The type of sound vibrations traveling in the same direction as air particle movement are called compressional, or longitudinal vibrations. The vibrations travel horizontally and produce a highly compressed sound. Visually, longitudinal vibrations resemble a Slinky toy. The vibrations are the easiest type to produce. Earthquakes and explosions create longitudinal sound vibrations. Vibrations could consist of single or multiple pulses, or energy bursts that create sound. Longitudinal vibrations travel quickly and create the highest sound frequency.
Transverse, or shear vibrations are the type of sound vibrations that move at right angles to air particle movement. Transverse sound vibrations create slower sound waves. A vibrating string represents transverse sound vibrations. Move the string in an up-and-down motion to create pulses along the entire length. The pulses travel at varied speeds, like true transverse sound vibrations. Secondary earthquake vibrations and the wave motion performed by sporting crowds are additional examples of transverse vibration action.
Surface sound vibrations occur close to the surface of the Earth. A refractive index measures the speed of light traveling through a particular substance. Earth and its atmosphere posses two different refractive index ratings, creating an interface that supports surface sound vibrations. Lower-frequency radio waves on the amplitude modulation (AM) band use surface vibrations for broadcasting. Due to solid earth being a poor electrical conductor, surface vibration sound is easily blocked or interrupted. Surface vibrations are incredibly difficult to produce and move at a slower rate than vibrations of a transverse nature.
Only the movement of extremely thin materials produce the final type of sound vibration, called plate waves. Nondestructive Testing uses plate vibrations to test structures and ensure that they function properly. Ultrasonic testing transmits plate waves into a material to detect imperfections or changes in material properties. NDT commonly uses ultrasonic testing on metals. Lamb waves are plate vibrations commonly used in NDT testing. Symmetrical waves travel along the surface, creating sound. Asymmetrical waves produce surface movement with some parallel movement.