Discussion

How does sound travel?

Sound comes from a series of vibrations, and all the sounds you heard in the experiment occurred because of vibrations and energy. Sound travels in waves. When a source, or something that produces sound, vibrates, it transfers its energy to the surrounding particles causing them to vibrate. Those particles then bump into the ones next to them and so on. This causes the particles to move back and forth but waves of energy to move outward in all directions from the source. Your vocal chords and the strings on a guitar are both sources which vibrate to produce sounds. Without energy, there would be no sound. Let's take a closer look at sound waves.

What do waves consist of?

Waves are made up of compressions and rarefactions. Compression happens when molecules are forced, or pressed, together. Rarefaction is just the opposite, it occurs when molecules are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the molecules are pressed together, they pass the kinetic energy to each other. Thus sound energy travels outward from the source. These molecules are part of a medium, which is anything that carries sound. Sound travels through air, water, or even a block of steel, thus, all are mediums for sound. Without a medium there are no molecules to carry the sound waves. In places like space, where there is no atmosphere, there is no sound.

Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, vibrates back and forth and creates 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 diaphragmÂ’s 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 pressure versus time. The pressure 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 pressure. 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 pressure force is the least, or the most of a pull rather than a push, when the diaphragm moves through its starting position heading the opposite way. When the pressure force is exerting a pulling force, we assign negative values to it. A graph of the pressure 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 hand 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.

Longitude

Shear

Surface

Plate(Symmetric)

Plate(Asymmetric)

Different types of waves

As the diaphragm vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that move in the same direction, or are parallel to their source 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 the wave propagates 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 the end and moving it up and down, this causes the slinky to create a wave (which looks more like the oceans waves you see) to move down to the other end. Another type of wave is the surface wave. Surface waves travel at the surface of a material and particles move in elliptical orbits. They are slightly slower than shear waves but difficult to make. A final type of sound wave is the plate wave. These waves also move in elliptical orbits but are much more complex. They can only be created in very thin pieces of material.

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