(LSES) Science of Sound Lesson

Science of Sound

A wave of any kind has certain components that we can use to describe how that wave behaves. Sound waves are no different, and there are many components to a sound wave that we need to understand. Most sound travels in a wave form called a longitudinal wave or a compression wave. While all waves propagate energy in one direction, compression waves occur in a series of alternating pressures that only move in the same direction as the energy.

A Compression Wave

Other waves called transverse waves, oscillate and transmit energy in one direction, but through an up and down motion.

A Transverse Wave

To get an idea of how each type of wave behaves we can use two examples:

1. A compression wave can be demonstrated with a slinky. A slinky is a large spring that can be pushed in a forward direction where the spring compresses and decompresses along its length. As each compression occurs, energy is propagated to the next part of the spring allowing energy to travel from one end to the other.
2. In contrast, a transverse wave can be demonstrated by shaking a rope up and down creating a high and low point along the way. Each high point is called a crest and each low point is called a trough. Even though the movement of energy is in the forward direction, it is transmitted down the length of the rope in an oscillating motion. Both types of waves transfer energy, but each carries that energy in a different way.

Like other waves, compression sound waves are a form of energy that causes molecules to vibrate at certain frequencies. Frequency is the number of waves that pass a given point in a certain amount of time. The frequency of a sound wave determines how much energy the sound is carrying. If you have ever been in front of a large speaker or other device that emits sound, you can feel that energy through vibration. As energy moves from the speaker to the listener, air is compressed and the shock wave from the energy source can be detected by not only your ears, but the hairs on your skin that can detect movement.

Different Frequencies (from low to high)

Another component of a compression wave is wavelength (sometimes called a cycle). Wavelength is the distance between two points of a wave. The wavelength in a compression wave is different from the wavelength of a typical transverse wave. In a transverse wave there are two crests and two troughs and the distance between them are equal to the wavelength. In a compression wave energy causes molecules to move closer together (compression) and then further apart (rarefaction) and the wavelength is the distance between two compressions.

Now that we know the basic parts of a compression wave and a transverse wave, we can move on to investigate how sound moves through different materials. There are many factors that determine how fast sound waves move and how we hear sounds as we do. In the next lesson we see how sound is propagated and why it moves through different materials in unique ways.

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