It is a theory in psychology that our perception of objects, both visual and auditory, is determined by certain principles. These principles function so that our perceptual worlds are organized into the simplest pattern consistent with the sensory information and with our experience.
In hearing, we also tend to organize sounds
into auditory objects or streams and use the principles of grouping to help us to segregate those components we are interested in from others. We are thus able to focus our listening attention to a particular noise source and distinguish an auditory object from the background noise.
The human ears can detect not only changes in the overall sound pressure level but are so sophisticated that they can detect sound, the sound pressure level of which is well below the background noise level.
While there are variations in individual perception of the strength of a sound, studies have shown that to a good approximation, the sound is perceived twice as loud if the sound level increases by 10 dB. Similarly, a 20 dB increase in the sound level is perceived as four times as loud by the normal human ear.