After reading this section you will be able to do the following:
- Summarize how sound travels and explain what the energy is that allows it to occur.
- Describe the different components waves have.
- List and discuss the different types of waves that exist.
- In each case, what is the energy that makes the sound happen?
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What is sound and how does it travel?
All of the sounds you can hear from plucking the strings above occur because mechanical energy produced by your computer speaker was transferred to your ear through the movement of atomic particles. Sound is a pressure disturbance that moves through a medium in the form of mechanical waves. When a force is exerted on an atom, it moves from its rest or equilibrium position and exerts a force on the adjacent particles. These adjacent particles are moved from their rest position and this continues throughout the medium. This transfer of energy from one particle to the next is how sound travels through a medium. The words "mechanical wave" are used to describe the distribution of energy through a medium by the transfer of energy from one particle to the next.
Waves of sound energy move outward in all directions from the source. Your vocal chords and the strings on a guitar are both sources which vibrate to produce sound waves. Without energy, there would be no sound. Let's take a closer look at sound waves.
What do waves consist of?
Sound or pressure waves are made up of compressions and rarefactions. Compression happens when particles are forced, or pressed, together. Rarefaction is just the opposite, it occurs when particles are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the particles are moved from their rest position, they exert a force of the adjacent particles and pass the kinetic energy. Thus sound energy travels outward from the source.
Sound travels through air, water, or a block of steel; thus, all are mediums for sound. Without a medium there are no particles to carry the sound waves. The word "particle" suggests a tiny concentration of matter capable of transmitting energy. A particle could be an atom or molecule. In places like space, where there is no atmosphere, there are too few atomic particles to transfer the sound energy.
Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, is caused to vibrate using electromagnetic energy. When the diaphragm moves to the right, its energy pushes the air molecules on the right together, opening up space for the molecules on the left to move into. We call the molecules on the right compressed and the molecules on the left rarefied. When the diaphragm moves to the left, the opposite happens. Now, the molecules to the left become compressed and the molecules to the right are rarefied. These alternating compressions and rarefactions produce a wave. One compression and one rarefaction is called a wavelength. Different sounds have different wavelengths.
What do sound waves look like?
We cannot see the energy in sound waves, but a sound wave can be modeled in two ways. One way is to create a graph of the diaphragm's position at different times. Think of a number line. We call the diaphragm's rest position zero. As it travels to the right, it moves to an increasingly positive position along the number line. As is travels to the left, its position becomes more and more negative. The graph of the diaphragms position as it vibrates looks like the sine graph, with its highest point when the diaphragm is the farthest right and its lowest point when it is farthest left.
Another graph can be made using the amount of force on the molecules versus time. The force is greatest when the diaphragm is moving through its original position. This is similar to the way we feel the greatest force on a swing as we move through the center, where we started. As the diaphragm moves to the right, there is less and less force. At its rightmost position, it is exerting no force (due to pressure) and begins its trip the opposite way. Similarly, the diaphragm is exerting no force at its leftmost position. For our graph, we say the force is least when the diaphragm moves through its starting position heading the opposite way. When the force is exerting a pulling force, we assign negative values to it. A graph of the force versus time also resembles the sine graph.
More about compression and rarefaction
Compression and rarefaction are terms defining the molecules near the diaphragm. Compression is the point when the most force is being applied to a molecule and rarefaction is the point when the least force is applied. It is important to note that when a molecule to the right of the diaphragm is experiencing compression, a molecule to the diaphragm's left is experiencing rarefaction. For right side molecules, compression occurs when the diaphragm is in its original position, moving towards the right. This is where the molecule experiences the most force. Rarefaction happens when the diaphragm is once again in the center, this time moving towards the left. At this point, the molecule is experiencing the least force. Of course, this is the opposite for molecules to the diaphragm's left.
Different types of waves
As the diaphragm vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that travel in the same direction as the particle movement are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed. However, it is possible to produce other types. Waves which move perpendicular to the direction particle movement are called shear waves or transverse waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Think of a stretched out slinky, you can create a longitudinal wave by quickly pushing and pulling one end of the slinky. This causes longitudinal waves for form and propagate to the other end. A shear wave can be created by taking one end of the slinky and moving it up and down. This generates a wave that moves up and down as it travels the length of the slinky.
Another type of wave is the surface wave. Surface waves travel at the surface of a material with the particles move in elliptical orbits. They are slightly slower than shear waves and fairly difficult to make. A final type of sound wave is the plate wave. The particles of these waves also move in elliptical orbits but plate waves can only be created in very thin pieces of material.