APU Software

TrueGain

Gain control with equal-loudness compensation

$40.00

APU TrueGain adjusts gain and frequency balance together, using equal-loudness contours such as ISO 226. The compensation accounts for changes in hearing sensitivity at different listening levels. Adaptive SPL tracks changes in the input level and updates the compensation as the audio plays.

Calibration connects the plug-in’s level estimates to your monitoring setup. A real-time spectrum analyzer shows the source, target, and output signals.

TrueGain can be operated in four modes:

Compensated Gain Control

Adjust the True gain parameter to raise or lower the volume with frequency compensation based on your calibrated listening level and the selected contour model.

Compensated AGC

Set a target SPL and let TrueGain adjust the gain and frequency compensation as the input level changes. Attack and release controls set how quickly it responds.

Translation Check

Use the target SPL parameter to hear the tonal changes predicted by the contour model at a different listening level, without changing your monitoring volume. This is an approximation of level-dependent hearing, not a simulation of a particular room or playback system.

Gain Control

Adjust gain without frequency compensation. Gain changes still use the plug-in’s ballistics controls.

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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

Calibration gives TrueGain a reference for estimating the sound pressure level at your listening position. Place TrueGain before room/headphone correction or crossfeed in your monitoring chain, and keep those processors enabled during calibration.

Calibration for Speaker Monitoring:

Place an SPL meter at your listening position, using C-weighting and a slow response.
Click “Calibrate SPL ...”, then select Speakers: Pink Noise (Band-Limited) from the calibration signal type drop-down. A 1 kHz sine wave and full-bandwidth pink noise are also available.
Play one speaker at a time using your monitor controls. At your chosen monitoring volume, set Reference SPL to the meter reading. If you are using a reference-level preset, adjust each speaker’s playback level to match it.
Click “Stop calibration” and restore normal speaker routing. Keep your monitor volume and downstream processing at their calibrated settings.

For Bob Katz’s K-System, the K-20 reference is 83 dBC SPL per speaker, measured one speaker at a time with pink noise at -20 dBFS RMS (+3). The reference-level drop-down offers common examples; choose the level appropriate to your monitoring setup.

Calibration Signal Levels:

The built-in speaker signals are normalized to -20 dBFS on a sine-referenced RMS (+3) meter, equivalent to approximately -23.01 dBFS mathematical RMS. This RMS reference does not define a fixed LUFS value for program material.

The built-in band-limited pink noise covers 500-2500 Hz. Its stored mono K-weighted calibration measurement is approximately -22.22 LUFS. The EBU reference listening signal covers 500-2000 Hz and is normalized to -23 LUFS. Their bandwidths and normalization targets differ. Adaptive SPL uses the stored measurement for the selected calibration signal and loudness type.

Calibration for Headphone Monitoring:

Keep your headphone volume at your chosen listening setting, with headphone correction and crossfeed enabled if you normally use them.
Click “Calibrate SPL ...”, then select Headphones: Dialog (Male) or Headphones: Dialog (Female) from the calibration signal type drop-down.
Adjust Calibration offset until the spoken dialog sounds like a normal conversation from about 3 feet away in a quiet room, then click “Stop calibration”.

While headphone calibration plays, Reference SPL updates automatically using Reference SPL = 65 - Calibration offset, in dB SPL. The 65 dB anchor is the plugin’s conversational reference. Dialogue matching provides a subjective estimate of listening level, not a direct measurement of headphone SPL.

Keep the selected calibration signal type after stopping playback so Adaptive SPL uses the same baseline. Calibration anchors the contour model to your playback setup; its loudness compensation remains an estimate. Repeat calibration after changing monitor volume or downstream processing.

Translation Check in Practice

Check the tonal changes predicted at another listening level while keeping your monitor volume at its calibrated setting.

For monitoring calibrated at 75 dB SPL, choose Translation Check, set Reference SPL to 75 and Target SPL to 83, and turn Adaptive SPL off for a fixed comparison. TrueGain applies the selected contour model’s 75-to-83 difference with no uniform gain adjustment. The EQ can still change measured loudness.

Shown with ISO 226:2023 and Minimum Phase.

Explore the full worked examples for automatic gain control, Adaptive SPL, and output-trim comparisons.

Contour Types

TrueGain applies the difference between two equal-loudness contours, rather than applying a single contour as an EQ curve. It interpolates between stored curves as the reference level and gain change.

ISO 226 (2003)

ISO 226:2003 describes equal-loudness contours for pure tones under controlled listening conditions. These curves show how hearing sensitivity varies with frequency and level. TrueGain’s 2003 contour model supports reference levels from 20 to 120 dB SPL, with interpolation between curves.

ISO 226 (2023)

ISO 226:2023 is a newer edition of the equal-loudness standard. TrueGain includes a separate 2023 contour model with the same 20 to 120 dB SPL reference range and interpolation controls.

Fletcher-Munson

Fletcher-Munson contours come from the 1933 equal-loudness study. TrueGain uses digitized curves from that work, interpolated across its 20 to 120 dB SPL reference range.

Mini Mode

Mini mode keeps the main level control visible in a small window. It shows the True gain knob in Compensated Gain Control mode and the Target SPL knob in Compensated AGC mode.

Use the mini-mode button in the upper-right of the editor to switch views. The expand button returns to the full editor.

Credits

Developed by APU Software, LLC with the following libraries:

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).