Noise floor is the level of background sound during silent passages of your recording, measured in dBFS. ACX (Audiobook Creation Exchange) requires the noise floor to sit below -60 dBFS in the final submitted file.[1]
Noise floor is the third of the three ACX audio requirements that cause automated rejection, alongside RMS loudness and true peak. It includes everything your microphone picks up when you are not speaking: HVAC hum, computer fans, electrical interference, and room ambience. ACX measures this level during the silent sections of your chapter file.[1]
Why does louder audio fail ACX for noise floor?
The noise floor paradox catches many narrators out. To meet ACX loudness requirements, you raise the gain on your recording. Raising the gain raises the noise floor by the exact same amount.
If your raw recording sits at -63 dBFS and you add 6 dB of gain to reach the required loudness, your noise floor lands at -57 dBFS. That is a fail. The raw file passed the noise floor check, but the mastered file did not. The mastering process raised everything, including the noise.
| Step | Raw level | After gain to -20 dBFS RMS | Pass/Fail |
|---|---|---|---|
| Clean recording | -70 dBFS | -62 dBFS (+8 dB gain) | Pass |
| Average home studio | -65 dBFS | -57 dBFS (+8 dB gain) | Fail |
| Noisy environment | -55 dBFS | -47 dBFS (+8 dB gain) | Fail |
How much noise can you afford in your gain budget?
ACX's noise floor limit is -60 dBFS.[1] The 60 dB figure is not arbitrary. It is the level below which continuous background sound stops being audible to a listener over spoken narration at normal playback volume. At -50 dBFS, hum or hiss is clearly present between sentences. At -60 dBFS, most listeners cannot hear it even in quiet passages.
Every decibel of gain you apply during mastering moves the noise floor up by the same amount. The gain budget matters. If you need 8 dB of gain to reach the -23 to -18 dBFS RMS loudness window, your raw noise floor must start below -68 dBFS. The maths is inescapable. This is why narrators with clean, quiet recordings still fail after mastering.
How do you measure your noise floor?
Measure after mastering, on the final MP3 you will submit. Select 2 to 3 seconds of silence. Choose a natural pause between sentences, not the very start or end of the file where artificial silence might have been added. Measure at least three different silent passages and use the worst (highest) value. If that number is above -60 dBFS, your file will fail.[1]
| Audio Editor | Measurement Path |
|---|---|
| Audacity | Select silence > Analyse > Contrast > Measure selection |
| Reaper | Select silence > Actions > Calculate loudness of selected items |
| Adobe Audition | Select silence > Window > Amplitude Statistics > Scan Selection |
| Logic Pro | Insert Multimeter plugin on master bus > Play silent section > Read RMS |
If you would rather not open a DAW, the Free ACX Check reports the noise floor of any file you drop in, against the ACX target, directly in your browser.
What are the common noise sources in home studios?
Most narrators record at home. These are the noise sources that push noise floors above -60 dBFS after mastering:
HVAC and air conditioning. The most common offender. Central heating, air conditioning, and fans produce continuous broadband noise between -55 and -45 dBFS depending on the room. Turn them off during recording. If you cannot due to climate constraints, record during the quieter part of the cycle.
Computer fans. Desktop computers are loud. Even laptops produce audible fan noise when running audio software. Position your microphone as far from the computer as your cable allows, or move the computer outside the recording space. Some narrators record on a laptop with fans spinning up during the session. If possible, pre-cool the room, then record in short bursts before the laptop heats up.
Electrical hum. A 50 Hz or 60 Hz hum from poorly shielded cables, ground loops, or nearby power supplies. Use balanced cables (XLR) between your microphone and interface. Keep audio cables away from power cables. A hum filter can help, but prevention is better.
Room reflections. Not technically noise floor, but reflections make noise reduction less effective because the reflected signal has a different frequency profile from the direct noise. Soft furnishings, blankets on hard walls, and commercial acoustic panels all help. The goal is not a dead room, just one where reflections do not colour the sound noticeably.
External noise. Traffic, neighbours, construction, weather. These are intermittent and harder to manage. Record during the quietest hours. Close windows. If your building has thin walls, a closet full of clothes is genuinely one of the best recording environments available.
Why does noise reduction often make audiobooks sound worse?
The instinct is to reach for noise reduction. Audacity has one built in.[3] It works by capturing a noise profile from a silent section, then subtracting those frequency bands from the entire recording.
The problem is that it subtracts from the entire signal, not just the silent parts. Your voice passes through the same filter. At moderate settings, this can sound acceptable. At the settings needed to pull a -57 dBFS floor down to -60 dBFS, it creates metallic, watery vocal artefacts that listeners notice immediately.
There is also the opposite problem. Aggressive noise reduction can push the noise floor too low. Passages that should contain natural room tone become unnaturally dead. Reviewers flag this as over-processing, and it can trigger a quality rejection even though the specs technically pass.
Broadband noise reduction treats every sample the same regardless of whether it contains speech. That is the root of both problems.
What is the difference between noise gating and noise reduction?
These are fundamentally different tools solving different problems.
Noise reduction analyses frequency content and subtracts noise bands from the entire recording. It alters your voice because your voice occupies some of the same frequencies as the noise.[3]
Noise gating reduces volume only during silent passages. When you are speaking, the gate is open and the signal passes through untouched. When you stop speaking, the gate closes and attenuates the silence.[2] Your voice is never processed.
A standard noise gate uses a fixed threshold. Below a set level, the signal gets reduced. This works but creates problems when the recording has dynamic range. Quiet passages may get gated when they should not. Loud breaths may pass through when they should not.
Adaptive gating takes this further. Instead of a fixed threshold, it classifies each audio block based on its characteristics and adjusts the gating per passage. This distinction between speech, breath, and background noise is what separates a tool that works on chapter one from one that works on every chapter.
What are the limits of noise gating?
Gating works on silent passages. It cannot separate noise from voice within the same audio block. If your recording environment is noisy enough that the background sound is audible under speech, improving the environment will give you better results than any post-processing.
A quieter recording space makes the biggest difference. Close the window, turn off the HVAC, move away from the computer, and treat the room with absorption.
For recordings that are clean but failing after mastering, the issue is almost always the gain budget. That is fixable. See fix ACX rejection for the step-by-step decision tree, or revisit the ACX audio requirements for how noise floor sits alongside loudness and peak.
What are the most common questions about audiobook noise floor?
Why does my noise floor get worse after mastering?
Because mastering raises the overall loudness, and loudness gain raises everything, including the noise floor. If you need 8 dB of gain to reach -20 dBFS RMS, your noise floor moves up by the same 8 dB. This is the gain budget problem. Manage noise before raising loudness.
What is a good noise floor for audiobooks before mastering?
Below -65 dBFS gives you headroom for loudness correction. Below -70 dBFS is comfortable. A professional treated booth typically measures -70 to -80 dBFS. A closet with soft furnishings is often -62 to -70 dBFS. These are approximations that vary with room size, construction, and equipment.
What is the difference between noise reduction and noise gating?
Noise reduction analyses frequency content and subtracts noise from the entire recording, including sections where you are speaking. Noise gating reduces volume only during silent passages and leaves your voice untouched. Gating is safer for audiobooks because it does not alter your narration.
Will Apple Books or Audible reject my file if the noise floor is too low?
Yes, they can. If you gate the silence to absolute zero (digital silence), it sounds unnatural and can cause audible clicks when speech resumes. ACX reviewers may fail the file for over-processing. Always leave a natural room tone floor, just ensure it sits below -60 dBFS.
Your first file is free at ChapterPass. You upload a file, and ChapterPass analyses it against every ACX requirement, manages the noise floor, and masters it to spec. The file stays in your browser, and you leave with audio ready to submit.