The figure also illustrates the relationships between ultrasonic wavelength, frequency, and propagation velocity. The top wave is call a longitudinal wave because oscillations occur in the longitudinal direction, the direction of propagation. Since compressional and dilational forces are active in it, it is also called a pressure or compressional wave. Because its particle density fluctuates, it has also been given the name density wave . This is a sound wave because it transmits the oscillations of a source of acoustic energy through the air to our ears.
Research shows that the same wave also transmits sound through liquid or solid bodies.
It can be seen that in this case the particles no longer oscillate in the direction of propagation but at right angles to it, that is transverse. Among the properties of propagating waves are the quantities, wavelength , frequency , and velocity . The are related by the equation in the figure above.