A null point, in acoustics and signal processing, refers to the point on a sound wave, at which there is zero displacement of the medium. This is also called a node.
Null points can also refer to points in space which, due to the way sound travels and interacts with itself, can be unintentionally (or intentionally) created through the reflection and later destructive interference of sound originating from one or more sources. This is something that you should take into account when planning your next home entertainment center or concert hall.
The term, "null" point is also used occasionally in specific applications to describe a point along a gradient, function, surface, or some other curve where a dependent value either reaches zero or an equilibrium value. Examples of this are the point of minimum signal reception in communications technology (satellite dishes, antennae, and so on), and the point of equilibrium in a Wheatstone Bridge.