Simultaneous Masking is a frequency domain phenomenon where a low level signal, e.g, a smallband noise (the maskee) can be made inaudible by simultaneously occuring stronger signal(the masker), e.g, a pure tone,if masker and maskee are close enough to each other in frequency. A masking threshold can be measured below which any signal will not be audible. The masking threshold depends on the sound pressure level (SPL) and the frequency of the masker, and on the characteristics of the masker and maskee. The slope of the masking threshold is steeper towards lower frequencies,i.e., higher frequencies are more easily masked.
Figure: Threshold in quite and masking threshold.
Without a masker, a signal is inaudible if its SPL is below the
threshold of quiet, which depends on frequency and covers a dynamic
range of more than 60 dB as shown in the lower curve of Fig 
.
We have just described masking by only one masker. If the source
signal consists of many simultaneous maskers, a global masking
threshold can be computed that describes the threshold of just
noticeable distortions as a function of frequency. The calculation of
the global masking threshold is based on the high resolution short
term amplitude spectrum of the audio or speech signal, sufficient for
critical band based analysis, and is determined in audio coding via
512 or 1024 point FFT. In a first step all individual masking
thresholds are calculated, depending on signal level, type of
masker(noise or tone), and frequency range. Next the global masking
threshold is determined by adding all individual thresholds and the
threshold in quiet.( Adding this later threshold ensures that the
computed global masking threshold is not below the threshold in
quiet). The effects of masking reaching over critical band bounds must
be included in the calculation. Finally the global signal-to-mask
ratio (SMR) is determined as the ratio of the maximum of signal power
and global masking threshold as shown in Fig .
Figure: Masking threshold and SMR