ChapterPass is a browser-based audiobook mastering tool that adjusts technical measurements to meet ACX and other platform requirements.[1] No account. Files never uploaded. First file free. You drop in a chapter file, ChapterPass processes it locally in your browser using WebAssembly, and you download the mastered file. Your audio never leaves your device.
What ChapterPass Does
ChapterPass performs six operations on each file, in a coordinated pipeline where the output of each stage feeds the next.
1. Adjusts loudness
To -23 to -18 dBFS RMS, targeting -20 dBFS (the centre of the ACX audio requirements (RMS, peak, noise floor) range).[1] The engine analyses the full file before applying gain, so the correction is based on the actual content, not a fixed setting. Quiet passages and loud passages are measured together to produce an accurate average.
2. Limits true peaks
Below -3 dBFS, including peaks that fall between digital samples. Standard peak meters read only the loudest individual sample. The actual analogue waveform can peak higher between samples. ChapterPass detects these inter-sample peaks that simpler tools miss, matching the measurement standard that ACX uses (ITU-R BS.1770). Applied automatically with no configuration.
3. Manages noise floor
Below -60 dBFS using speech-aware gating that classifies audio blocks as speech, breath, or noise, then attenuates only the noise blocks. This is gating, not broadband noise reduction. Gating reduces volume during silent sections where noise is audible, leaving your voice completely untouched. Broadband noise reduction subtracts a noise profile from the entire signal, including speech, which at aggressive settings creates metallic artefacts. ChapterPass targets well below ACX's -60 dBFS threshold, leaving a safety margin that absorbs the gain applied during loudness correction. See the audiobook noise floor guide for why that distinction matters.
4. Converts format
To 44,100 Hz mono MP3 at 192 kbps CBR. This is the exact format ACX requires. If your input file is stereo, it is mixed down to mono. If it is at a different sample rate (48,000 Hz is common for files recorded with video production settings), it is resampled. The MP3 encoding uses a constant bit rate, not variable, because ACX rejects VBR-encoded files.
5. Adds silence padding
1 second of head silence, 1.5 seconds of tail silence. ACX requires 0.5 to 1 second at the head and 1 to 5 seconds at the tail.[1] The silence is generated as room tone level, not digital zero, because an instant transition from absolute silence to audio creates an audible click on playback.
6. Verifies all 8 ACX specs
Before download, ChapterPass measures the output file against all 8 ACX requirements: RMS loudness, true peak, noise floor, sample rate, channel count, bit rate, head silence, and tail silence. If any measurement is outside the allowed range, it re-enters the pipeline for correction and re-measurement until all specs pass. Only files that meet every spec are available for download.
Deterministic: the same input produces the same output every time. No randomness, no creative decisions, no variation between runs. Run it twice on the same file and you get identical results. This matters for narrators who process chapters across multiple sessions, because every chapter receives the same treatment regardless of when you process it.
How It Works in Practice
The workflow is three steps:
- Drop your file in. WAV, MP3, FLAC, AIFF, or M4A. For best results, start with uncompressed WAV or lossless FLAC.
- Wait for processing. A typical chapter takes a few minutes. The engine runs entirely in your browser via WebAssembly, so speed depends on your device. Keep the tab visible, because browsers throttle background tabs.
- Download the mastered file. The output is a verified, ACX-compliant MP3. The verification report shows every measurement and its pass/fail status.
You can upload multiple chapters at once. Each is processed in sequence. There is no account to create, no upload queue on a server, and no waiting for someone else's files to finish.
Browser Compatibility
ChapterPass runs in any modern desktop browser that supports WebAssembly: Chrome, Firefox, Safari, and Edge. It requires a desktop or laptop computer. Mobile browsers do not have enough memory or CPU headroom for reliable audio processing at the sustained throughput an audiobook chapter demands.
For the fastest results: keep the ChapterPass tab visible while processing, close other tabs and applications competing for memory, and plug in your laptop if you are on battery. Browsers reduce processor speed on battery power to conserve energy. Chrome typically delivers the fastest processing times due to its V8 WebAssembly engine, but all four browsers produce identical output. The processing is deterministic regardless of browser choice.
A typical 30-minute chapter takes 2 to 4 minutes depending on your device. Longer chapters (60+ minutes) take proportionally longer. If processing stalls or slows dramatically, check that the tab is in the foreground and that no other application is consuming heavy CPU.
Your Files Never Leave Your Device
The processing happens entirely in your browser using WebAssembly. No audio data is sent to a server at any point. For narrators working under non-disclosure agreements or handling unreleased manuscripts, this matters. Your files stay on your device throughout the entire workflow. The trade-off is that processing speed depends on your hardware rather than a server, but for a single audiobook the difference is negligible.
Signal Processing, Not AI
ChapterPass uses deterministic digital signal processing (DSP). It does not use machine learning, neural networks, or generative AI. Every operation is a defined mathematical transformation: gain adjustment, filtering, peak detection, gating thresholds. The same input always produces the same output.
This is the same type of processing that hardware limiters, compressors, and mastering chains have used for decades. The difference is that ChapterPass automates the configuration, coordinates the stages so they do not conflict, and verifies the result against all 8 specs.
For narrators concerned about platform policies on AI-processed audio: ChapterPass applies signal processing to human-recorded narration. It does not generate, synthesise, or alter the spoken content. ACX prohibits AI-generated narration.[2] ChapterPass does not generate narration. It masters it.
What ChapterPass Does Not Do
Each of these boundaries is deliberate.
- Does not edit audio content.
- Does not remove clicks, pops, or mouth noise.
- Does not add opening or closing credits.
- Does not fix echo or reverb.
- Does not improve narration quality or pacing.
- Does not guarantee ACX acceptance. Human review is a separate stage that evaluates performance, not specs.[1]
- Does not upload your files to a server. All processing happens in your browser via WebAssembly.
- Does not require an account.
If your file has content problems (clicks, echo, pacing), those need to be fixed before mastering. See fix ACX rejection for the full decision tree on what is a spec failure versus a quality failure.
Mastering vs Editing
Editing fixes content problems. A click in the middle of a sentence, a misread word, a long pause that disrupts pacing. These require human judgement. You listen, you decide, you cut. No software can replace that ear.
Mastering adjusts technical measurements. Loudness, peak levels, noise floor, format, silence padding. These are deterministic: the ACX audio requirements (RMS, peak, noise floor) define the targets, the tool hits them. There is no artistic interpretation involved, only maths.
ChapterPass does mastering, not editing. It takes a clean, edited recording and adjusts the numbers. It does not make creative decisions about your content. If you are unsure whether your problem is a mastering issue or an editing issue, see fix ACX rejection for the distinction.
What This Means for You
Start with a clean, edited recording. Fix any clicks, mouth noise, or misreads before you master. ChapterPass handles the technical mastering: loudness, peaks, noise floor, format, silence, and verification.
Your files never leave your browser. There is no server upload, no cloud processing, no account creation. Drop a file in, download the mastered version. The processing runs entirely in WebAssembly on your own machine. Your first file is free at ChapterPass.
For the manual Audacity approach, see Audacity audiobook mastering. For a side-by-side look at all five mastering tools, see audiobook mastering tools compared. For quick answers on pricing and privacy, see the FAQ.