What AU format is and when you need it

AU format is an uncompressed audio file type developed by Sun Microsystems. It stores sound as raw data without the compression that MP3 or AAC use, which means AU files are larger but play on almost any device without needing special software. You'll encounter AU files mostly in older systems, some Unix-based programs, and certain online tools that specifically request this format.

The main reason to create an AU file is because something you're trying to use requires it. Unlike MP3, which you might choose for its smaller file size, AU format is usually a requirement rather than a preference. If you're working with legacy software, a university system, or a web application built years ago, you may need to convert existing audio or record directly into AU format.

Creating an AU file is straightforward once you know which tool to use. You have three main paths: convert an existing audio file using free software, record directly into AU format, or use an online converter. The method you choose depends on whether you're starting from scratch or converting something you already have.

Key Takeaways

  • AU is an uncompressed audio format that works on older systems and some specialized software, and you typically need it because a specific program requires it rather than by choice.
  • Audacity is the most reliable free tool for both recording audio directly as AU and converting existing files to AU format on Windows, Mac, or Linux.
  • When exporting from Audacity, you must select "Other uncompressed formats" and then choose AU as the specific format, because AU does not appear in the main export menu.
  • Online converters can turn MP3, WAV, or other audio files into AU format in your browser without installing software, though you should not use them for sensitive audio.
  • AU files are much larger than MP3 files of the same audio because they store sound without compression, so check your file size limits before creating one.

Converting an existing audio file to AU using Audacity

Audacity is a free, open-source audio editor that runs on Windows, Mac, and Linux. It handles AU format natively and is the most reliable way to convert files you already have. Start by downloading Audacity from audacityteam.org and installing it on your computer.

Open Audacity and go to File > Open. Select the audio file you want to convert — it can be MP3, WAV, FLAC, or most other common formats. Audacity will import the file and display it as a waveform on the screen. You do not need to edit or change anything; you can convert it as-is.

To export as AU, go to File > Export > Export As. A dialog box opens. At the bottom, you will see a dropdown menu that says "Save as type" or "File type." Click it and scroll down to find "Other uncompressed formats." Select that option. Now change the filename to end with .au (for example, mysound.au) and click Export. Audacity will ask you to confirm the encoding settings — the defaults are fine for most uses, so click Export again. Your AU file is now ready.

Recording audio directly as AU format

If you need to record new audio and save it as AU, Audacity is again the simplest choice. Open Audacity, make sure your microphone is connected and selected (you can check this in the device dropdown at the top), and click the red Record button. Speak or play audio into your microphone. Click the pause button when you are done.

The recording appears as a waveform. You can trim the beginning or end by selecting the part you want to remove and pressing Delete, or you can export it as-is. Go to File > Export > Export As, choose "Other uncompressed formats" from the file type dropdown, name your file with a .au extension, and click Export twice. Your recorded AU file is saved.

If you do not want to install software, some online recording tools let you record directly in your browser and download as AU, though these are less common. Search for "online audio recorder AU format" to find current options, but be aware that browser-based tools may have limits on recording length or audio quality.

Using an online converter

If you already have an audio file and do not want to install Audacity, online converters can turn it into AU format in your web browser. Search for "audio converter AU format" and you will find several free options. Most work the same way: you upload your file, select AU as the output format, and download the result.

Common converters include CloudConvert, Online-Convert, and Zamzar. Upload your MP3, WAV, or other audio file, make sure AU is selected as the output format, and click Convert. The process usually takes a few seconds to a few minutes depending on file size. Download the converted AU file to your computer.

Online converters are convenient if you only need to convert one or two files and do not want to learn new software. However, do not use them for audio you consider private or sensitive, since the file is uploaded to someone else's server. For anything confidential, use Audacity instead, which works entirely on your own computer.

Understanding AU file size and quality

AU files are significantly larger than compressed formats like MP3. A three-minute song in MP3 might be 5 to 10 megabytes, but the same song as AU could be 30 to 40 megabytes. This matters if you are uploading to a system with file size limits or if you need to email the file.

The trade-off is that AU format preserves the original audio quality without any loss from compression. If the program requiring AU format is old or specialized, this uncompressed quality may be exactly what it needs to work correctly. Check whether the system you are sending the file to has any size restrictions before you create your AU file.

If file size is a problem, ask whether the system will accept WAV format instead. WAV is also uncompressed and nearly identical to AU in terms of quality and file size, but it is more widely supported by modern software. Some older systems will take either one.

Troubleshooting common problems

If Audacity will not export as AU, the most common cause is that you selected the wrong file type. Make sure you chose "Other uncompressed formats" from the dropdown, not "WAV" or another option. AU does not appear as its own line item in the export menu on most versions of Audacity.

If the AU file plays but sounds wrong — distorted, too quiet, or at the wrong speed — the issue is usually the encoding settings that Audacity asks about during export. For most uses, the default settings (16-bit PCM, 44100 Hz sample rate) work fine. If the program that needs the AU file specifies different settings, Audacity will let you change them in that export dialog.

If an online converter fails or times out, try uploading a smaller file or using a different converter. Some services have limits on file size or may be temporarily overloaded. Audacity is more reliable for large files since it works on your own computer with no upload required.

Choosing between Audacity, online converters, and other tools

Audacity is the best choice if you plan to create AU files regularly, need to edit audio before converting, or want to keep files private. It is free, works on any operating system, and handles AU format reliably. The learning curve is minimal — you only need to know File > Open, File > Export As, and the file type dropdown.

Online converters are best if you have just one or two files to convert and do not want to install software. They are fast and require no setup, but they upload your file to the internet and may have size limits.

Some audio editing software like Adobe Audition or GarageBand can also export as AU, but these are paid programs or limited to Mac. Unless you already own them, Audacity is the more practical choice.

Frequently Asked Questions

Can I convert an AU file back to MP3?

Yes. Open the AU file in Audacity (File > Open), then go to File > Export As and choose MP3 from the file type dropdown. You will need to install the MP3 encoder if Audacity prompts you, which is free. The resulting MP3 will be smaller than the original AU file.

What is the difference between AU and WAV format?

Both are uncompressed and produce similar file sizes and audio quality. AU was developed by Sun and is less common today, while WAV is the Windows standard and more widely supported. If the program you are working with accepts either one, WAV is usually the safer choice.

Why does my AU file sound different from the original?

AU format itself does not change sound quality if you use the standard settings (16-bit, 44100 Hz). If audio sounds different, check whether the original file was a different sample rate or bit depth. You can see this in Audacity at the top of the waveform window and change it before export if needed.

Can I create an AU file on my phone?

Most phone audio apps do not support AU format natively. Your best option is to record on your phone as MP3 or WAV, transfer the file to a computer, and convert it using Audacity or an online converter.

Is AU format still used anywhere?

AU is mostly found in older systems, some Unix and Linux applications, and occasionally in academic or research software. Most modern programs use MP3, WAV, or AAC instead. You typically only need AU because a specific legacy system requires it.