In acoustics, simultaneous masking is masking between two concurrent sounds. Sometimes called frequency masking or spectral masking since it is often observed when the sounds share a frequency band e.g. two sine tones at 440 and 450 Hz can be perceived clearly when separated. They cannot be perceived clearly when simultaneous.
The "unmasked threshold" is defined as the quietest level of the signal which can be perceived without any masking present, and a "masked threshold" is the quietest level of the signal perceived when combined with a specific masking noise. The amount of masking is the difference between the masked and unmasked thresholds.
Gelfand provides a basic example.[1] Let us say that for a given individual, the sound of a cat scratching a post in an otherwise quiet environment is first audible at a level of 10 decibels (dB). However, in the presence of a "masking" noise—for example, a vacuum cleaner that is running simultaneously—that same individual cannot detect the sound of the cat scratching unless the level of the scratching sound is at least 26 dB. We would say that the unmasked threshold for that individual for the target sound (i.e., the cat scratching) is 10 dB, while the masked threshold is 26 dB. The amount of masking is simply the difference between these two thresholds—in our example, 16 dB.
The amount of masking will vary depending on the characteristics of both the target signal and the masker, and will also be specific to an individual listener. While the person in the example above was able to detect the cat scratching at 26 dB, another person may not be able to hear the cat scratching while the vacuum was on until the sound level of the cat scratching was increased to 30 dB (thereby making the amount of masking 20 dB).