RACK4MASTER is a professional web‑based audio mastering rack that runs entirely in your browser.
It allows you to load any audio file, build a custom chain of processors (dynamics, filters, spatial effects, saturation, etc.), adjust parameters in real time, and export the processed audio as a WAV file.

AND MOST IMPORTANTLY... IT'S COMPLETELY FREE!

(NOTE: No data is sent to any server – everything happens locally on your machine).

If you find RACK4MASTER useful, help us grow by giving a star on GitHub:

You can also donate via PayPal to keep improving the tool.

• Load any audio (WAV, MP3, etc.) via drag & drop or file picker.
• Interactive waveform with click‑to‑seek, loop handles, and real‑time playback position.
• 18 built‑in audio modules:
  • Dynamics: Gate, Compressor, Limiter, Tremolo, De‑esser (classic), De‑esser Pro, Multiband Compressor.
  • Filters: HP/LP Filter, 4‑band Equalizer.
  • Spatial: Chorus, Flanger, Reverb, Delay, Widener, Mid/Side.
  • Saturation: Saturator, Soft Clipper, Harmonic Exciter.
• A/B comparison slots – maintain two independent mastering chains and switch between them instantly.
• Solo mode – isolate a single module to hear its effect alone, available on both thumbnails and module cards.
• Spectrum analyzer – real‑time frequency visualization with bar/line modes and labeled axes (Hz / dB).
• Intelligent analysis assistant – automatically detects frequency imbalances and recommends a custom mastering chain.
• Modular signal chain – drag & drop modules from the sidebar to the chain area, reorder them, bypass individually or globally.
• Full parameter control via analog‑style knobs and toggles, with real‑time audio processing.
• Module presets (where available) for quick setup.
• 30+ instrument & genre presets – optimized chains for acoustic/classical/electric guitars, pianos, Rhodes, synths, male/female vocals, choirs, rap, acoustic/studio/vintage/jazz/electronic drums, percussion, strings, brass, woodwinds, and mixes by genre (Rock, Blues, Country, Folk, Jazz, Urban, Latino, Pop, Ballad, Mastering).
• Preset system – save / load your entire chain (including both A/B slots) + loop settings + output gain.
• Export the processed audio as a 16‑bit WAV file (offline rendering, respects the whole chain and bypass states).
• Professional dithering – Triangular PDF dithering applied on export to 16-bit WAV, eliminating quantization distortion.
• VU meters with peak hold and LUFS approximation.
• Global bypass to compare processed vs. raw audio instantly.
• Dark / Light theme and multilingual UI (English, Spanish, Catalan) – no external dependencies for translations.
• 100% client‑side – no tracking, no data collection.

Drag & drop a file onto the waveform area, or click the waveform and select a file.
The waveform will be drawn, and the transport controls become active.

• Open the right sidebar (MÓDULOS DEL RACK).
• Click the ⊕ button of any module to add it to the chain.
• You can also drag a module from the sidebar directly into the chain area.
• Modules are processed from left to right (INPUT → module 1 → module 2 → ... → OUTPUT).
• Rearrange modules by dragging their thumbnails (drag handle ⠿).

• Click on any thumbnail in the chain area to open its editor in the central panel.
• Adjust knobs, toggles, or select a preset. Changes are heard immediately.
• Use the S button on any module thumbnail or card to solo it – all other modules will be temporarily bypassed.

• Use the A and B buttons in the chain toolbar to maintain two independent mastering setups.
• The first time you switch to an empty slot, you'll be asked whether to copy the current chain.
• Switching between A and B is instant — the app remembers both configurations automatically.
• When saving a preset, both slots are stored in the JSON file.

• Click the ▼ SPECTRUM toggle to open the frequency analyzer panel (200px high).
• Switch between bar and line mode with the button in the top‑right corner.
• The graph displays frequency labels (Hz) and amplitude labels (dB) for precise reading.
• Press 🔍 ANALYZE & RECOMMEND to scan the full audio file.
• Detected issues (sub‑bass excess, resonance peaks, sibilance, lack of air, etc.) appear in the right panel.
• Click APPLY SUGGESTED CHAIN to replace your current modules with the custom‑tailored mastering chain.

• Each module has a BYP button on its thumbnail and in its editor. Click to toggle bypass (LED turns red).
• The Global Bypass button (in the chain toolbar) disables all modules at once for A/B comparison.

• Play / Pause / Stop buttons.
• Enable loop and drag the loop handles in the waveform to set start/end points.
• Click anywhere on the waveform to seek.

• Adjust the output gain knob (right sidebar) to increase or decrease final level.
• The VU meters show approximate loudness (RMS) and peak values. Click RST PICO to reset peak indicators.

• Use ↓ GUARDAR to save a JSON preset containing module chain (both A/B slots), all parameters, loop settings, and output gain.
• Use ↑ CARGAR to load a previously saved preset.

• Click the WAV button. The app will render the entire chain offline and download a 16‑bit stereo WAV file.

• The dropdown in the header offers ready‑made chains for 35+ sources:
  • Guitars (6), Pianos/Keys (4), Vocals (5), Drums/Percussion (6), Orchestral (3), Mix by Genre (Rock, Blues, Country, Folk, Jazz, Urban, Latino, Pop, Ballad, Mastering).

• Web Audio API – real‑time audio processing graph.
• Sortable.js – drag & drop reordering of modules.
• Canvas API – waveform drawing and overlays.
• Google Fonts – Orbitron, Rajdhani, Share Tech Mono.
• No frameworks – vanilla JavaScript modules.

For questions, suggestions or bug reports, please contact:

Author: Francesc Llorens Cerdà

Email: rackmaster@proton.me

Rack4Master is a free, open source software under a MIT Licence. This software is provided "as is", without warranty of any kind.
You must own or have permission to use the audio files you process.
All rights reserved. © 2026

• Inspired by classic analog mastering consoles and modular racks.

• Thanks to the Web Audio API community for making real‑time audio in the browser possible.

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🚀 Want a quicker workflow? If you are looking for a simplified, fully automated experience, check out our compact version: rack4master-mini.

If you find RACK4MASTER useful, you can support the project with a donation: