MultiBandEQ

June 22, 2026 · View on GitHub

A multi-band parametric equalizer audio plugin built with JUCE. It starts with three bands and lets you add or remove bands on the fly (up to eight), each with a selectable filter type, and shows a live frequency-response curve.

Builds as Standalone, VST3, and AU.

Features

  • 3 bands by default, expandable to 8 via the Add Band / Remove Band buttons.
  • 5 filter types per band: Low Cut, Low Shelf, Peak, High Shelf, High Cut.
  • Per-band Frequency (20 Hz–20 kHz), Gain (±24 dB), and Q (0.1–10) controls.
  • Live frequency-response curve that reflects the combined effect of all enabled bands.
  • Full state save/restore via JUCE's AudioProcessorValueTreeState, so settings persist in DAW sessions and presets and are fully automatable by the host.
  • Stereo processing with efficient IIR filters (juce::dsp).

How "adding bands" works

Plugin formats (VST3/AU) require a fixed parameter layout for host automation and preset recall, so parameters cannot be created at runtime. MultiBandEQ instead pre-declares a fixed maximum of 8 bands, each with an on/off switch. Add Band enables the next dormant band and Remove Band disables the last active one. This keeps automation and saved state stable while still feeling like you are adding and removing bands.

Project layout

source/
  EqConstants.h             Shared config: band count, filter-type enum, parameter-ID helpers
  PluginProcessor.{h,cpp}   DSP: APVTS parameter layout, per-band IIR filters, state save/restore
  PluginEditor.{h,cpp}      Main editor: response curve, band strips, add/remove buttons
  BandComponent.{h,cpp}     One control strip per band (on/off, type, freq/gain/Q)
  ResponseCurveComponent.{h,cpp}  Live magnitude-response display
modules/                    JUCE and cmake-includes submodules
CMakeLists.txt              Build configuration

Building

Prerequisites

  • A C++20 compiler (Xcode/Clang on macOS, MSVC on Windows, GCC/Clang on Linux)
  • CMake ≥ 3.23
  • On Linux: webkit2gtk-4.0 and the usual JUCE Linux dependencies

Clone (with submodules)

JUCE and the CMake helpers are git submodules, so clone recursively:

git clone --recurse-submodules <repo-url>
cd MultiBandEQ
# already cloned without submodules?
git submodule update --init --recursive

Configure and build

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build --config Release

Where the artefacts land

After a successful build the plugin/app bundles are under:

build/MultiBandEQ_artefacts/<Config>/
  Standalone/MultiBandEQ.app      (or .exe)
  VST3/MultiBandEQ.vst3
  AU/MultiBandEQ.component         (macOS only)

The build also copies the VST3/AU into your user plugin folder (COPY_PLUGIN_AFTER_BUILD is enabled) so DAWs pick them up automatically.

Usage

  1. Launch the standalone app, or load the VST3/AU in your DAW.
  2. Each band strip has an on/off toggle, a filter-type menu, and Freq / Gain / Q knobs. (Gain has no effect on the Low Cut and High Cut types.)
  3. Use Add Band / Remove Band to change how many bands are active.
  4. Watch the curve at the top update as you adjust the controls.

Tech stack

  • JUCE audio plugin framework
  • juce_dsp IIR filters (ProcessorDuplicator for stereo)
  • AudioProcessorValueTreeState for parameters, automation, and state

License

See LICENSE.