RMS Level
Root Mean Square level measures the average perceived loudness of an audio signal, with ACX requiring audiobook chapters to fall between -23dB and -18dB RMS.
RMS (Root Mean Square) level is a measurement of average audio loudness over time. Unlike peak level, which captures the single loudest sample, RMS reflects how loud audio actually sounds to the human ear. It does this by squaring all the sample values, averaging them, and taking the square root, which gives more weight to sustained energy than to brief transients.
For audiobook production, RMS level is one of the most important technical specifications. ACX (Audible’s publishing platform) requires each chapter to have an RMS level between -23dB and -18dB. Falling outside this range means your submission will be rejected during quality review. Too quiet and listeners will struggle to hear narration over background noise. Too loud and the audio becomes fatiguing and risks clipping on playback devices.
Achieving consistent RMS across an entire chapter requires attention to microphone technique, distance, and gain staging. Narrators who vary their distance from the mic or shift their projection significantly between quiet dialogue and loud passages will see wider RMS swings. Many narrators apply gentle compression during mastering to bring their RMS into the target range, but the best results come from consistent recording technique in the first place.