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 5: 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 5. 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 6.
Figure 6: Masking threshold and SMR