ACX Rejection for Noise Floor: Causes, Measurement, and Fixes

Of all the ACX audio requirements, noise floor is the one that catches the most people off guard. Your recording room sounds quiet to you. But ACX's automated check says your noise floor exceeds -60 dBFS, and your audiobook is rejected. This guide explains exactly what's happening, how to measure your noise floor accurately, what causes it to exceed the limit, and specific fixes that bring it below -60 dBFS without introducing artefacts.[1]

What Is Noise Floor and Why Does -60 dBFS Matter?

The noise floor is the level of background sound present in your recording when nobody is speaking. It includes everything your microphone picks up that isn't the narrator's voice: room tone, HVAC systems, computer fans, electrical hum, traffic rumble, and the cumulative ambience of your recording environment.[1]

ACX requires this background level to stay below -60 dBFS, measured as the RMS level of silent sections in your file. To put this in perspective:

• -70 dBFS: Excellent. A well-treated professional studio.
• -65 dBFS: Good. A well-treated home studio with equipment off.
• -60 dBFS: The threshold. Marginal, you're at the edge.
• -55 dBFS: Failing. Audible noise in silent passages.
• -50 dBFS: Significantly failing. Clearly audible background noise.

On earbuds in a quiet room, the most revealing listening scenario, a noise floor above -60 dBFS produces a noticeable hiss or hum during pauses.

How Do You Measure Noise Floor Correctly?

Incorrect measurement is a common source of confusion. The precise method matters.[2]

Step 1: Find a Representative Silent Section

You need a section of your recording where no speech is present, at least 1 second long, ideally 2–3 seconds. This section should represent typical room tone, not an unusual moment. Don't use the very beginning or very end of the file, as these may have been padded with artificial silence, which gives a misleadingly low measurement.

Step 2: Select and Measure

In Audacity:

1. Select the silent section precisely (zoom in to verify no speech is included)
2. Go to Analyze → Contrast
3. Read the "RMS of selection" value. This is your noise floor
4. Alternatively, if you have the ACX Check plugin: Analyze → ACX Check, which measures the noise floor of the quietest 0.5-second window automatically[3]

In Adobe Audition:

1. Select the silent section
2. Go to Window → Amplitude Statistics
3. Read "Average RMS Amplitude"

Step 3: Measure at the Right Point in Your Signal Chain

This is crucial and the source of most confusion. Measure your noise floor on the final mastered MP3 file, not on the raw recording.

Why this matters: Your raw recording might have a noise floor of -65 dBFS. But if you boost gain by 8 dB during mastering to meet the RMS loudness target, your noise floor is also boosted by 8 dB, bringing it to -57 dBFS, above the limit.[4] Always verify noise floor on the final output.

What Are the Five Major Causes of Noise Floor Failure?

Cause 1: HVAC and Climate Control

Heating, ventilation, and air conditioning systems are the single most common source of noise floor problems. Central air produces a consistent broadband noise. Window units add mechanical vibration. Forced-air heating creates duct rumble.

The insidious part: HVAC noise is constant and uniform, which makes your brain tune it out. You literally stop hearing it. But your microphone doesn't, and ACX's measurement doesn't either.

How to identify: Turn off all climate control. Record 10 seconds of room tone. Turn it back on. Record another 10 seconds. Compare the RMS levels.

Fix: Turn off all HVAC during recording sessions. If this isn't practical, record in short sessions between climate control bursts.

Cause 2: Computer and Electronic Equipment

Computers generate noise from fans, hard drives, and power supplies. Monitors produce faint electronic whines.

Fix:

• Move the computer as far from the microphone as possible (a long USB cable lets you put it in another room)
• Use an SSD instead of a spinning hard drive (SSDs are silent)
• Run a laptop on battery power during recording to eliminate power supply fan noise
• Use dynamic microphones in noisy environments, as they're less sensitive to ambient noise than condensers

Cause 3: Electrical Interference and Ground Loops

Electrical hum at 50 Hz (Europe, Asia, Africa) or 60 Hz (Americas) is caused by electromagnetic interference from power cables, transformers, and grounding issues.

Fix:

• Use balanced (XLR) cables instead of unbalanced for microphone connections
• Keep audio cables away from power cables. Never run them parallel
• Plug all audio equipment into the same power strip to avoid ground loops
• A narrow notch filter at 50/60 Hz (and harmonics) can remove hum surgically in post-production

Cause 4: External Environmental Noise

Traffic, construction, aircraft, neighbours, barking dogs, wind, rain on the roof. These are sounds that originate outside your recording space.

Fix:

• Record during the quietest time of day (typically late evening or early morning)
• Add mass to walls and windows (heavy curtains, acoustic blankets)
• Seal gaps around doors and windows (weatherstripping, door sweeps)
• Record in an interior room with no exterior walls
• Use a microphone with a tight cardioid or hypercardioid polar pattern, aimed away from noise sources

Cause 5: Gain Boost During Mastering

This isn't a recording problem. It's a mastering problem. Your raw recording might have a perfectly acceptable noise floor, but meeting the RMS loudness target of -23 to -18 dBFS requires boosting the gain, which raises the noise floor proportionally.[4]

Example: Raw recording has -65 dBFS noise floor and -30 dBFS RMS. You need to boost by about 10 dB to reach -20 dBFS RMS. Your noise floor becomes -55 dBFS, above the limit.

Fix:

• Apply noise treatment before loudness adjustment in your mastering chain
• Use a noise gate to silence pauses before boosting overall gain
• Start with a cleaner recording (lower raw noise floor = more headroom for gain)

What Are the Specific Fixes for Noise Floor?

Fix 1: Noise Gate

A noise gate silences audio below a threshold. When the narrator speaks, the signal passes through. During pauses, the signal drops below the threshold and gets attenuated.

Audacity settings for ACX compliance:

• Gate threshold: -40 dBFS
• Level reduction: -60 dBFS or lower
• Attack: 10 ms
• Hold: 50 ms
• Decay: 100 ms

Pros: Clean, artefact-free silence during pauses. Simple to configure. Cons: Doesn't reduce noise during speech. Can create a contrast between "speech with background noise" and "dead silence in pauses", though this is usually acceptable for ACX.

Fix 2: Spectral Noise Reduction

Spectral noise reduction analyses a profile of your room noise and subtracts that noise pattern from the entire recording, both silent sections and speech sections.

Audacity procedure:

1. Select 1–2 seconds of pure room tone (no speech)
2. Effect → Noise Reduction → Get Noise Profile
3. Select all audio
4. Effect → Noise Reduction
5. Conservative settings: 6–8 dB reduction, Sensitivity 6, Frequency Smoothing 3[3]
6. Click Preview and listen critically for artefacts
7. If clean, click OK

Pros: Reduces noise during both speech and silence. Cons: Can introduce artefacts at high reduction levels: watery, metallic, or bubbly sounds. Multiple passes at low settings produce better results than one pass at high settings.

Fix 3: High-Pass Filter

Low-frequency noise below 80 Hz (rumble, HVAC vibration, traffic) contributes to noise floor but isn't audible as "noise" on most playback systems. Removing it lowers the measured noise floor without any audible change.

Settings: High-pass filter at 80 Hz, 12 dB/octave slope.

Pros: No artefacts. Removes energy that doesn't contribute to speech. Cons: Only helps if low-frequency energy is a significant portion of your noise.

Fix 4: Notch Filter (For Electrical Hum)

If your noise is primarily electrical hum at 50 or 60 Hz, a narrow notch filter removes it surgically.

Settings:

• Centre frequency: 50 Hz or 60 Hz (depending on your electrical system)
• Q (bandwidth): Narrow (30–50)
• Gain: -20 to -40 dB at the notch frequency

Also apply notches at harmonics: 100/120 Hz, 150/180 Hz, 200/240 Hz.

Pros: Removes hum completely with minimal impact on speech. Cons: Only effective for tonal noise (hum/buzz). No effect on broadband noise.

Fix 5: What Is the Best Combined Approach?

For most home studios, the optimal noise treatment combines multiple techniques:[3]

1. High-pass filter at 80 Hz: removes low rumble without artefacts
2. Notch filter(s) if hum is present: removes electrical noise surgically
3. Light spectral noise reduction (6–8 dB): reduces remaining broadband noise
4. Noise gate: ensures pauses are below -60 dBFS even after gain boost

Apply in this order, before loudness adjustment and peak limiting in your mastering chain.

How Do You Prevent Noise Floor Issues?

The best fix is prevention. Every decibel of noise you prevent in recording is a decibel you don't need to remove in post-production. Post-production noise removal always involves tradeoffs: artefacts, processing time, complexity. Clean recording does not.

Recording Environment Checklist

• ☐ HVAC off during recording
• ☐ Computer fans minimised (laptop on battery, desktop in another room)
• ☐ Refrigerator off (or in another room with door closed)
• ☐ Phone on silent (not vibrate)
• ☐ Exterior noise minimised (windows closed, curtains drawn)
• ☐ No fluorescent lighting (use LED or incandescent)
• ☐ Soft surfaces in room (carpet, curtains, bookshelves, acoustic treatment)
• ☐ Mic positioned away from noise sources
• ☐ Balanced (XLR) cables used for mic connection

Pre-Session Noise Floor Test

Before every recording session:

1. Set up your equipment as you would for recording
2. Record 30 seconds of silence (room tone only)
3. Measure the RMS of that recording
4. If it's above -65 dBFS, identify and address the noise source before narrating

Targeting -65 dBFS or lower in your raw recording gives you approximately 5 dB of headroom for gain boosts during mastering.

How Do You Verify Before Re-Submitting?

After applying noise treatment:

1. Measure noise floor on the final MP3 (not the intermediate WAV)
2. Verify it's below -60 dBFS
3. Re-check RMS loudness, as noise treatment can lower the overall level slightly
4. Re-check true peak. It shouldn't be affected, but verify
5. Listen critically for treatment artefacts (watery speech, metallic sounds, pumping)
6. Compare treated and untreated versions

If the treatment introduced audible artefacts, reduce the aggressiveness and re-process. A file that passes the noise floor check but sounds unnatural may get flagged by ACX's human quality review for recording quality issues.

The ACX audio requirements guide covers all specs in a systematic order. For the broader guide to fixing all types of rejections, see how to fix every ACX rejection issue.