The five main tools for mastering audiobooks to ACX spec are Audacity, Auphonic, Hindenburg Narrator, Source (formerly Ohelo Studio), and ChapterPass.[5] They differ on five criteria that matter to narrators: ACX compliance, automation level, privacy (whether your audio leaves your machine), price, and true peak handling. This guide compares all five honestly, including where each one falls short.
Audacity
Audacity is a free, open-source audio editor with a large community and extensive documentation.[1] It is the most widely used tool for audiobook mastering among independent narrators, and for good reason: it costs nothing and gives you full control over every parameter.
What it does well. Audacity handles every step of the mastering chain: noise reduction, compression, normalisation, and limiting. You can save the entire chain as a macro for batch processing across all your chapter files. The community has produced tutorials for nearly every audiobook scenario. It runs on Windows, macOS, and Linux. For narrators who want to understand what each effect does and why, Audacity is the best learning tool available. Everything happens locally on your machine, so there are no privacy concerns with client audio.
Where it falls short. Every setting is manual. You configure four separate effects, each with multiple parameters. There is no automated verification step, so you master your files, then measure them separately, then compare against the ACX spec table. If any measurement is out of range, you adjust and re-process. Audacity's limiter measures sample peaks only, not true peaks, which can miss 0.5 to 1 dB of inter-sample overshoot. See the Audacity mastering guide for the exact settings and workarounds.
Best for: narrators who want full manual control and are willing to learn the technical details. Also the right choice if you are on a strict budget and willing to invest the time.
Cost: free and open source.
Auphonic
Auphonic is a cloud-based audio post-production service that uses intelligent algorithms to normalise loudness, reduce noise, and balance levels automatically.[2] It was built for podcasters but works for audiobooks.
What it does well. Auphonic's loudness normalisation is sophisticated. It analyses the full file before processing, rather than applying fixed settings. This means it adapts to the specific dynamics of each recording rather than using a one-size-fits-all compressor threshold. The results are generally good, especially for speech content. It offers a REST API for integration into production workflows, which is valuable for studios processing high volumes. The interface is straightforward: upload a file, set your target loudness, and download the result. Auphonic also handles noise reduction and can level multiple speakers in dialogue.
Where it falls short. Your audio is uploaded to Auphonic's servers for processing. For narrators working under NDA or handling sensitive content, this may be a concern. Auphonic is not built specifically for ACX. It targets EBU R128 and other broadcast standards, and while you can set ACX-compatible loudness targets, it does not verify all eight ACX specs. The free tier is limited to 2 hours of audio per month. A single audiobook can run 8 to 12 hours, so the free tier covers a fraction of one project. Beyond that, you purchase processing credits or subscribe to a monthly plan.
Best for: podcasters and narrators comfortable with cloud processing who want intelligent loudness normalisation.
Cost: 2 hours per month free. Paid plans start at a monthly subscription for additional hours, with one-time credit packs also available.
Hindenburg Narrator
Hindenburg Narrator is a desktop application designed specifically for audiobook production.[3] It is a full production suite, not just a mastering tool.
What it does well. Hindenburg includes recording, editing, and mastering in one application. Its voice profiler analyses your microphone and room to create a custom processing profile. The clipboard feature is useful for long-form narration: it lets you mark, find, and replace sections across a full manuscript. Hindenburg targets professional narrators and publishers, and the feature set reflects that scope.
Where it falls short. Hindenburg is more tool than most narrators need. If you already record in another application and only need the mastering step, you are paying for a full production suite when you only use a fraction of it. It runs on Windows and macOS only, so Linux users are excluded. The subscription model means ongoing cost whether you are actively producing or between projects.
Best for: professional narrators who produce regularly and want recording, editing, and mastering in a single application.
Cost: subscription-based. Free trial available.
Source (Formerly Ohelo Studio)
Source is a browser-based audio editor and mastering tool built for audiobook production.[4] Originally launched as Ohelo Studio, it rebranded in 2025.
What it does well. Source runs entirely in the browser with no installation required. It includes punch-and-roll recording, a channel strip with EQ and dynamics, and ACX compliance features. The interface is designed for narrators, not audio engineers.
Where it falls short. As a newer platform, the community and documentation are smaller than Audacity's or Hindenburg's. If you run into an unusual issue, there are fewer forum threads and tutorials to draw from. The feature set is still growing, so check whether your specific needs (batch export, specific format options) are supported before committing to a full project.
Best for: narrators who want a browser-based workflow that includes both recording and mastering without installing desktop software.
Cost: currently free.
ChapterPass
ChapterPass is a browser-based audiobook mastering tool that processes files locally. Your audio never leaves your machine. It is built specifically for ACX compliance.
What it does well. ChapterPass applies the full mastering chain in one step: loudness adjustment, true-peak-aware limiting, noise floor management via speech-aware gating, format conversion to 192 kbps CBR MP3, and silence padding. Every output file is verified against all eight ACX specs before you download it. Processing is deterministic: the same input always produces the same output. Your audio never leaves your browser. First file free.
Where it falls short. ChapterPass masters files. It does not edit them. There is no noise removal under speech, no click removal, no echo reduction, no EQ, and no creative processing. If your recording has problems that need editing, you need an editor (Audacity, Hindenburg, or Source) first. ChapterPass is the last step, not the only step. See what ChapterPass does for the full scope.
Best for: narrators who want automated mastering with ACX verification, without uploading their audio to a server.
Cost: per file. First file free.
Key differences at a glance
| Audacity | Auphonic | Hindenburg | Source | ChapterPass | |
|---|---|---|---|---|---|
| Price | Free | 2 hr/month free, then paid | Subscription | Free | Per file, first free |
| Privacy | Local | Cloud upload | Local | Browser | Browser, local only |
| True peak | Sample peak only | Yes | Yes | Undocumented | Yes |
| Automation | Manual (macros) | Fully automated | Semi-automated | Semi-automated | Fully automated |
| ACX-specific | General editor | Broadcast standards | Audiobook workflow | Audiobook workflow | Audiobook workflow |
| Editing | Full editing | No editing | Full editing | Full editing | No editing |
| Verification | Manual | Partial | Partial | Partial | All 8 ACX specs |
Which one should you use?
The right tool depends on what you need from it.
Full control over every parameter: Audacity. You set every value. You learn what each effect does. You verify the results yourself. It costs nothing and it works. Start with the Audacity mastering chain.
Automated processing, comfortable with cloud upload: Auphonic. Upload your file, set your targets, download the result. Good loudness normalisation. Be aware that your audio is processed on their servers.
Full production suite for regular production: Hindenburg Narrator. Recording, editing, and mastering in one application. Worth the subscription if you produce audiobooks regularly.
Browser-based recording and editing: Source. No installation, projects sync across devices, built for audiobook workflows.
Automated mastering without upload: ChapterPass. Drop a file in, get a compliant file out. No editing, no creative processing. Just the technical mastering, verified against every ACX spec. First file free.
Most narrators use more than one tool. A common workflow: record and edit in Audacity or Hindenburg, then master the final files with ChapterPass or Auphonic. The editor handles creative decisions (pacing, retakes, mouth noise removal). The mastering tool handles technical compliance (loudness, peaks, noise floor, format, silence).
No tool on this list fixes a bad recording environment. If your room has audible HVAC, reflections, or electrical hum baked into the speech, mastering cannot remove it. The best investment any narrator can make is a quiet, treated recording space. Once your raw files are clean, any of these five tools can get them across the ACX line. If your files were rejected, see ACX Rejection: How to Fix Every Issue for the decision tree. For quick answers on pricing, privacy, and what ChapterPass does not do, see the FAQ.