RACK4MASTER is a professional web-based audio mastering rack that runs entirely in your browser.
Load any audio file, build a custom chain of processors, adjust parameters in real time, and export the processed audio as a WAV file.
AND MOST IMPORTANTLY... IT'S COMPLETELY FREE!
No data is sent to any server β 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.
- 15 professional audio modules (higher DSP quality):
| Module | Description |
|---|---|
| Gate | Real downward expander with envelope follower and Hold time. Eliminates noise in silences. |
| Compressor | Dynamic range control with adjustable knee, attack, release and makeup gain. |
| Limiter | Brickwall limiter with 1 ms attack, aligned with streaming targets (β1 dBTP). |
| De-esser | Dynamic sibilance control: only cuts when the level exceeds the threshold β not a static notch. |
| Multiband | 3-band compressor with LR4 Linkwitz-Riley crossover for perfectly flat band summation. |
| Tremolo | Amplitude modulation for rhythmic effects. |
| Module | Description |
|---|---|
| Filter HP/LP | Independent High-Pass and Low-Pass filters with ON/OFF toggles. |
| EQ 4-Band | Low shelf + 2Γ Peaking + High shelf. Interactive frequency curve with draggable nodes. |
| Module | Description |
|---|---|
| Reverb | Improved IR with frequency-dependent HF decay (air absorption). New Damping control. |
| Delay | Echo with LP filter in the feedback path for tape-style degradation. |
| Modulation | Chorus + Flanger + Vibrato in one module with mode selector. |
| Exciter | Adds brightness and air via even-order harmonics. Oversample 4Γ. |
| Widener | Haas stereo widening. Proper stereo channel routing + Mono Bass for mono compatibility. |
| Mid/Side | Process center and sides independently. Normalized M/S matrix (unity = 0 dB). |
| Module | Description |
|---|---|
| Harmonic Drive | Soft (tanh normalized) Β· Tape (asymmetric, even harmonics) Β· Hard (polynomial clip). Oversample 4Γ on all modes. |
- A/B comparison slots β maintain two independent mastering chains and switch instantly.
- Solo mode β isolate a single module, available on thumbnails and cards.
- Spectrum analyzer β real-time frequency visualization (bar/line modes, labeled Hz/dB axes).
- Intelligent analysis assistant β detects frequency imbalances, dynamics issues and stereo problems; recommends a custom chain.
- Modular signal chain β drag & drop from sidebar, reorder thumbnails, bypass individually or globally.
- Full parameter control via analog-style knobs and toggles with real-time audio processing.
- Module presets β 7+ presets per module for quick setup.
- 35+ instrument & genre presets with explicit parameters for all modules:
- Guitars (6): Acoustic, Classical, Clean, Drive, Metal, Fuzz
- Pianos/Keys (4): Grand Piano, Studio Piano, Rhodes, Synth
- Vocals (5): Lead, Male, Female, Choir, Rap
- Drums/Percussion (6): Acoustic, Studio, Vintage, Jazz, Electronic, Percussion
- Orchestral (3): Strings, Brass, Woodwinds
- Mix by Genre (10): Rock, Blues, Country, Folk, Jazz, Urban, Latino, Pop, Ballad, Mastering
- Preset system β save/load entire chain (both A/B slots) + loop settings + output gain as JSON. Backward-compatible with presets from older versions.
- WAV Export β choose 16 bit (with TPDF dithering) or 24 bit, at original or 48 kHz sample rate.
- Professional TPDF dithering β applied on 16-bit export to eliminate quantization distortion.
- VU meters with peak hold and LUFS approximation.
- Global bypass β compare processed vs. raw audio instantly.
- Dark / Light theme and multilingual UI (English, Spanish, Catalan).
- 100% client-side β no tracking, no data collection, no server uploads.
Drag & drop a file onto the waveform area, or click it to select a file.
- Open the right sidebar (RACK MODULES).
- Click
βor double-click any module to add it to the chain. - Drag a module from the sidebar directly into the chain area at any position.
- Modules are processed left to right: INPUT β module 1 β module 2 β ... β OUTPUT.
- Reorder modules by dragging their thumbnails via the
β Ώhandle.
- Click any thumbnail to open its editor in the central panel.
- Adjust knobs (drag up/down or use mouse wheel), toggles, or choose a preset.
- Double-click any knob to reset it to its default value.
- Changes are heard in real time.
Press S on any module thumbnail or card to solo it β all other modules are temporarily bypassed. Press S again to restore the full chain.
Use A and B buttons in the chain toolbar to maintain two independent setups. Switching is instant; both slots are saved together in a preset file.
- Click βΌ SPECTRUM to open the frequency analyzer.
- Press π ANALYZE & RECOMMEND to scan the full audio and detect problems.
- Click APPLY SUGGESTED CHAIN to load the recommended modules automatically.
- Use β COPY RESULTS to copy the analysis text to clipboard.
- Each module has a
BYPbutton β LED turns red when bypassed. - Global Bypass (chain toolbar) disables all modules at once.
- Play / Pause / Stop controls.
- Enable loop with the Loop button, then drag the loop handles on the waveform.
- Click anywhere on the waveform to seek. Keyboard shortcuts available (see below).
- Adjust the output gain knob (right sidebar) β double-click to reset to 0 dB.
- VU meters show RMS and peak values.
RST PEAKresets peak indicators.
β SAVEβ exports a JSON file with the full chain, both A/B slots, loop settings, and output gain.β LOADβ loads a previously saved preset. Old presets (with renamed modules) are migrated automatically.
Click WAV β choose bit depth (16 or 24) and sample rate β the entire chain renders offline and downloads automatically.
The dropdown in the header offers 35+ ready-made chains. All presets include explicit parameter values for every module β not just default settings.
| Key | Action |
|---|---|
Space |
Play / Pause |
S |
Stop and rewind |
L |
Toggle loop |
β / β |
Seek Β±5 seconds |
Home |
Go to start |
End |
Go to end |
B |
Toggle global bypass |
R |
Reset peak meters |
Delete / Backspace |
Remove selected module |
| Double-click knob | Reset to default value |
- Gate: proper downward expander (ScriptProcessorNode), not a compressor. Correctly attenuates signals below threshold.
- Limiter: 1 ms attack (was 30 ms in previous versions) to catch fast transients.
- Mid/Side: normalized M/S matrix β unity processing produces exactly 0 dB gain change.
- Widener: correct stereo channel routing β both L and R channels preserved; Mono Bass parameter for low-end phase coherence.
- Harmonic Drive / Exciter:
oversample: '4x'on all WaveShaperNodes to minimize aliasing. - Reverb: frequency-dependent IR decay (HF absorbs faster than LF, like real rooms).
- Multiband: LR4 Linkwitz-Riley crossover (two cascaded 2nd-order Butterworth at Q=0.707) for flat magnitude summation.
- Web Audio API β real-time audio processing graph.
- Sortable.js β drag & drop reordering.
- Canvas API β waveform and spectrum visualization.
- Google Fonts β Orbitron, Rajdhani, Share Tech Mono.
- No frameworks β vanilla JavaScript ES modules.
Author: Francesc Llorens CerdΓ
Email: rackmaster@proton.me
For questions, suggestions or bug reports, please open an issue on GitHub.
RACK4MASTER is free, open source software under the MIT License.
Provided "as is", without warranty of any kind.
You must own or have permission to use the audio files you process.
Β© 2026 Francesc Llorens CerdΓ . All rights reserved.
- 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.
If you find RACK4MASTER useful, please consider supporting the project:
