When Sounds Disappear
Auditory masking is the phenomenon where one sound renders another inaudible — not by physically canceling it, but by overwhelming the neural response in the cochlea. When you cannot hear your phone ring in a noisy restaurant, that is masking at work. The effect is frequency-dependent: a 1 kHz tone at 70 dB can completely hide a 1.2 kHz tone at 50 dB, even though 50 dB is well above the absolute threshold of hearing.
The Masking Pattern
A single pure tone produces a characteristic masking pattern: the threshold elevation is greatest near the masker frequency and falls off on both sides, but asymmetrically. The upward spread (toward higher frequencies) is broader than the downward spread, and both slopes become shallower at higher masker levels. This pattern mirrors the excitation pattern on the basilar membrane.
Critical Bands: The Ear's Frequency Resolution
The cochlea decomposes sound into approximately 24 overlapping frequency bands (critical bands or Bark bands). Within a single critical band, energy is summed before masking is computed. Two tones within the same critical band interact strongly; tones in separate bands are largely independent. This frequency resolution — about 1/3 octave at mid frequencies — sets the fundamental limit on spectral detail the ear can resolve.
Perceptual Audio Coding
The entire MP3 revolution rests on masking. By computing which spectral components are masked and therefore inaudible, perceptual codecs allocate bits only to audible components. A 1411 kbps CD signal can be compressed to 128 kbps with near-transparent quality because roughly 90% of the spectral detail is perceptually irrelevant. This simulation lets you explore the masking patterns that make modern audio streaming possible.