Adding tags to an mp3 file

I have an mp3 file that I want to add tags to without changing the audio content. I proceeded as follows:

Open Audacity
Import the file
Export the file with added tags (saved the tags to a template file).

Everything seemed to work as expected, except that the size of the new file is about 6.9 KB while the original was about 17.4 KB. What do you think happened here?

If “all” you are doing is changing the id3 tags, don’t use Audacity.
When Audacity imports an MP3 file (or any other compressed audio format) the file is decoded to uncompressed audio data. Exporting as MP3 reduces the sound quality (always to some extent) and may lead to a different file size (depending on the MP3 settings).

It is generally best to use the application that you will be using to read the file for adding the id3 tags, so as to ensure compatibility.
If you have a lot of files to tag, you could try a batch tagging program such as MP3tag.

Thanks for your help. I went into iTunes and added the tags that way. Easy enough.

I’m a little concerned though about the secondary issue of the sound quality getting diminished by processing through Audacity. Is there a way of getting the mp3 settings to match those in the source file, so that the audio in the exported file is the same or at least comparable to what was in the source?

Yes you can change the MP3 settings:

There will always be some loss of sound quality when you export from Audacity in MP3 format because MP3 is a “lossy format”, which means that some of the audio data is discarded during the encoding process. The amount of discarded data (and thus the loss of sound quality) can be minimised by using a high bit rate for the MP3 file, but that will produce a larger file. Ideally, exporting in a “lossless” format (such as WAV or FLAC) will totally avoid these losses, but the resulting WAV/FLAC file will be even bigger.

There are a few programs that can perform simple editing tasks (such as trimming, splitting or deleting) to MP3 files without decoding the files, thus there is no requirement to re-encode the file, so no additional loss of sound quality. However, such programs are not capable of complex processing tasks. Two such programs are mp3DirectCut and MP3Split.