Frequency Masking

Broadband white noise can mask test tones. White noise has a spectral density that is independent of frequency. Other types of noise and signals, such as pink noise, narrow-band noise, pure tones, and complex tones, also can mask a test signal. When narrow-band noise is the masking signal, masked thresholds show a very steep rise greater than 100 dB per decade as the test tone increases in frequency up to the center frequency of the narrow-band noise. This test tone increase is independent of the level of the masking noise. For frequencies greater than the center frequency of the noise, the masked threshold decreases quickly for low levels of masking noise but more slowly for high levels of masking noise. When pure tones are the masking signal, the signal needs additional filters to remove measurement artifacts such as audible beating and difference tones. The following figure shows the masked threshold for a masking signal at a frequency of 1 kHz.