Understanding ACX Audio Standards

ACX (Audiobook Creation Exchange), the platform that connects authors, narrators, and publishers, imposes strict technical requirements to ensure a consistent, high-quality listening experience across all devices and distribution channels. Background noise is one of the most common reasons audiobook submissions are rejected. ACX specifies that background noise must be below −60 dBFS (decibels relative to full scale) throughout the recording. This noise floor includes any hum, hiss, electrical buzz, room echo, or ambient sounds—traffic, air conditioning, appliances, even your computer’s fan. Meeting this requirement demands careful attention at every stage: preparation, recording, and post-processing.

The Noise Floor: What −60 dBFS Really Means

The −60 dBFS threshold is extremely quiet—about the level of a barely audible whisper in a dead‑silent space. In practical terms, if you record silence in your environment, the noise you capture should be at least 60 dB below the loudest peaks of your narration. Most digital audio workstations (DAWs) have a level meter; you can record a few seconds of silence in your space and observe the RMS (root mean square) level. If it hovers around −70 dBFS or lower, you’re safe. If it creeps above −60 dBFS, your submission risks rejection.

ACX also requires that the audio have an average RMS level between −23 dBFS and −18 dBFS, with peak levels not exceeding −3 dBFS. Noise below that −60 dBFS floor ensures it stays hidden beneath the narration and doesn’t distract listeners. For a deeper dive into ACX standards, refer to the official ACX Audio Submission Requirements.

Setting Up Your Recording Space for Minimal Noise

Your environment is the single biggest factor in noise control. A treated room can make an affordable microphone sound professional; an untreated one can ruin even the most expensive equipment.

Choosing the Right Room

Avoid large, open, or echo‑prone spaces like living rooms or kitchens. Small closets filled with clothes act as natural sound absorbers. Walk‑in closets, carpeted bedrooms with heavy curtains, or even a dedicated vocal booth are ideal. Key qualities:

  • Minimal hard surfaces: Hard walls, floors, and windows reflect sound and create reverb that contributes to noise.
  • No mechanical sounds: Turn off HVAC systems, refrigerators, computers, and any buzzing electronics. Record during low‑traffic hours (late night or early morning).
  • Isolation from outside: Heavy drapes, weatherstripping on doors, and acoustic sealant can block street noise.

Acoustic Treatment Options

You don’t need to build a professional studio. Affordable DIY solutions work:

  • Fabric panels and moving blankets: Hang them on walls and over windows to absorb mid and high frequencies.
  • Bass traps: Place in corners to reduce low‑frequency rumble (e.g., from traffic or heating systems).
  • Reflection filters: Portable panels that mount behind your microphone to block reflections from your immediate surroundings.
  • Carpet and rugs: Cover hard floors to minimize footfall noise and slap echo.

If you’re looking for a comprehensive guide to DIY acoustic treatment, see this detailed resource: Sound on Sound’s Beginner’s Guide to Acoustic Treatment.

Monitoring Your Environment

Before you begin a session, listen critically with closed‑back headphones. Walk around the room to identify intermittent noises—a creaky chair, a ticking clock, a nearby elevator. Record a 30‑second silence and examine the waveform and spectrum. Use a spectral analyzer (most DAWs have one) to see exactly where noise lives. Common culprits: 60 Hz electrical hum (or 50 Hz in some countries), high‑frequency hiss from computer fans, and low‑frequency rumble from HVAC systems.

Equipment Choices That Reduce Noise

Your gear chain—microphone, preamp, audio interface, cables—significantly affects how much noise you capture.

Microphone Selection: Condenser vs. Dynamic

Condenser microphones (e.g., Rode NT1, Audio‑Technica AT2020) are highly sensitive and capture detail, but they also pick up more ambient noise and can be prone to self‑noise (the mic’s own electronic hiss). Dynamic microphones (e.g., Shure SM7B, Electro‑Voice RE20) have lower sensitivity and reject off‑axis sound better, making them excellent for untreated spaces. Many professional audiobook narrators prefer dynamic mics for home studios because they naturally reduce room noise.

Whichever type you choose, ensure its self‑noise specification is as low as possible. A good condenser will be around 4–6 dB SPL (sound pressure level) equivalent noise; a dynamic is often even quieter in that regard.

Positioning and Proximity Effect

Place the microphone 6–12 inches from your mouth, slightly off‑axis to reduce plosives. Too far away, and you’ll capture more room ambience; too close, and the proximity effect boosts low frequencies, potentially amplifying low‑frequency noise from your environment. Use a pop filter to block breath pops and sibilance that can be mistaken for noise.

Preamp and Interface Quality

A noisy preamp adds hiss that pushes your noise floor up. Invest in a clean audio interface (e.g., Focusrite Scarlett, Universal Audio Apollo, or RME Babyface) with good preamps and high‑gain headroom. Keep gain levels appropriate—too low and you’ll raise the noise floor when you later normalize; too high and you risk clipping. Aim for peaks around −6 dBFS during recording.

Recording Techniques to Avoid Noise

Proper Gain Staging

Set your preamp gain so that the loudest part of your narration hits about −6 dBFS on your DAW meter. This leaves headroom and keeps the signal well above the noise floor. Recording too quietly forces you to boost levels in post‑processing, which amplifies any noise captured.

Recording in Short Takes vs. Long Sessions

Long takes increase the chance of capturing intermittent noises—a car horn, a door slam, a cough. Record in manageable chunks (3–5 minutes) so you can easily retake sections that have detectable noise. This also makes editing more efficient and reduces the cumulative fatigue that can cause you to miss noise during monitoring.

Handling Plosives and Sibilance

These aren’t background noise per se, but they can mask low‑level hums and hisses when you apply noise reduction later. Use a pop filter and practice consistent mic technique. If sibilance is strong, try a de‑esser plugin during editing rather than relying solely on noise reduction.

Post‑Processing Noise Reduction

Even with the best preparation, some noise may remain. Post‑processing tools can salvage a recording, but careless use can create artifacts—metallic sounds, “underwater” effects, or pumping—that are worse than the original noise.

Using Spectral Editing

Tools like iZotope RX (popular among audiobook professionals) or Audacity’s built‑in spectral selection let you visualize noise and remove it selectively. For a fixed noise profile (e.g., constant hum), sample a few seconds of room tone and use a noise reduction effect to subtract that profile. In Audacity, go to Effect > Noise Reduction, capture the noise profile, then apply with moderate settings (noise reduction: 12–18 dB, sensitivity: 6–12 dB). For an excellent walkthrough, see this Audacity Noise Reduction Guide.

Advanced Plugin Workflows

If you can budget for it, iZotope RX’s Voice De‑noise module is industry‑standard for spoken word. It automatically adapts to changing noise profiles and rarely introduces artifacts. For budget users, plugins like Brusfri or Accusonus ERA (now part of Meta‑Sound) also offer easy one‑knob noise removal.

Balancing Noise Reduction and Artifacts

Rule of thumb: never try to remove more than 20–25 dB of noise in a single pass. If the noise floor is at −45 dBFS and you need it below −60 dBFS, you’re asking for trouble. Instead, tackle it at the source: better acoustic treatment, closer mic placement, or a different microphone. Noise reduction should only clean up the last few decibels.

Final Checks Before Submission

Before you export your final file, run it through ACX’s free ACX Checker Tool. This plugin for DAWs (or standalone) validates that your audio meets all technical requirements, including noise floor, RMS level, peaks, and sampling rate.

ACX Audio Submission Guidelines – Key Points

  • File format: WAV at 96 kHz, 24‑bit or MP3 at 192 kbps, constant bit rate. 96 kHz is preferred for mastering; ACX will convert if needed.
  • Noise floor: Must be −60 dBFS or lower throughout.
  • RMS level: −23 dBFS to −18 dBFS average.
  • Peak level: Must not exceed −3 dBFS.
  • Uniformity: No volume jumps between chapters; loudness should be consistent.
  • No clipping or distortion.
  • Approved intro and outro: Exactly “Chapter [number]” at the start and “End of Chapter [number]” at the end (unless directed otherwise).

Common Mistakes and How to Avoid Them

  • Over‑processing: Applying too much compression or limiting can raise the noise floor as it brings up quiet sections. Use gentle compression (2:1 ratio, threshold around −20 dBFS).
  • Ignoring low‑frequency rumble: A high‑pass filter at 80 Hz can remove sub‑sonic noise without affecting vocals. Do this before noise reduction.
  • Inconsistent room tone: If you record in different sessions or locations, the background noise profile changes. Capture room tone for each session and apply noise reduction consistently.
  • Skipping the ACX Checker: This tool catches problems early. Run it after each chapter, not just at the end.

Conclusion

Achieving a noise floor below −60 dBFS is challenging but entirely feasible with a methodical approach. Invest time in your recording environment, choose gear that suits your space, and master a few post‑processing techniques. Narrators who consistently meet ACX standards build a reputation for reliability and produce audiobooks that listeners love. Remember: every hour spent optimizing your recording chain saves many hours of editing and resubmission later. With practice, clean audio becomes second nature.