A podcast is usually built from voices, and a voice recorded on one microphone is a mono signal. Publishing it as stereo doubles the channels without adding information; publishing it as mono saves data but changes how loudness is measured. This guide explains when each choice makes sense and what Apple Podcasts, Spotify, YouTube and ACX expect.
What mono and stereo mean in a file
A mono file has one channel. A stereo file has two, left and right. If both stereo channels carry identical audio, the result is called dual mono: it sounds the same as mono played on two speakers, but takes twice the data to store uncompressed. Real stereo carries differences between the channels, such as a voice panned to one side, a wide music bed or room ambience captured with two microphones.
Players generally send a mono file to both earbuds or speakers, so listeners hear the voice in the centre either way.
When mono makes sense
- Every voice was recorded on its own mono microphone, and nobody is panned.
- The show is talk only, without music beds or sound design that relies on width.
- You want the smallest files for a given quality, for listeners on limited data plans.
When stereo is worth it
- Music is a real part of the show, not just a short intro.
- Sound design, field recordings or ambience use the stereo width.
- Voices are panned deliberately, for example in fiction or drama.
- You upload WAV or FLAC to Apple Podcasts Connect, which requires two channels (see below).
Bit rate and file size
Apple's audio requirements give the same bit-rate ranges for AAC and MP3, with the mono range at half the stereo range:
| Channels | 22.05 or 24 kHz | 44.1 or 48 kHz |
|---|---|---|
| Mono | 40–80 kbps | 64–128 kbps |
| Stereo | 80–160 kbps | 128–256 kbps |
File size follows bit rate directly. One hour at 64 kbps is 64,000 bits per second × 3,600 seconds ÷ 8, about 29 MB; the same hour at 128 kbps is about 58 MB. Following Apple's ranges, a mono episode needs about half the data of a stereo one.
Apple strongly recommends AAC over MP3 for episodes in RSS feeds. For Apple Podcasts Connect, which handles subscriber audio uploads, Apple recommends 96–128 kbps for mono MP3 and 128–256 kbps for stereo MP3. Codec choices are covered in podcast and audiobook file formats.
Loudness: -16 or -19 LUFS?
This is where mono gets confusing. ITU-R BS.1770 measures loudness by adding up the energy of each channel. A mono file has one channel; the dual-mono version of the same audio has two. So the mono file measures 3.01 LU lower than its dual-mono copy, even though both sound the same when the mono file is played on two speakers.
That leaves two ways to master a mono episode:
- -19 LUFS, measured as mono. On two speakers it plays at about the same level as a -16 LUFS stereo episode. This is the common convention. Its documented origin is Google's loudness guidance for Assistant content, now deprecated, which recommended -16 LUFS for stereo and -19 LUFS for mono because the two "need to be offset by 3.01 LUFS".
- -16 LUFS, measured as mono. It reads exactly -16 on a single-channel BS.1770 meter, but on players that do not compensate it plays about 3 LU louder than stereo episodes at -16.
Apple publishes one figure, -16 LKFS ±1 dB, with no separate number for mono. AES TD1008 notes that mono content played on two channels "will measure 3 LU higher" and recommends that players use channel-mode metadata, but it gives no -19 figure, so -19 should not be credited to the AES. EBU Tech 3344 places the mono compensation in the playback device rather than in the file, but you cannot count on every player applying it. If you want a mono episode to sit level with stereo shows at -16, -19 LUFS is the convention to follow. What loudness should a podcast be covers the wider picture.
Platform channel rules
Apple Podcasts
Episodes in RSS feeds can be mono or stereo, in AAC or MP3. For Apple Podcasts Connect uploads, WAV and FLAC files must be stereo: Apple states that "single-channel audio will not be accepted for WAV or FLAC files." If your master is mono, duplicate it to two identical channels before exporting WAV or FLAC for Connect. MP3 uploads to Connect can be mono.
Spotify
Spotify for Creators accepts audio episodes "either in mono or stereo". For video episodes, Spotify's specs call for stereo audio and upmix mono.
YouTube
YouTube's recommended upload settings list 128 kbps for mono and 384 kbps for stereo audio, with AAC-LC or Opus at 48 kHz.
ACX audiobooks
ACX requires all files in a title to share one channel format, all mono or all stereo. The current requirements page does not prefer either; the "strongly recommend mono" advice comes from a 2014 ACX blog post. If you deliver stereo, note that ACX does not accept joint stereo MP3 files, so export plain stereo.
Converting between mono and stereo
- Stereo to mono: mix the two channels into one, then listen. If two microphones picked up the same voice at different distances, the mix can sound hollow or phasey. If one channel is silent or only noise, keep the good channel instead of mixing.
- Mono to stereo: duplicate the channel, then measure again. The stereo file reads about 3 LU higher, as AES TD1008 points out.
A quick decision
- Voice-only show, one microphone per person, nobody panned: mono at -19 LUFS works well, or stereo at -16 LUFS if your workflow is built around stereo.
- Music, sound design or panned voices: stereo at -16 LUFS.
- WAV or FLAC for Apple Podcasts Connect: stereo, duplicating a mono master if needed.
- Audiobook for ACX: either, as long as every file in the title matches.
LufsReady has separate podcast presets for both cases: stereo at -16 LUFS, and mono at -19 LUFS delivered as 128 kbps MP3, both with a -1 dBTP ceiling. You can keep the original channels, mix down to mono or output stereo when you upload. To see how your current files measure, try the free LUFS meter, or compare targets in the loudness targets reference.
Sources
- Apple Podcasts for Creators: Audio requirements
- Google Assistant: Audio loudness guidelines (deprecated)
- AES TD1008: Loudness guidelines for internet audio streaming and on-demand distribution (archived copy)
- EBU Tech 3344: Practical guidelines for distribution systems in accordance with EBU R 128
- ITU-R BS.1770: Algorithms to measure audio programme loudness and true-peak audio level
- Spotify for Creators: Publishing audio episodes
- Spotify for Creators: Video specs
- YouTube Help: Recommended upload encoding settings
- ACX: Audio Submission Requirements
- ACX: Audio terminology glossary
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