APU TrueGain

The perceived frequency balance of your mix changes dramatically with playback volume. APU TrueGain is a utility designed to address this fundamental challenge by applying precise, frequency-compensated gain based on established equal-loudness contours like ISO 226.

To ensure its perceptual modeling is accurate, TrueGain features a straightforward calibration process that aligns the plug-in with the true Sound Pressure Level at your listening position. Using any standard SPL meter, you can match the plug-in to your physical environment, ensuring all loudness simulations are precise and repeatable.

Its two primary workflows are:

• Compensated Gain Staging: Apply gain adjustments that maintain the perceived tonal balance of your source audio.
• Translation Checking: Without changing your monitor volume, use the target SPL parameter to accurately simulate how your mix sounds at different levels-from quiet home listening to loud club playback.

A comprehensive, real-time spectral analyzer provides clear visual feedback on your source, target, and output signals throughout the process.

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System requirements: macOS 10.14 (x64, ARM), Windows 10 (x86, x64), OpenGL 3.2.
Supported software formats: Standalone application, VST, AU, AAX (Pro Tools 11+).

Calibration

To ensure TrueGain’s perceptual modeling is accurate for your specific environment, a simple calibration process aligns the plugin with the true Sound Pressure Level at your listening position.

The process is straightforward. First, place an SPL meter at your listening spot, using a C-weighted, slow response setting. In the plugin, click the “Calibrate SPL …” button to play a band-limited pink noise signal (a 1 kHz sine wave option is also available). Now, simply adjust your speaker volume and/or the reference SPL parameter until the reading on your external SPL meter matches the reference level set within TrueGain (for example, 83 dB SPL).

For convenience, you can also toggle through standard reference levels using the drop-down next to the reference SPL parameter.

Once calibrated, all of the plugin’s loudness simulations and compensations are precisely matched to your physical listening environment.

Contour Types

This plug-in supports multiple frequency weighting models, each designed to reflect how human hearing perceives sound at different loudness levels. The frequency compensation feature of this plug-in applies the delta between Phon levels, so each contour type contains dozens of frequency response curves which are smoothly interpolated as a function of the current reference SPL and true gain parameters.

ISO 226 (2003)

ISO 226:2003 defines standardized equal-loudness-level contours based on international listening tests, capturing how the sensitivity of human hearing changes across frequencies at different loudness levels (measured in phons).

In APU TrueGain, the 2003 edition of ISO 226 provides frequency response curves that adapt dynamically based on a user-selected reference level from 20 dB SPL to 120 dB SPL. This allows for precise emulation of how the ear perceives sound at varying playback volumes.

The reference SPL parameter allows you to specify phon levels from 20-120 dB SPL with full interpolation.

ISO 226 (2023)

ISO 226:2023 is the latest revision of the international standard for equal-loudness contours, offering improved accuracy over the 2003 edition through updated measurement techniques and expanded listening data. It reflects more recent research into how human hearing perceives frequency balance across a wide range of sound pressure levels.

In APU TrueGain, ISO 226:2023 contours can be interpolated smoothly between 20 dB SPL and 120 dB SPL, allowing detailed modeling of loudness perception at any listening level.

The reference SPL parameter allows you to specify phon levels from 20-120 dB SPL with full interpolation.

Fletcher-Munson

Fletcher-Munson contours represent one of the earliest attempts to map human equal-loudness perception, originating from the seminal 1933 study by Harvey Fletcher and W.A. Munson. Though later refined by ISO standards, these original phon curves remain historically significant and are still referenced in perceptual audio work.

In APU TrueGain, the Fletcher-Munson curves are faithfully reconstructed from the original research data, carefully digitized and interpolated to provide smooth transitions between 20 dB SPL and 120 dB SPL. This allows for the exploration of classic loudness perception models with modern flexibility.

Credits

This software was developed by APU Software, LLC and is available as VST (windows x64/x86, macOS universal), Audio Unit (macOS universal), Pro Tools (windows x64/x86, macOS universal), or Standalone Application. The software libraries below are utilized for portions of the software:

• JUCE (cross-platform audio and user interface framework)
• Boost (header-only algorithms)
• melatonin_blur (blur effects)
• PFFFT (FFT library)

Demo video song credits:

• Slytek - Body Heat (Radio Edit).