mp3 (MPEG Layer 3) tips for Podcasting

Mp3 (MPEG Layer 3) tips for PodcastingFollow these detailed tips and recommendations when making MP3s for podcasting to maintain high sound quality while avoiding possible playback issues.

mp3 Basic Tips

  • Do not encode in Variable Bit Rate (VBR). Use a constant bit rate (CBR) such as 64Kbps, 96Kbps, or 128kbps.
  • Use either 22.05kHz or 44.1kHz (44.1kHz  is CD quality and recommended) sample rate. Other sample rates will playback in some players incorrectly, typically making you sound like one of the Chipmunks.
  • Make sure your files have the appropriate file extensions (e.g. file.mp3). Applications such as Apple Podcasts and others determine the file content type based on the file extension.
  • Only use the 26 English letters, 0-9 numbers and underscore/dash characters for file names and folders. Some applications will not capture the special characters — such as dollar signs, blank spaces, or commas — correctly from the filename, which could make the saved file unplayable on the target device.
  • Make sure you write ID3 tags to the mp3 files so artwork appears during playback on devices and in Apple Podcasts.
  • “Normalize” your audio to make sure each recording has the same volume level. (Use the same normalized level for every episode.)
  • Edit and save your audio in the audio editor’s recommended format (not directly as an mp3 as the quality will change) and only export to mp3 when you are done.

mp3 Bit Rates in More Detail

The bit rate has a direct collaboration to the mp3’s file size, every increase to the bit rate will increase the size of the file. For example, going from 64 to 128kbps will double the file size. For that reason, it is important to pick the bit rate that is appropriate for the content. See the list of minimum bit rates below as a guide.

64Kbps – AM radio quality, best for talk only with the exception of speakers who have very soft or deep voices. Record in mono to maximize the sound quality.

96Kbps – FM radio quality, best for talk with enough quality to play bumpers and music. Record in mono to maximize the sound quality.

128Kbps – Compact disk (CD) quality, best for distributing recordings with music. This is a good starting point for recording in stereo.

The values above are a guide and reflect the results using the popular open source Mp3 LAME encoder. Better quality encoders might provide better quality with lower bit rates. Refer to the software’s manual for recommendations, or experiment with different bit rates until you find the quality you desire.

Can’t decide? Recommendation: 128 kbps minimum (stereo) or 96 Kbps minimum (mono)

Variable (VBR) vs Constant (CBR) Bit Rate in mp3’s

As noted above, never use a Variable Bit Rate (VBR) for podcasting. Reasons to avoid a VBR include:

  • Some hardware and software may not play VBR media correctly, if at all
  • Streamed playback (playing within a web browser) complications. Firefox browser can have issues playing VBR mp3 files.
  • Many players display the duration (total time in seconds) wrong for VBR media. Sometimes this appears as a constantly changing value during the loading of the media file in the media player.
  • Older versions of Flash media players (commonly found on older Web sites) cannot play VBR media files

Can’t decide? Required: Constant Bit Rate

Typical File Sizes from Bit Rate in mp3’s

Below is a list of typical recording bit rates and file sizes.

10-minute recording
64 kbps ~ 5MB
96 kbps ~ 8MB
128 kbps ~ 10MB

30-minute recording
64 kbps ~ 15MB
96 kbps ~ 23MB
128 kbps ~ 30MB

1-hour recording
64 kbps ~ 30MB
96 kbps ~ 45MB
128 kbps ~ 60MB

Larger file sizes negatively impact your file hosting costs, while lower bit rates can impact the quality of your recordings. Picking the right bit rate is a balance between quality and what you can afford.

As a rule of thumb, a 64Kbps file is about 0.5MB per minute, a 96Kbps file is about 0.75MB per minute, and a 128Kbps file is about 1MB per minute. If you do decide to use a larger bit rate, remember that playback when streaming from a Web page will be further delayed until enough of the file is downloaded, which might make some of your listeners impatient.

mp3 Sample Rate

Picking the sample rate is easy. The sample rate is the number of samples per second, the more samples the clearer the sound will be. If you’re using a cheap microphone or tape recorder for example, using either 22,050 and 44,100 will not make much of a difference. A typical digital video camera or Flip-type camera will record somewhere between the two sample rates, in that case you want to use 44,100 kHz. If you record at CD quality (most of us do), 44,100 kHz will always sound better than 22,050 kHz.

Can’t decide? Recommendation: 44,100 kHz

Mono, Stereo and Joint Stereo mp3’s

In the past, we only recommended using either “stereo” or “joint stereo” for your media. As of 2012, we believe “mono” can now safely be used as well. Mono is ideal if you are not concerned about stereo sound in your episodes as it will result in your media files being half the size in bytes compared to stereo.

Older versions of Flash, a handful of generic brand mp3 players and first generation iPods will not play mono mp3 files. Flash issues with mono files were solved sometime between 2008-10 and the first generation iPod is now 10+ years old (sold from 2001-2003). We believe that saving your media in “mono” no longer poses an issue.

If you use professional audio recording software to create your mp3 files, you can use the “joint stereo” feature to optimize the file size and/or sound of your mp3. If you are using the LAME mp3 encoder with your audio software, using “joint stereo” or “stereo” will not make a difference with your media file size, but the sound quality will be improved. With the LAME encoder specifically, you must use the option “forced joint stereo” if your goal is to both improve the sound and shrink the file size. If your bit rate is more than 128Kbps, the sound quality improvements and file size difference using joint stereo are minimal at best.

Last thing to remember is the difference between mono and stereo. Think of it as two separate recordings that play at once. Two 128kbps recordings together make 256kbps. When converting a recording from mono to stereo, or stereo to mono, keep in mind that you either need to double or half the bit rate as well.

Still Confused? Here’s What We Suggest

If you’re recording from one microphone (in mono): use “mono” (perfect for interviews and talk)

If you’re recording in stereo and using bit rate lower than 128kbps: use “joint stereo” (ideal for talk recorded in stereo and some music)

If you’re recording in stereo and using 128kbps or higher bit rate: use “stereo” (perfect when sound quality is of the utmost importance)