The browser provides one means of compression, which is also how I’ve compressed an MP3 audio file. But it doesn’t magically shrink the file in some way that you’re not authorized to share, or compressing a larger file for use within certain limits. That’s a re-encoding, as described below.
What an online compressor changes
An online compressor will take your existing MP3 and produce another lossy MP3 file. The new MP3 will be smaller, using either a lower bitrate (e.g., 128 kbps) or sample-rate setting (e.g., 44.1 kHz). The size reduction comes from removing details, however, so you lose some quality. It’s possible to do this with a number of different options — both constant-bitrate (CBR) and variable-bitrate (VBR) encoders are supported by online services.
If you already have an MP3 file encoded using a higher bitrate than what you want, then recompressing won’t preserve all the original details, because that second lossy generation won’t preserve the original quality. If possible, instead start from an original, uncompressed audio file or one encoded at a higher-quality level.
But if you must compress an already-compressed file, the general advice is to pick a target that makes sense. For example, if you’re trying to keep the file under 10 MB, then you’ll need to estimate what a constant-bitrate would need to be to reach that target. In addition, there is headroom for headers, metadata, etc. Also, note that VBR may vary based on the program material: for example, a recording of birdsong might have more variation in loudness than a speech recording. Still, this approach can help provide a starting point.
Let’s assume that we have a file that lasts two minutes, and our goal is to compress it down to 6 MB, with about 5 MB (40,000,000 bits) being available for audio data after leaving room for headers and metadata. Then we can divide 40,000,000 by 120, giving us roughly 333,000 bits per second (or about 333 kbps).
As an alternative, you can start with what you want and work backwards: say that you want to encode it at 96 kbps. Multiply 96 by the duration in seconds to get the total number of kilobits required, then divide by 8,000 to get the size in decimal megabytes, and add some extra for headers and metadata. Note that VBR may not match up exactly, but again, it can serve as a useful starting point.
Practical examples
When selecting a constant-bitrate, bear in mind that most people use 128–160 kbps to encode mid-range music, although they’ll probably tolerate slightly lower bitrates. Some services offer 64kbps as a default for streaming and speech, which could easily fit within an MP3 file for someone listening to a podcast, for instance. Mobile phone calls, by comparison, usually use much lower bitrates. If unsure, start with the highest bitrate you think makes sense, then download the compressed file and preview it before deciding to upload it elsewhere.
When looking for a service, check the controls available to set the target-size (a percentage, like 40% which turns 10 MB into 4 MB) and target-quality. Some sites allow you to choose an explicit bitrate, too, which may be more convenient. Others may allow you to change the sample rate used. While I recommend using the highest sample rate supported when possible, there may be cases where the choice of sample rate has no bearing on the overall sound.
You should always know how much of a file you can upload. Most online MP3 compressors document their upload limits and access constraints, but you should confirm these before waiting through an upload.
Some files shouldn’t go anywhere near an online compressor
Not all MP3s can be processed online, even those that are otherwise fine. Encrypted audio (such as some forms of digital rights management) may be unsupported, for example. Protected audio (e.g., protected by password encryption) is also often unsupported, and other services are likely to reject it as well. A number of services will process unprotected media, as long as you own the rights. If not, they may refuse your file. Other times, they simply ask whether you’d like to sign up.
Owning the rights doesn’t remove that technical protection: a service can still reject the file. It may ask whether you want to sign up, but that won’t necessarily make the upload work. Some services, such as OnlineConverter, impose file-size limits—its maximum upload is 200 MB—and slow uploads may need to be cancelled and resubmitted, which could affect performance. In short, it’s important to make sure that the file you upload is not only compatible with the web browser in question, but also that it isn’t restricted in any way.
Before uploading, ensure that the service uses appropriate privacy checks, too. FreeConvert promises 256-bit SSL encryption in transit between your device and its servers, storing files securely on Amazon’s AWS S3 server located in Ireland. The site says that files are stored temporarily until manually deleted, or permanently deleted after eight hours if not accessed. The latter option is manual, too, but you can still control who accesses them: “Your file(s) will only be accessible with your consent, unless otherwise stated by law.” There are similar privacy protections offered by others.
So, what should you look for? Check documented upload limits, access constraints, and the controls available for compressing your audio files. Make sure that you own the rights to the file, too, and understand the terms for protected files. Finally, ensure that the site’s security and privacy statements align with your expectations, and consider the tradeoffs in realistic terms: no matter what, you won’t be able to guarantee that the same-quality file will be shrunk, nor will you be able to guarantee that the output size will be fixed.