Noise Floor
The level of background noise present in a recording when no one is speaking, with ACX requiring -60dB or lower for audiobook submissions.
The noise floor is the level of ambient sound captured by your microphone when you are not speaking. It includes everything from air conditioning hum and computer fan noise to electrical interference in your signal chain and the self-noise of the microphone itself. In audiobook production, the noise floor must be low enough that listeners do not hear distracting background sound during pauses between sentences and paragraphs.
ACX requires a noise floor of -60dB or lower for all audiobook submissions. This is measured during a section of room tone, a deliberate silent passage at the head of each chapter where the microphone captures only the ambient environment. Failing this check is one of the most common reasons for ACX rejection, especially among home studio narrators who may not have adequately treated their recording space.
To achieve a clean noise floor, start with your recording environment: close windows, turn off HVAC, silence phones, and move away from noisy electronics. Acoustic treatment with absorption panels reduces reflections and standing waves. On the equipment side, choose a microphone with low self-noise, use a quality audio interface with clean preamps, and set your gain so your voice peaks around -6dB to -3dB without pushing the preamp into a noisy range. A quiet room and proper gain staging will get you well below -60dB without any need for noise reduction processing.