Waves and Energy

Science background provides teachers with more in-depth information about the phenomena students explore in this unit on sound waves. Here is an excerpt of the background information on waves and energy.

Because waves transfer energy, they require a source since energy cannot be created or destroyed. Raindrops falling on a still lake are a good illustration of this. When the raindrops hit the water’s surface, they transfer their kinetic energy to the water. This creates water waves that carry that energy as they ripple outward. Once the ripples die down, the water returns to the same position.

Scientists classify waves according to how the disturbance moves relative to the wave itself. In some kinds of waves, the disturbance moves perpendicular to the direction of the wave itself. In other words, if the wave moves from left to right, the disturbance moves up and down. This kind of wave is called a transverse wave. When raindrops fall on the water, they hit the water vertically, while the wave moves horizontally. (Water waves also move in a longitudinal motion, which we’ll explore next, so they are categorized as their own kind of wave.)

When a crowd of people in a stadium do “the wave,” they are modeling a transverse wave. The individual people move up and down, while the wave moves from left to right (or right to left). This is an example of how waves move energy, not matter. After the disturbance passes through, the people go back to where they were before the wave moved through.

Sound waves move differently. Sound waves are patterns of vibrating molecules caused by the movement of sound through a medium. They are a type of wave called a longitudinal wave, in which the disturbance moves parallel to the direction of the wave itself. If the wave moves from left to right, the disturbance also moves from left to right.

As sound waves move through a medium, the molecules of the medium collide with one another in the same direction the sound wave is moving.

Think again of the sounds made by a breaching whale. When the whale crashes into the water, it transfers kinetic energy from its moving body to the surface of the water. This makes the molecules of water begin to vibrate and bump into the molecules closest to them. This passes on the energy and makes them vibrate too. Then those molecules bump into more particles, and so on.

As the sound wave moves from left to right through the water, the water molecules also vibrate to the right and left as the energy of the sound wave passes through it. If another whale is in the range of these vibrating molecules, it will hear the vibrations as sound.

As sound energy moves through matter, it causes molecules to press together. This is called compression. When this happens, the molecules on either side of the compression spread out. This is called rarefaction.

How compressed the molecules become as the sound wave moves through determines the wave’s amplitude. Amplitude is a measure of the wave’s displacement from its resting position. Displacement refers to the movement of a substance from its resting position. A water wave’s amplitude is its height above the water’s surface. A sound wave’s amplitude is the amount of compression that occurs as it moves through the medium.

The larger the force that causes the disturbance, the greater a wave’s amplitude will be. Because of this, amplitude is closely related to the amount of energy a wave carries. The greater the amplitude of a wave, the more energy it is carrying.

In sound waves, the amplitude of a wave also determines the sound’s volume, which is how loud or soft a sound seems. For example, the scientists who studied the behaviors of humpback whales believe that whales breach to produce loud sounds that can travel long distances through the water by creating a large disturbance that carries more energy, similar to how a drum makes sounds that can be heard from far away.

The wavelength is the distance spanned by one cycle of the motion of the wave. In longitudinal waves, it is the distance from one compression to the next or from one rarefaction to the next. In transverse waves, it is the distance from crest to crest (the top of the wave) or from trough to trough (the bottom of the wave).