Can I reduce mp3 file size?

[cyrano]

A 22050 sample rate's files are half the size when compared to 44100...

44100 has 22050 Hz, or 22 KHz as highest recorded tone. A bit less, really, as the filters need to be some distance from the top. Well beyond human hearing.

Half of the sampling frequency is the top of the frequency range.

22100 has 11 KHz as highest range. That leaves off a tiny bit of high tone. The recording will sound a little bit duller. Nothing to worry about as it is for use on a website and 99% of listeners won't be using quality headphones or speakers.

[steve]

There's not really any benefit in reducing the sample rate when producing MP3s unless you are aiming for extremely small (and low quality) files. The MP3 algorithm is highly optimised to handle everything for you so you only need to be concerned with 3 things:
1) Mono or stereo
2) CBR or VBR ("Constant" or "Variable" bit-rate) see below.
3) Size vs quality. Smaller size = worse sound quality.

For a podcast that is mostly speech, the biggest saving you can make is to do the show in mono and use the lowest bit-rate that still sounds OK.
The "CBR" ("Constant" bit-rate) mode is best for compatibility and generally recommended for podcasts. VBR tends to give sound quality that is a bit better, but some players display the playing time incorrectly or not at all. The "Presets" in the Audacity MP3 settings are VBR setting (the "Extreme" preset is excellent quality and well suited for high quality stereo music, but the files are relatively large for MP3s).

For speech recordings, "64 kbps CBR mono" is usually considered a good choice. If you need your files to be even smaller, you can go down to 32 kbps CBR mono, but the sound quality will suffer. I wouldn't recommend going below 32 kbps because is sounds crap

Always do your production in high quality and save high quality backups (at least 16-bit 44100 Hz WAV). Encode as MP3 as the final step of the production after you have finished editing / processing and whatever else. Do not be tempted to re-compress an MP3 to make a smaller file. If you need a smaller file, go back to your original high quality version (or high quality backup) and make the smaller MP3 from that.

[Gideon]

For speech recordings, "64 kbps CBR mono" is usually considered a good choice. If you need your files to be even smaller, you can go down to 32 kbps CBR mono, but the sound quality will suffer. I wouldn't recommend going below 32 kbps because is sounds crap

I did my last job at 16 kbs CBR. I'll change it to 32 in this case. As for 'mono' the original recording was mono. However, the Export Options box only gave the choice of 'stereo' or 'joint stereo'. I chose 'joint' but I wonder if a 'mono' button should be included there to avoid confusion?

[steve]

If the Audacity project is entirely mono, then the export will be mono. That's the same for all export formats. When exporting a mono file, the stereo / joint stereo option has no effect. For high quality stereo recordings, the "joint stereo" option generally gives better quality.

I'd expect 16 kbps CBR to be a bit low. I' expect there to be a very noticeable swirly or bubbly metallic tone to the sound. It's probably worth doing some test - get a nice clean recording and export it as 8, 16, 32, 64 and 128 kbps CBR, then load them all into Audacity and use the track Solo buttons to listen to one at a time and compare them. For a mono track, I'd expect the 128 kbps version to sound virtually identical to the original. The difference between the original and the 64 kbps version may be just a little more noticeable, but still pretty good. At 32 kbps you will probably notice that it "sounds like an mp3". 16 kbps sounds pretty bad to me, and 8 kbps is dreadful. When doing this you will notice that for very low bit-rates, you will be prompted to select a lower sample rate - the LAME encoder will then convert the sample rate on the fly as it converts.