How to Fix a Noise Floor That Fails ACX
Noise floor is the single most common reason ACX returns an audiobook. The requirement is −60 dB RMS or quieter, measured in the gaps between words, and a surprising number of otherwise good recordings sit at −50 or −45.
This is how to work out where your noise is coming from and what can actually be done about it — including the cases where the honest answer is to re-record.
First, measure it properly
Install the free ACX Check plug-in for Audacity. Select a section of silence — two or three seconds of room tone with no speech, no breath, no page turn — and run the check on that selection alone.
Running it on the whole file measures the average of speech and silence together, which tells you nothing useful about the noise floor. Measure the quiet parts on their own.
Note the number before you change anything. The gap between where you are and −60 dB decides which of the fixes below is realistic.
Find out what the noise actually is
Different noise needs different treatment, and the wrong treatment makes things worse.
Steady hum at 50 or 60 Hz
Electrical. Usually a ground loop, a dimmer switch, or a power supply too close to the microphone cable. This is the easiest kind to remove in post, because it sits at a known frequency and its harmonics are predictable. A notch filter takes it out with almost no effect on the voice.
Better still, find it at the source: unplug things one at a time while recording and listen back.
Broadband hiss
This is preamp noise or air movement. It covers the whole frequency range, which is why it is much harder to remove without taking some of the voice with it.
The usual cause is gain. A quiet microphone pushed to high gain amplifies its own noise along with your voice. A dynamic microphone on a cheap interface is the classic combination.
Air conditioning, fans, traffic
Low-frequency rumble, mostly below 100 Hz, sometimes with a steady mid-range component. A high-pass filter at 80 Hz removes the rumble safely — there is nothing in a human voice below that worth keeping. The mid-range part is harder.
Computer fans
Often overlooked because you stop hearing them. If your machine is in the same room as the microphone, it is in the recording.
Fix the room before you fix the file
Every decibel you remove at the source is a decibel you do not have to fight in post, and source fixes cost nothing in audio quality.
- Turn off the air conditioning while recording. Record in shorter sessions if the room gets warm.
- Move the computer out of the room, or at least behind you and as far away as the cable allows.
- Get closer to the microphone. Doubling your distance from the mic costs about 6 dB of signal, and you make that up with gain — which raises the noise floor by the same 6 dB. Speaking at 15 cm instead of 30 cm is worth more than any plug-in.
- Soften the room. Duvets, a wardrobe full of clothes, heavy curtains. This reduces reflections rather than noise, but reflections are what make noise reduction sound bad.
- Record at a quieter time. Traffic and neighbours follow a daily pattern.
What can be fixed in post, and what it costs you
Here is the part most guides skip: noise reduction is not free. It works by identifying the noise and subtracting it, and the subtraction takes some of the voice too.
| Starting noise floor | Realistic outcome |
|---|---|
| −55 to −60 dB | Passes with gentle processing. No audible cost. |
| −50 to −55 dB | Passes. Slight loss of air on sibilants if pushed carelessly. |
| −45 to −50 dB | Can be made to pass, but the voice starts to sound processed. Judgement call. |
| Above −45 dB | Passing the measurement means failing the quality review. Re-record. |
That last row is the one worth taking seriously. ACX rejects for over-processing as readily as for noise, and a book that passes the numbers while sounding thin, phasey or underwater comes back anyway — just with a vaguer rejection note.
The right order of operations
Doing these in the wrong order is why some files never seem to pass.
- High-pass filter at 80 Hz. Removes rumble that would otherwise be amplified in the next steps.
- Notch out any hum at 50 or 60 Hz and its harmonics, if present.
- Noise reduction, gently. Capture a noise profile from clean room tone. Start at around 6 dB of reduction, not the default. Listen to the voice, not the silence.
- Set RMS last. Raising the file to −18 dB RMS raises the noise floor by the same amount, so this has to come after the cleaning, never before.
- Check peaks. Limit to −3 dB.
- Re-measure the noise floor. Step 4 may have pushed it back over the line.
If step 6 fails after step 4, you do not have enough margin. Go back to the room.
A note on noise gates
A gate silences everything below a threshold, which makes the measurement pass instantly. It is also the fastest way to get rejected.
Gating produces digital silence between phrases, which ACX explicitly fails, and it chops the natural tails off words. The result is unmistakable to a reviewer who listens to audiobooks all day. Use a gate for nothing in audiobook work.
When to stop and re-record
If your noise floor is above −45 dB, the honest advice is to fix the room and record again. A re-recorded chapter takes a few hours. A book that fails review twice costs weeks of calendar, and the second rejection is harder to diagnose than the first.
The full specification, including RMS and peak, is covered in our guide to ACX audio requirements.
If you would rather not make that call yourself, send us a sample. Our audiobook editing service measures the file and tells you plainly whether it can be saved in post or needs re-recording — before you commit to a full production.