Sound waves traveling in fluids, such as air and water, are longitudinal mechanical waves.   Solid media can support longitudinal and transverse sound waves.  In this course, we will only consider longitudinal sound waves and, for simplicity, we say sound waves are longitudinal waves.

Sound waves are very important to us; without them we would not be able to communicate.  The vibrations that you produce in your mouth are carried by sound waves to your friends' ears. Human ear can detect vibrations in the range of 20 to 2000 Hz.  Sound waves, also, are used in many applications. For example, ships use sound waves to detect underwater obstacles and fishes.  Vibrations with frequencies higher than 20000 Hz are called ultrasound waves.  Ultrasound waves  are used by doctors  to image fetuses in his mother's womb and they are used by engineers to detect defects in structures.  Vibrations with frequencies below 10 Hz, can travel over very long distances and because of this they are used to probe Earth's crust, for example, for oil.

Mechanical waves are waves that require medium to travel, they cannot travel in vacuum.  We call mechanical waves transverse waves when the medium particles oscillate in a direction perpendicular to the direction in which the wave travels and we call mechanical waves longitudinal waves when the medium particles oscillate in a direction parallel to the direction in which the wave propagates.