Nature of a Mechanical Sound Pressure Wave

Sound is an everyday part of our life. Hearing is one of our five human senses, normally only preceded by sight in importance. It is interesting to notice how fully sound can take the place of sight in the case of a sightless person. In our normal lives, we rarely take the time to question how sounds are produced, how they travel and how they are detected.

Sound is a mechanical pressure wave traveling through a medium. On this site we will look at some of the physical properties of sounds and how they are produced and detected. But first, let us look at what sound really is.

A Mechanical Pressure Wave Traveling through a Medium

Sound is a pressure disturbance that travels through a medium, thereby transporting energy. The medium is whatever matter the pressure wave is traveling through. When you stand by the side of train tracks and hear a train coming, the medium is air. If instead, you place your ear on the train tracks, the medium would be the steel rails.

Another good example of a mechanical pressure wave is a slinky. If you look at the patterns of the slinky coils in the picture below, you'll see the "pressure" wave as is travels down the slinky medium. You'll notice the disturbance travels from one coil to the next, gradually dissipating the vibrating energy.

Sound pressure waves are similar to the slinky pressure wave above with the main difference being that the medium for sound is typically air, although it could really be any medium.

Read more about longitudinal waves and their properties.

The Source of Sound

The source of sound is typically any vibrating matter. It could be the vocal cords of a person speaking, a guitar string, a vibrating speaker or the legs of a tuning fork. The disturbance vibrations then travel through the medium, away from the source. In air, the vibration disturbance is moved along in a domino effect as each particle is moved from its equilibrium position momentarily, followed by its neighbor moving, followed by the next neighbor moving, as the neighboring particles interact with one another. Since the sound involves actual movement of air, we call it a mechanical sound pressure wave.

Light, on the other hand, is an electromagnetic wave, and does not require a medium to travel. Therefore, light can travel through space.

What Happens When a Sound is Made in a Vacuum?

Sound is a pressure wave traveling through a medium. If you place a ringing bell inside a vacuum, the bell can still ring and you could see it ringing, but you will not be able to hear it since their is no air around the bell to carry the pressure waves to your ears. Mechanical pressure waves require a medium to transport the vibration and without a medium, the pressure waves will not travel.

Longitudinal and Transverse Waves

Sound is a longitudinal mechanical pressure wave. Longitudinal means that the particles of air travel back and forth, toward and away from the source. Another type of wave is the transverse wave. In this case, the particles travel up and down. A good example of a transverse wave is a wave of water. The actual water droplets travel up in waves and down in crests and troughs, while the wave travels along the surface of the water. Look at the pictures below to see the animated demonstration of both pressure and transverse waves.

Read more about longitudinal waves and their properties.