Another type of mechanical LONGITUDINAL WAVE is seen in long, thin rods or bars. If you take a hammer and hit the end of a rod, a longitudinal type wave called an extensional wave is sent down the rod. The speed of this wave is given by
(Picture of hammer hitting a rod on end.)
As you can now see, one can have mechanical waves both in fluids, such as our atmosphere or water, and in solids. All of these types of waves are important for noise control. They also are all described using similar mathematical formulae.