👂 Psychoacoustic Masking

Psychoacoustic masking is the phenomenon where a louder sound (the masker) raises the effective hearing threshold for other, quieter sounds (the probe) that fall near it in frequency and time. If the probe's level is below this raised threshold, it becomes inaudible even though it would be perfectly audible in silence.

Yes. The masking curve has a steep low-frequency skirt (roughly -27 dB per Bark below the masker) and a much shallower high-frequency skirt. This means a low-frequency masker spreads upward and masks higher frequencies more effectively than a high-frequency masker masks lower ones, a property called the asymmetry of masking or upward spread of masking.

Perceptual audio codecs compute a masking threshold across the spectrum for every short block of audio, then allocate fewer quantization bits (or none at all) to frequency components that fall below that threshold, since a listener could not hear the added quantization noise there anyway. This lets the codec discard or coarsely encode inaudible information, shrinking file size with minimal perceived quality loss.

Temporal masking is the time-domain counterpart of simultaneous masking. A loud sound can mask a softer sound that follows it for tens to hundreds of milliseconds (post-masking, or forward masking) and, more surprisingly, can even mask a softer sound that precedes it by a few milliseconds (pre-masking, or backward masking) because the auditory system needs time to process loud onsets. This simulation focuses on simultaneous (frequency-domain) masking only.

The cochlea behaves like a bank of overlapping bandpass filters called critical bands; sounds within the same critical band interact and mask each other far more strongly than sounds in different bands. The Bark scale (24 bands spanning the audible range) rescales frequency so that each unit step corresponds to roughly one critical bandwidth, which is why masking curves are drawn and computed on a Bark axis rather than a linear Hz axis.

The masking curve's height scales directly with masker level, and its high-frequency slope also becomes shallower as masker level increases, so a louder masker not only raises the threshold more but spreads that raised threshold across a wider frequency range, particularly upward in frequency.

Different playback systems and rooms alter the relative levels and spectral balance of simultaneous sounds. If a change in equipment shifts the level of a masker relative to a probe tone, a sound that sat below the masked threshold on one system can rise above it on another, making previously masked details suddenly audible.

Clinical pure-tone audiometry must present a test tone to one ear while sometimes masking the other ear with noise, to prevent the non-test ear from "cross-hearing" the tone through bone conduction and giving a false threshold. Audiologists use calibrated masking noise levels derived from the same masking principles modeled in this simulation.

This simulation uses a simplified piecewise-linear spreading function on the Bark scale, based on the classic model used in early MPEG perceptual audio coding (after Schroeder, Atal and Hall). It approximates the masking curve's shape with different slopes below and above the masker frequency, and is an approximation of the true auditory filter shape rather than an exact physiological measurement.