I have been reading a couple of definitions on signal such as:
Any quantity measurable through time over space or any higher dimension can be taken as a signal.
In the physical world, any quantity exhibiting variation in time or variation in space (such as an image) is potentially a signal that might provide information on the status of a physical system
Taking human voice as example,
A sound signal is converted to an electrical signal by a microphone, generating a voltage signal as an analog of the sound signal, making the sound signal available for further signal processing. Sound signals can be sampled at a discrete set of time points.
I can understand where times comes in but where's the "space" come here according to the definition of signal?