APU TrueGain

APU TrueGain is a powerful utility plugin which corrects for a fundamental challenge in audio engineering: a mix’s tonal balance changes depending on the listening level. By modeling the human ear’s frequency sensitivity at different Sound Pressure Levels (SPL), TrueGain allows you to audition and control your audio with perceptual consistency.

The core of TrueGain is its ability to perform “translation checks.” After a simple one-time calibration to your monitoring setup, you can use TrueGain to simulate how your mix will sound at different loudness levels, all without changing the actual volume of your monitors or headphones. This allows you to quickly check how the bass will hold up or if the vocals will become harsh in quieter listening environments, ensuring your mix translates reliably.

Alternatively, TrueGain can function as a perceptually-compensated gain control. In this mode, as you adjust the output gain, the plugin automatically applies the corresponding frequency compensation. This preserves the perceived tonal balance of your mix as it gets louder or quieter, making it an invaluable tool for your master bus, monitoring chain, or in live sound applications.

The frequency compensation is meticulously modeled on psychoacoustic research, featuring the modern ISO 226:2023 and ISO 226:2003 equal-loudness contours for state-of-the-art accuracy. For more of a vintage character, the classic Fletcher-Munson curves are also included. All filters are validated with rigorous tolerance requirements.

All gain and spectral adjustments are interpolated smoothly, with configurable smoothing times to control the responsiveness and help prevent any audible artifacts. To ensure maximum fidelity in any scenario, the plugin offers both linear phase and minimum phase filter types.

To provide detailed visual feedback, TrueGain includes a comprehensive real-time spectrum analyzer. It features configurable FFT sizes, window types, and time averaging. The frequency display can be switched between linear, logarithmic, or musical note scales, with an adjustable dB range and auto-zoom to help you focus on what matters.

Note: If you have already purchased another product and would like to receive a discount price for TrueGain, click the icon in the bottom-right corner to contact the developer.

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)
• libebur128 (loudness measurements)
• melatonin_blur (blur effects)
• PFFFT (FFT library)

Demo video song credits:

• Slytek - Body Heat (Radio Edit).