Keynote sound

In music, a keynote identifies the key or tonality of a particular composition. It provides the fundamental tone around which the composition may modulate, but from which other tonalities take on a special relationship. In soundscape studies, keynote sounds are those that are continuously or frequently heard by a particular society to form a background against which other sounds are perceived. Examples might be the sound of the sea for a maritime community or the sound of the internal combustion engine in the modern city. Often keynote sounds are not consciously perceived, but they act as conditioning agents in the perception of other sound signals. They have accordingly been likened to the ground in the figure-ground grouping of visual perception. 

(Source: Schafer, R. Murray: Our Sonic Environment and the soundscape: the tuning of the world. Destiny Books 1994).