Peak: the loudest single instant
Peak is the simplest measurement and the least useful on its own — it only reports the single highest point the waveform reaches. Two files can share an identical peak and sound completely different in overall loudness, since peak says nothing about everything else in between.
RMS: the average level
RMS (root mean square) averages the signal's level over time, which represents loudness far better than peak alone. It's still a raw measurement of the waveform rather than something tuned to human hearing — which is exactly why ACX specifies RMS directly for audiobooks: it needs a fixed, technical number to check files against, not a perceptual estimate.
LUFS: loudness tuned to how you actually hear
LUFS (Loudness Units relative to Full Scale) weights the signal by frequency to match how sensitive human hearing actually is at each one, then averages that over time. It's the standard modern streaming and podcast platforms use, because they're normalising perceived loudness across a huge catalogue of very different content — music, speech, sound effects — where a raw RMS number wouldn't compare fairly across all of it.
Why different platforms specify different ones
Audiobook platforms check individual files against fixed technical limits, so RMS and peak make sense — they're simple, objective and easy to enforce. Podcast and streaming platforms are normalising playback across millions of unrelated files in real time, so LUFS — built specifically to estimate perceived loudness — is the better tool for that job.
Quick reference
- Audiobooks (ACX): RMS between -23 dB and -18 dB, peaks below -3 dB.
- Podcasts: around -16 LUFS integrated, true peak below -1 dBTP.
- YouTube and Spotify: normalise toward roughly -14 LUFS.
Doing it automatically
Optivox targets the right number for the job depending on the mode you choose — LUFS-based mastering for podcasts and general recordings, or ACX's RMS and peak targets specifically in Audiobook mode.