One sound can mask another even when their spectra do not overlap.

Evidence

The effect of spectral masking occurs when a sound with certain spectral content makes the detection of another sound with different spectral content harder, even though the spectra do not necessarily overlap. Spectral masking, that is, how the masker sound affects the detection threshold of the test sound, can be best described by plotting the masking threshold as a function of frequency. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 991

Categorization

• DEFINE: Clarifies that masking does not require spectral overlap — a non-obvious mechanism underpinning codec design.