/44.1kHz occupies roughly 10MB. AAC compression in the M4A container is highly efficient, reducing that same file to around 1MB at 128kbps. This size reduction is the primary practical benefit of this conversion for storage and sharing purposes.
AU files have an extremely minimal header that can contain only a basic annotation string — there are no standardized fields for title, artist, or album. FFmpeg will attempt to copy any annotation text present, but M4A supports rich iTunes-style metadata tags (title, artist, album, artwork, etc.) that AU simply cannot provide as a source. You will likely need to tag the output M4A file manually using a tool like MusicBrainz Picard or iTunes after conversion.
Adjust the value after -b:a to set the AAC bitrate. For example, replace 128k with 256k for higher quality: ffmpeg -i input.au -c:a aac -b:a 256k -vn output.m4a. Available options include 64k, 96k, 128k, 192k, 256k, and 320k. Use 64k or 96k for voice recordings where smaller file size matters, and 192k or higher for music where fidelity is a priority.
Yes. On Linux or macOS, use a shell loop: for f in *.au; do ffmpeg -i "$f" -c:a aac -b:a 128k -vn "${f%.au}.m4a"; done. On Windows Command Prompt, use: for %f in (*.au) do ffmpeg -i "%f" -c:a aac -b:a 128k -vn "%~nf.m4a". This processes every AU file in the current directory and saves a corresponding M4A file with the same base name.
AU files can contain PCM variants including 16-bit big-endian (pcm_s16be), 8-bit signed (pcm_s8), 8-bit unsigned (pcm_u8), G.711 A-law (pcm_alaw), and G.711 mu-law (pcm_mulaw). FFmpeg automatically detects and decodes all of these formats, so the conversion command works correctly regardless of which PCM encoding your AU file uses. The output AAC audio in the M4A file will be consistent quality regardless of the source PCM variant.
Technical Notes
The Sun AU format uses a four-byte magic number (.snd) and a simple fixed header, making it one of the easiest audio formats for FFmpeg to parse. However, its lack of a standardized sample rate field or multi-channel support beyond stereo means some very old or non-standard AU files may need inspection before conversion. AAC encoding in FFmpeg uses the built-in native AAC encoder by default; if you have libfdk_aac compiled into your local FFmpeg build, substituting -c:a libfdk_aac will produce slightly better quality at equivalent bitrates, which is worth considering for archival-quality conversions. The -vn flag in the command is technically redundant for AU input (which carries no video), but it is included as a best practice to explicitly exclude any unexpected non-audio streams and ensure a clean M4A output. M4A does support chapter markers, but since AU has no chapter data to transfer, the output will contain none. The MPEG-4 container also enables gapless playback metadata, which is beneficial if the AU file is part of a continuous recording.