If someone listens to a soft and a loud sound at the same time, he or she may not hear the soft sound. The soft sound is masked by the loud sound. The loud sound has a greater masking effect if the soft sound lies within the same frequency range, but masking also occurs when the soft sound is outside the frequency range of the loud sound.
Masking of soft sounds is more pronounced among hearing-impaired people.
Masking can occur both ways - forwards and backwards, and can spread upwards as well as downwards.
When the masking sound precedes the masked sound, it is called forward masking. When the masking is backwards, the masking sound comes milli-seconds after the masked sound. When the masking and masked sounds come at the same time, masking is simultaneous.
Upward spread of masking is low-frequency sounds masking high-frequency sounds. Downward spread of masking occurs when low-frequency sounds are masked by an intense level of high-frequency sounds.
Masking is more effective when the loud sound is a low-frequency sound and the soft sound is a high-frequency one, rather than the opposite.