Recording pristine audiobook audio for ACX (Audiobook Creation Exchange) compliance means conquering two of the most persistent audio gremlins: sibilance and plosives. These distortions not only jar the listener but also trigger ACX quality rejections. This guide goes beyond surface-level tips, offering a deep dive into practical, studio-proven techniques to eliminate sibilance and plosives, ensuring your narration meets ACX’s stringent technical standards and sounds professional.

Why Sibilance and Plosives Matter for ACX

ACX specifications demand clean, noise-free audio with no distracting artifacts. Sibilance—those harsh ‘s’, ‘sh’, ‘ch’, and ‘z’ sounds—causes listener fatigue and makes the audiobook sound amateurish. Plosives—the explosive energy from ‘p’, ‘b’, ‘t’, ‘d’, and ‘k’ consonants—create low-frequency thumps that overload the microphone and produce audible pops. Both defects can lead to ACX quality control flags, forcing you to re-record or edit problematic sections. Understanding the acoustics behind these issues is the first step to permanently fixing them.

Understanding the Physics of Sibilance

Sibilance occurs when high-frequency air turbulence from the teeth and tongue hits the microphone capsule directly. The energy is concentrated between 5 kHz and 8 kHz, often peaking around 6–7 kHz. This frequency range is where the human ear is most sensitive, making even mild sibilance stand out. Poor microphone technique, a bright-sounding microphone, or a reflective room can amplify these frequencies. The goal is not to eliminate all ‘s’ sounds—which would make speech unnatural—but to tame the harsh peaks.

Microphone Technique: The Core Solution

Your microphone positioning is the single most effective tool against sibilance. Place the microphone at a 15–30 degree angle off-axis to your mouth—not directly in line. This angle reduces direct high-frequency air blasts hitting the capsule. Also, maintain a consistent distance of 6–8 inches from the mic. Too close and plosives and sibilance intensify; too far and you lose clarity and increase background noise. Experiment with the mic height: positioning the capsule slightly above or below mouth level can reduce sibilance further without affecting tone quality.

Choosing the Right Microphone for Your Voice

Not all microphones handle sibilance equally. Large-diaphragm condenser microphones, common in home studios, can exaggerate sibilance because of their extended high-frequency response. If you struggle with sibilance, consider a microphone with a darker voicing or one that has a built-in high-frequency roll-off switch. Dynamic microphones like the Shure SM7B or Electro-Voice RE20 are popular for voiceovers because of their natural, forgiving high-end. Ribbon microphones also smooth out sibilance but require more gain. Test several microphones with your voice before committing to one for your ACX workflow.

De-Essing: Precision Post-Processing

Even with great technique, some sibilance may remain. De-essers are audio processors designed to detect and reduce sibilant energy without affecting the rest of the voice. Use a frequency-specific de-esser (not a simple EQ cut) to target only the problematic band. Set the threshold so that only the loudest ‘s’ sounds trigger the reduction—typically a reduction of 3–6 dB. Over-de-essing makes speech sound lispy and unnatural. Many DAWs include a built-in de-esser; third-party plugins like iZotope RX De-ess or Waves Sibilance offer more control. Always process in a copy of your audio and listen at low volume to judge the effect.

Mastering Plosive Control

Plosives are bursts of low-frequency air pressure that cause the microphone diaphragm to move abruptly, creating a pop. The energy is mainly below 200 Hz. The solution combines mechanical barriers, mic placement, and vocal technique.

Pop Filters and Windscreens: The Physical Barrier

A pop filter placed 2–4 inches from the microphone diffuses the air burst from plosives. Use a dual-layer pop filter for best results; single-layer fabric pop filters may not catch all low-frequency energy. Foam windscreens (often called “dead kittens”) are less effective for plosives but good for reducing wind noise. For broadcast-style mics like the SM7B, the built-in windscreen is usually sufficient, but adding an external pop filter provides an extra safety buffer.

Advanced Mic Placement for Plosives

If you position the microphone so the capsule is not directly in the air stream of your mouth, you drastically reduce plosives. Place the mic off-axis horizontally (left or right) by about 15–20 degrees. Alternatively, try a “over-and-down” placement: angle the mic slightly downward, aiming at your chin rather than your lips. This uses gravity to deflect the air burst. Remember to adjust your head position slightly to speak past the capsule rather than into it.

Breath Control and Vocal Technique

Plosive pops are essentially explosive breaths. Train yourself to release plosives with less force by imagining a gentle puff of air rather than a sharp burst. Practice words like “pop,” “bat,” “top” while keeping your diaphragm relaxed and your outflow steady. Some voice actors use a technique called “unvoiced plosive,” where they aspirate the consonant more softly. Also, avoid leaning forward during emphatic phrases; maintaining a stable posture keeps your mouth at the same distance from the mic.

High-Pass Filtering in Post

After recording, a high-pass filter (HPF) at 80–100 Hz can remove the low-frequency rumble of plosives that still made it to the recording. However, the HPF should be used gently—cutting too high removes the natural warmth of the voice. Some DAWs allow you to apply a HPF only during plosive moments using automation, but that requires a very clean edit. A better approach is to manually remove plosive bursts by zooming in on the waveform and either replacing the pop with a clean breath or using a declicker tool (e.g., iZotope RX De-click).

Recording Environment: The Overlooked Factor

Sibilance and plosives are often worsened by a room with reflective surfaces. Hard walls, glass windows, and bare floors cause high-frequency reflections that reinforce sibilance and even create comb filtering. Treat your recording space with acoustic panels, blankets, or a portable vocal booth. The goal is not total deadness but controlled early reflections. A room that sounds “live” will exaggerate any sibilance in your voice. Use a reflection filter around your microphone (like the sE Electronics RF-X) to reduce ambient high-frequency buildup.

ACX-Specific Technical Compliance Beyond Sibilance/Plosives

While eliminating sibilance and plosives is critical, ACX also enforces overall audio quality thresholds. Ensure your recordings meet these requirements:

  • Noise floor: ACX requires a noise floor of -60 dBFS or lower. Use a quiet preamp, avoid computer fan noise, and apply noise reduction carefully.
  • RMS level: Maintain an average RMS level of -23 dB to -18 dB for spoken word. Peaks should not exceed -3 dBFS. Consistent levelling reduces the need for heavy compression.
  • Sample rate and bit depth: 44.1 kHz, 16-bit WAV or 320 kbps MP3. Always upload the correct format to avoid rejection.
  • Editing completeness: Remove mouth clicks, breaths, and other extraneous noises. ACX expects a clean, seamless listen.
  • Check with ACX Audio Meter: Use the free ACX Audio Submission Guidelines and third-party audio checkers like Auphonic or ACX Check to validate your files before upload.

Troubleshooting Common Issues

Scenario: Sibilance persists despite de-essing and mic positioning

Try adding a notch filter around 6.3 kHz with a narrow Q to reduce the specific peak. Or test a different microphone—some voices naturally match better with certain capsule types. Also, check your mouth moisture; a dry mouth can produce more sibilance. Drink water and consider using a lubricating mouth spray designed for voice actors.

Scenario: Plosives appear even with a pop filter

The pop filter might be too close to the microphone. Move the filter further away (3–6 inches) and ensure it’s angled slightly. If you still get pops, increase the distance between you and the mic by 2 inches. Another cause is a low-cut filter on your preamp that isn’t steep enough—engage a HPF at 80 Hz on your audio interface.

Scenario: Audio sounds dull after removing sibilance

You may have over-de-essed or used a wide EQ cut. Reduce the de-esser threshold and use a narrower frequency band. Alternatively, use a dynamic EQ that only engages when sibilance exceeds a certain level. Apply gentle high-frequency shelving (boost at 10 kHz) after de-essing to restore airiness without re-introducing harshness.

Invest in quality tools that speed up your workflow. For de-essing, the iZotope RX Suite offers advanced spectral editing. For plosive removal, Accusonus ERA Plosive Remover can automatically clean up pops. A good pop filter from brands like Stedman or Aokeo can cost under $20 and make a world of difference.

Daily Practice for Plosive and Sibilance Reduction

Incorporate these exercises into your warm-up routine:

  1. Soft plosive drills: Read a list of plosive-heavy words (e.g., “Peter Piper picked”) while whispering. Then increase volume gradually, maintaining the same relaxed breath.
  2. sibilance softening: Say sentences with many ‘s’ sounds (e.g., “She sells seashells”) using a lighter tongue contact. Record and check playback for harshness.
  3. Mic distance consistency: Place a mark on your desk or use a string to keep constant distance. Record a test and listen for any changes in sibilance as you vary energy.

Consistent, mindful practice will train your muscles to produce cleaner audio naturally, reducing your reliance on post-processing.

Final Words for ACX Narrators

Reducing sibilance and plosives is not a one-time fix but an ongoing combination of technique, equipment, and careful editing. Every voice is unique, so experiment with the tips above to find what works for you. By mastering these fundamentals, you’ll not only pass ACX quality checks consistently but also elevate your narration to a professional level that listeners appreciate. Remember: clean audio equals a better listening experience and higher ratings on Audible.

For more detailed ACX requirements, refer to the official ACX Audio Specifications and consider joining communities like the Audiobook Narrators Facebook group for peer feedback.