Sound is part of our everyday lives. Just like we have eyes for, we are given ears for hearing sound. We rarely take the time to think about the characteristics of sound and the ways that they are made. The basis for an understanding of sound and hearing is the physics of waves. Sound is a wave, which is created by vibrating objects and spread through a medium from one location to another. In this paper, we will look at the nature, properties and behaviors of sound waves.
A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location. The medium is simply the material through which the sound, or disturbance, is moving; it can be thought of as a series of interacting particles. The example of a slinky wave is often used to illustrate the nature of a wave. A disturbance is typically created within the slinky by the back and forth movement of the first coil of the slinky.
The first coil becomes disturbed and begins to push or pull on the second coil; this push or pull on the second coil will shift the second coil from its balanced position. As the second coil becomes shifted, it begins to push or pull on the third coil; the push or pull on the third coil displaces it from its balanced position. As the third coil becomes shifted, it begins to push or pull on the fourth coil. This process continues one after the other, each individual particle acting to displace the neighboring particle; eventually the disturbance travels all the way through the slinky. As the disturbance moves from coil to coil, the energy which was originally introduced into the first coil is transported along the medium from one location to another.
A sound wave is similar to a slinky for a couple of reasons. First, there is a medium, which carries the disturbance from one location to another. Usually, this medium is air; though it could be any material such as water or steel. Th medium is simply a series of interconnected and interacting particles. Second, there is an original source of the wave, some vibrating object capable of disturbing the first particle of the medium. The vibrating object, which creates the disturbance, could be the vocal chords of a person, the vibrating string and sound board of a guitar or violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a radio speaker.
Third, the sound wave is transported from one location to another by means of the particle interaction. If the sound wave is moving through air, then as one air particle is shifted from its balanced position, it exerts a push or pull on its nearest neighbors, causing them to be shifted from their equilibrium position. This particle interaction continues throughout the entire medium, with each particle interacting and causing a disturbance of its nearest neighbors. Since a sound wave is a disturbance, which is transported through a medium via the means of particle interaction, a sound wave is characterized as a mechanical wave.
The creation of sound waves are often demonstrated by using a tuning fork. A tuning fork is a metal object that has two tines (spikes) that vibrate if they are hit by a rubber hammer or mallet. As the tines of the tuning forks vibrate back and forth, they begin to disturb surrounding air molecules. These disturbances are passed on to neighboring air molecules by the means of particle interaction. The motion of the disturbance, originating at the tines of the tuning fork and traveling through the medium (in this case, air) is what is referred to as a sound wave. Sometimes, the tuning fork is mounted on a soundboard.
When this happens, the vibrating tuning fork, connected to the soundboard, sets the soundboard into a vibrating motion. In turn, the soundboard sets the air inside of the it into a vibrating motion. As the tines of the tuning fork, the structure of the soundboard, and the inside of the soundboard begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles which can be made to vibrate, the louder or more amplified the sound. This concept is also demonstrated by the placing the vibrating tuning fork against the glass panel of a window; the vibrating tuning fork sets the glass panel into a vibrating motion and results in an amplified sound. There are two different categories of waves: mechanical waves and electromagnetic waves.
Electromagnetic waves are waves that have an electric and magnetic nature and that are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy. Mechanical waves are waves that require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through regions of space which have no particles, so sound waves cannot travel through a vacuum. Sound is created by a vibrating object. The vibrations of the object set particles in the surrounding medium in a vibrating motion, transporting energy through the medium.
The vibrations of the particles are best described as longitudinal. Longitudinal waves are waves that the motion of the individual particles of the medium are in a direction which is parallel to the direction of energy transport. A longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal direction and the first coils of the slinky are vibrated horizontally. When this happens, each individual coil of the medium is set into a vibrating motion in directions parallel to the direction which the energy is transported.
So sound waves are longitudinal. A vibrating string can create longitudinal waves. As the vibrating string moves in the forward direction, it begins to push upon surrounding air molecules, moving them to the right towards their nearest neighbor. This causes the air molecules to the right of the string to be compressed into a small region of space. As the vibrating string moves in the reverse direction (leftward), it lowers the pressure of the air immediately to its right, thus causing air molecules to move back leftward. The lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large region.
The back and forth vibration of the string causes air molecules in the region to continually move back and forth horizontally; the molecules move rightward as the string moves rightward and then leftward as the string moves leftward. These back and forth vibrations are passed on to bordering neighbors by particle interaction; making other surrounding particles begin to move rightward and leftward, then sending a wave to the right. Since air molecules are moving in a direction which is parallel to the direction which the wave moves, the sound wave is referred to as a longitudinal wave. The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air. Regardless of the source of the sound wave - whether it is the vibrating string or the vibrating tines of a tuning fork - sound is a longitudinal wave. And the essential characteristic of a longitudinal wave is that the particles of the medium move in a direction parallel to the direction of energy transport.
So as you can see, sound waves are both mechanical and longitudinal. And they are constantly around us and effecting all that we do. Whether we are talking, singing or even listening to music, sound is a major part of our everyday lives and that will probably never change (unless we run out of AIR).