Noun
longitudinal wave (plural longitudinal waves)
(physics) Any wave in which the direction of disturbance is parallel to the direction of travel.
By contrast, a longitudinal wave is one in which the amplitude vector is parallel to k, such as for acoustic waves in a gas or fluid. Source: Internet
It is given by : The wave equations are also valid below the cutoff frequency, where the longitudinal wave number is imaginary. Source: Internet