“Signals are foreground sounds and they are listened to consciously. In terms of the psychologist, they are figure rather than ground. Any sound can be listened to consciously, and so any sound can become a figure or signal, but for the purposes of our community-oriented study we will confine ourselves to mentioning some of those signals which must be listened to because they constitute acoustic warning devices: bells, whistles, horns and sirens.” Sterne, J 2012, The Sound Studies Reader, Routledge

We call signals to the more meaningful sounds which happen in our near environment and are subject to a predeterminate cultural context. In psychology, these kind of sounds are more rapidly understood by ourselves. Any sound could really be understood as a signal, so any sound could become meaningful depending on our own different code, however, in terms of academical study we will just mention those sound which are culturally understood as acoustic alerts, like bells, horns, sirens or whistles.

References:

Sterne, J (2012) The Sound Studies Reader, Routledge