How It Works

A detailed explanation of how Equaliser processes audio on your Mac.

Overview

Equaliser provides system-wide audio equalisation for your Mac. This means every app — Spotify, Safari, Games, FaceTime — has its audio processed through the equaliser before reaching your speakers or headphones.

Achieving system-wide EQ on macOS requires a special approach. Unlike individual apps that only process their own audio, Equaliser needs to intercept all audio playing on your system. This is accomplished using a virtual audio driver.

The Audio Pipeline

When Equaliser is running, audio flows through this pipeline:

┌─────────────────┐     ┌──────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│  Your Apps      │     │  Equaliser       │     │  Equaliser      │     │  Output Device  │
│  (Spotify, etc.)│ ──▶ │  Driver          │ ──▶ │  App (EQ)       │ ──▶ │  (Speakers/     │
│                 │     │  (Virtual Device)│     │  (Processing)   │     │   Headphones)   │
└─────────────────┘     └──────────────────┘     └─────────────────┘     └─────────────────┘

How macOS Sees It

macOS treats the Equaliser Driver as a real audio device. When you set it as your system default output:

1. All apps send their audio to the driver
2. The driver passes audio to Equaliser for processing
3. Equaliser sends the processed audio to your chosen output

The Virtual Audio Driver

What Is It?

The Equaliser Driver is a virtual audio device that installs into macOS's audio system. It appears alongside your real audio devices (built-in speakers, headphones, etc.) but has no physical hardware — it exists purely to capture audio for processing. It's based on BlackHole by Existential Audio Inc., customised with dynamic device naming and shared memory capture for seamless integration with Equaliser.

Why Is It Needed?

macOS doesn't allow apps to intercept each other's audio. Each app can only access its own audio stream. The virtual driver solves this by becoming the system-wide output device, creating a common point where all audio converges.

Installation

The driver requires admin privileges to install. When you first enable Equaliser:

1. macOS prompts for your password
2. The driver installs to /Library/Audio/Plug-Ins/HAL/Equaliser.driver
3. Equaliser appears as an available audio device

Uninstallation: The driver can be removed from Settings → Driver → Uninstall Driver. This requires admin privileges again.

Automatic Updates

Equaliser checks the bundled driver version against the installed version. If a newer version is included with the app, you'll be prompted to update.

Audio Routing Modes

Equaliser offers two modes for handling audio devices.

Automatic Mode (Recommended)

The simplest way to use Equaliser. Set it once and forget about it.

How it works:

1. Set the Equaliser Driver as your default output device (Equaliser does this automatically)
2. Change your output device in macOS Sound settings
3. Equaliser detects the change and routes audio accordingly

Manual Mode

For advanced users who want complete control over device selection.

Use Manual Mode when:

• You want to use a different virtual audio driver as input
• You need to route audio from a specific application
• You're doing advanced audio routing with multiple virtual devices

Capture Methods

Equaliser supports two methods for capturing audio from the driver.

Shared Memory (Default)

The default and recommended capture method.

The driver writes audio to shared memory, and Equaliser reads it. No microphone permission needed, no system indicators.

HAL Input

An alternative capture method for specific use cases.

When to use HAL Input:

• Using Manual Mode with a third-party virtual driver
• Capturing from a real input device
• Advanced audio routing setups

The Equaliser Engine

Equaliser uses a custom biquad DSP engine for audio processing. This provides native performance through Apple's vDSP framework, supporting up to 64 bands of parametric EQ with low latency and real-time safety.

The engine is built on the industry-standard RBJ Audio EQ Cookbook and implements 7 filter types: parametric, low-pass, high-pass, low shelf, high shelf, band-pass, and notch. The Q parameter controls resonance for low/high-pass filters and slope for shelf filters.

Key features:

• Real-time safe — No allocations or locks on the audio thread
• Native performance — SIMD-optimised via Apple's Accelerate framework
• Independent stereo — Left and right channels can have completely different EQ curves
• Up to 64 bands — Far beyond typical 10-band equalisers

For a detailed explanation of how the biquad engine works, see The EQ Engine.

Parametric EQ

Equaliser provides up to 64 bands of parametric equalisation. Each band gives you precise control over:

Standard Band Frequencies

The default 10-band configuration uses logarithmically-spaced frequencies:

Gain Controls

In addition to per-band adjustments, Equaliser provides overall gain controls:

Processing Chain

Audio passes through this sequence:

[Input Audio] → [Input Gain] → [EQ Bands] → [Output Gain] → [Output Audio]

When bypassed (EQ toggle off), the EQ bands and input/output gains are skipped.

Compare Mode

Compare Mode lets you A/B test your EQ settings against a flat response:

The volume matching ensures fair comparison — EQ boosts naturally sound louder, which can bias perception.

Presets

Factory Presets

Equaliser includes 11 carefully crafted presets for common use cases:

See the EQ Presets Guide for detailed explanations of each preset.

Custom Presets

Create your own presets:

1. Adjust your EQ settings
2. Click the preset menu → Save As...
3. Name your preset

Custom presets are saved to:

~/Library/Application Support/Equaliser/Presets/

Import and Export

• Export: Share presets as .eqpreset files
• Import: Load presets from .eqpreset files
• EasyEffects: Import presets from Linux EasyEffects (compatible format)
• REW: Import filter settings from Room EQ Wizard

Device Management

Headphone Auto-Switch

When headphones are connected, Equaliser automatically switches output:

This matches macOS behaviour and prevents audio from unexpectedly playing through speakers.

When headphones are unplugged, Equaliser restores the previous output device from history.

Device History

Equaliser remembers your output device choices:

• When a device becomes unavailable, Equaliser finds a replacement
• When the device returns, Equaliser can restore your preference
• History is limited to prevent stale device references

Privacy & Permissions

What Equaliser Does NOT Do

All audio processing happens locally on your Mac. Equaliser has no network capability.

Microphone Permission

macOS may request microphone permission when Equaliser launches. Here's why:

The permission prompt appears at launch because macOS detects the audio-input entitlement in the app, even though the default mode doesn't need it. This is macOS behaviour and cannot be controlled.

If you only use Automatic Mode with Shared Memory capture, you can deny the permission — the app will work correctly.

Troubleshooting

No Sound After Enabling

1. Check that Equaliser Driver is set as default output in macOS Sound settings
2. In Automatic Mode, verify an output device is selected
3. Try toggling EQ off and on

Orange Microphone Indicator Appears

This happens when using HAL Input capture or Manual Mode. To avoid it:

1. Go to Settings → Driver
2. Select "Shared Memory" as capture mode
3. Ensure you're in Automatic Mode

Driver Installation Fails

Driver installation requires admin privileges. If installation fails:

1. Ensure you entered your password correctly
2. Check that no other audio apps are blocking the installation
3. Try restarting your Mac and installing again

Audio Sounds Distorted

Distortion usually indicates:

1. Input gain too high: Reduce Input Gain
2. Output gain too high: Reduce Output Gain
3. Excessive band boost: Reduce individual band gains

Watch the level meters for clipping indicators (red peaks).