The other major type of mechanical waves are called LONGITUDINAL WAVES. In contrast to TRANSVERSE WAVES, longintudinal waves have displacements in the same direction as the direction of propagation.
The most common example of longitudinal waves is sound waves in air. Although we can't see sound waves in air, like we can waves on a string, sound waves surround us everyday.
We live near the earth's surface where the AMBIENT PRESSURE P is
atmospheric pressure, roughly 
pascals (Pa) or 14.7 pounds
per square inch (psi).
As you recall pressure is defined as a force over an area and is
given by the units of pascals, equivalent to newtons per square meter.
Sound waves, as we will see, are most easily described in terms of an ACOUSTIC PRESSURE p, which is the deviation in pressure above or below the AMBIENT PRESSURE. The ACOUSTIC PRESSURE is created because acoustic ``particles" are displaced from their equilibrium positions in sound waves, and these displacements are in the same direction as the wave travels. This is why sound waves are LONGITUDINAL WAVES.