APU Loudness Limiter

The APU Loudness Limiter is a tool designed for loudness-based limiting. Its core function is to perform limiting across multiple loudness measurement types simultaneously, including modern LUFS standards (Integrated, Short-term, Momentary) and traditional metrics (RMS, Peak, and True Peak). The True Peak detection is designed to account for inter-sample peaks.

The limiter provides a range of controls to shape its response. In addition to standard Threshold, Ceiling, Attack, and Release parameters, it includes a look-ahead feature for preemptive gain reduction. A unique “Linear Release” mode offers a constant rate of gain change, differing from traditional ballistic envelopes, which can be used to achieve very fast responses or to set a maximum release speed. You can also select between “Natural” and more responsive “Inertial” ballistics types, with optional dithering.

A central feature is the real-time visualization system, which displays the loudness of the source, target, and output signals over time. You can switch between a rolling history view and a histogram that shows the distribution of loudness levels. This display is highly configurable, with options to adjust layouts, colors, and even add aesthetic shader effects. Diagnostic tools are also built into the visualizer, such as the “Breath residual” feature which helps identify where the limiter is working hardest to meet the target gain, and a snapshot system for comparing loudness histograms.

For workflow, the limiter includes presets that target the loudness and peak requirements of various streaming services and broadcast standards, such as Spotify, Apple, and EBU R128. It also supports external sidechain input for dynamics processing, and uses a “budget” system for look-ahead and attack times, which allows you to adjust these latency-inducing parameters in real-time without causing audio artifacts.

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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+).

Measurement Modes

One of the key features of this limiter is its support for a variety of loudness types. The limiter’s detection circuitry can be configured to use either LUFS and/or traditional measurement units (RMS, Peak). You can also choose to split or link channels, which is particularly useful for multi-channel audio formats such as Dolby Atmos®.

The limiter’s detection circuitry can be configured to use any of the following loudness types:

Momentary

Momentary is a short duration LUFS measurement (400ms), which is useful for processing the audio’s moment-by-moment dynamics. This mode is responsive but also transparent. K-weighting and channel weighting are applied to the compressor’s detection circuitry, improving perceptual accuracy.

Short-Term

Short-Term is a longer duration LUFS measurement (3 seconds), which is useful for processing the audio’s large scale dynamics. Given the longer duration, this mode is less responsive than momentary, but it can be useful for adjusting the bigger picture of the audio’s dynamics. K-weighting and channel weighting are applied as with momentary.

Integrated

Integrated is an infinite duration LUFS measurement. K-weighting and channel weighting are applied as with momentary, as well as both relative and absolute gating. This loudness type can be useful for continuously wrangling the audio’s integrated loudness toward a specific target.

RMS

RMS loudness types use a rolling time interval of 150ms by default, which can be adjusted. RMS is a traditional loudness measurement, which is a good first-order approximation of perceived loudness. This mode doesn’t apply K-weighting or channel weighting to the limiter’s detection circuitry, giving it a more traditional character.

True Peak

True Peak loudness types are similar to traditional peak measurements, but they are more accurate as they take into consideration inter-sample peaks. These measurements use a time interval equal to the configured block size (1ms by default). This mode prevents clipping inter-sample peaks, which can be useful for mastering.

Peak

Peak loudness types are a standard peak circuit, operating on a time interval equal to the configured block size (1ms by default). This is another traditional limiter style measurement. Technically, this mode is even slightly more responsive than True Peak, but it is less accurate as it doesn’t take into consideration inter-sample peaks.

Alternate Weighting

In addition to the standard K-weighting for LUFS measurements, the dynamics engine now supports several alternate weighting curves. These curves modify the detector’s frequency response, allowing the limiting to be tailored for specific perceptual goals or technical standards. This can be useful for achieving more transparent results, targeting specific frequency ranges, or emulating the behavior of classic hardware.

The screenshots below are courtesy of the Loudness Contour plugin, which allows visualization of the frequency response of each contour type. For the limiter, the supported weighting types are implemented as zero-latency IIR filters, which allows them to be used in real-time without introducing latency.

ECMA-418

ECMA-418 is a modern standard for perceptual frequency weighting, designed to reflect human hearing sensitivity in typical listening environments. Unlike legacy curves such as A- or C-weighting, ECMA-418 is based on contemporary psychoacoustic research. Using it in the detector can lead to limiting that aligns very closely with perceived loudness, resulting in transparent and natural-sounding results.

ITU-R 468

ITU-R 468 is a weighting standard designed for measuring noise in broadcast systems. It strongly emphasizes the 1-9 kHz range, making the detector highly sensitive to frequencies associated with harshness or sibilance. This can be creatively used as a frequency-selective limiter to tame harshness in cymbals, vocals, or distorted guitars without needing a separate EQ in the sidechain.

A-weighting

A-weighting approximates human hearing at moderate levels (~40 phon). It has a significant low-frequency roll-off and a broad peak in the mid-range (1-5 kHz). When used in the detector, it causes the optimizer to be less reactive to low-frequency content and more sensitive to the mid-range, which can be useful for managing vocal presence or preventing bass frequencies from dominating the optimization.

C-weighting

C-weighting has a much flatter response than A-weighting, with less aggressive low-frequency roll-off, and is intended for higher sound pressure levels (~100 phon). Using C-weighting results in limiting that is more responsive to low-frequency energy, making it suitable for full-range material where you want the bass to contribute more significantly to the overall dynamics control.

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)

Demo video song credits:

• Tea Time - Warm-Up Rhythm, licensed via ShutterStock.