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

Frequency spectrum

The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...

 e.g. two sine tones at 440 and 450 Hz can be perceived clearly when separated. They cannot be perceived clearly when simultaneous.

Unmasked threshold

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. 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).

See also

• Auditory masking
  Auditory masking
  Auditory masking occurs when the perception of one sound is affected by the presence of another sound.- Simultaneous masking :Simultaneous masking is when a sound is made inaudible by a "masker", a noise or unwanted sound of the same duration as the original sound.-Critical bandwidth:If two sounds...
  
  
• Psychoacoustics
  Psychoacoustics
  Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...
  
  
• Temporal masking
  Temporal masking
  Temporal masking or "non-simultaneous masking" occurs when a sudden stimulus sound makes inaudible other sounds which are present immediately preceding or following the stimulus...
  
  
• Spectral masking