Podcast audio optimization

Podcasting is a popular way to quickly get content onto your website, but audio files can be big and file storage limitations are a reality. On the blogs here the file uploads are limited to 20MB per file with a 100MB total file storage quota per blog site.

Recently a colleague wanted to upload some podcasts that were hour long recordings, each weighing in at about 40MB each. Not only is that twice the size of the single file size limit, with files that size they were going to run into the overall quota limit after only two podcasts!

These podcasts are teleconference recordings of people talking, so high fidelity audio isn’t a priority–we’re not trying to reproduce the full stage presence of the Boston Philharmonic–and they were a prime candidate for heavy optimization. After doing some quick optimization tests, I managed to get a 37MB file down to almost 7MB!

That’s an 80% savings! And it was done in just a few minutes with free software.

Tools

Audacity – Free, open source audio editing software that works on Windows, Mac, and Linux. I used the 1.3.12 (beta) version for this.

LAME MP3 encoder – This is required for saving MP3 files. (Don’t worry about the name. It’s a geek joke.)

Optional: QuickTime Player – Useful to see the encoding settings of audio files.

Quick Encoding Definitions

Sample rate – The number of samples per second taken during the recording process. It’s measured in Herz (Hz). A sample rate of 11025Hz means there about 11,025 samples taken per second of the audio. This is equivalent to speech on a phone. CD quality is 44100Hz.

Bitrate – The amount of data/space used in computer memory to store the audio in a given second. Measured in kilobits per second (kbps), 24 to 50 kbps is good for speech on a phone. CD quality is 128kbps.

Quick and Easy Steps to Optimization with Audacity

Open Audacity and from the file menu, open your audio file.

At the bottom of the window, there is a setting for Project Rate (Hz). This is the sample rate of the audio. For simple spoken voice like a teleconference, 11025Hz is good, so select that from the menu. (In older versions of Audacity, this is available under Preferences > Quality > Default Sample Rate.)

Next we’re going to convert this from a stereo recording to a mono recording. For simple speech like this there’s no need for stereo, so why double the file size? Next to the waveform there is a down triangle next to the name that is a menu. Open that menu and select “Split Stero Track to Mono”.  This makes the right and left tracks independently editable.

Now simply click the “X” next to the second track name to delete the extra audio track.

Select File > Export…

Give your new podcast file a filename, select MP3 from the audio type, and click the “Options” button.

Here are the last critical settings for a nice small audio file. Set Bit Rate Mode to Variable. Quality to “9, 45-85 kbsp (Smaller files)”. Variable Speed: Standard. Channel Mode: Joint Stereo.

Once you’re done, click OK and then Save to finally export your new, and hopefully much smaller, MP3 file!