APU Software Loudness Contour v5.1.8 (Win/macOS)

APU Loudness Contour is a real-time equal-loudness filter plugin that lets you shape your mix according to how human hearing actually perceives frequency balance at different volumes. It includes a wide range of perceptual contours such as ISO 226 (2003 and 2023), Fletcher-Munson, K-weighting, A/C-weighting, ECMA-418, and ITU-R 468. Each contour is normalized to 0 dB at 1 kHz and has a dedicated weight parameter that scales the response from flat (0%) to inverted (negative weights). For ISO 226 and Fletcher-Munson, you can set reference levels from 20 to 120 dB SPL with smooth interpolation between phon curves. The plugin offers both linear and minimum phase filter types, and built-in makeup gain compensation calibrated via pink noise LUFS measurements. A comprehensive real-time spectrum analyzer includes configurable FFT size, window type, bucket width, and time averaging, with linear, logarithmic, or musical note frequency scales. Ideal for hobby producers using FL Studio, Ableton, or Logic who want to check and correct tonal balance at realistic listening levels.