What is sound?
Sound is caused when objects vibrate (move back and forth very quickly). Vibrations create sound waves that can travel in all different directions through air, water, and lots of other materials. When sound waves are spread out, the sound we hear is quiet. When they are clumped together, the sound is much louder. When sound waves enter your ear, they hit your eardrum and make it vibrate. The tiny vibrations move through your ear like a light shining through a long tunnel until they get to some nerves at the end of your ear. The nerves take them to your brain where they turn into the sound that you hear! Because sound travels very quickly, this all happens before we even have a chance to think about it.

Sound can move through all states of matter—gas (such as air), liquid (such as water), and even solids (such as wood or metal). Since molecules in liquids are closer together than those in a gas, sound travels more quickly in water than in air—more than four times faster! That’s because vibrations pass more easily from one molecule to the next when they are close together.

Some solid objects absorb and stop sounds while some conduct and carry sound. Can you guess which types of objects absorb sounds and which ones carry sounds? Soft objects, such as cotton or wool, absorb sound waves while harder things, like wood or metal, carry them. You can test this out by touching one end of a hard object, such as a wooden or metal spoon, to a clock (or watch) and holding your ear near the other end. Does the ticking of the clock sound louder or quieter through the spoon than through the air? Now repeat the exercise using something soft, like a wadded up paper towel. You should have found that the soft object muffled the sound and the hard object carried it.

Sound waves are measured by their frequency. To find out the frequency of a sound, you need to know the number of vibrations an object has in a certain amount of time. You can think of it as being sort of like your pulse, which is a measurement of how many times your heart beats per minute. Instead of heartbeats per minute, we measure sound waves in vibrations, or cycles, per second, which is known at Hertz (or Hz for short). In music, we called the frequency of a sound its pitch (how high or low the sound or note is).

Sounds Underwater
Many ocean animals rely on sounds to communicate with each other, find food, navigate (tell where they are going), and keep themselves safe. Dolphins and other ocean mammals including orcas, sperm whales, and belugas, can create special clicking noises and listen for the echoes of their sounds to bounce off of an object and get back to them. They can tell how far away they are from the object by how the echoes sound! This process is called echolocation and is especially helpful in deep waters where there is little or no light. Other animals—especially bats, but also some types of birds—use echolocation, too. Since bats and hunting birds are nocturnal (active at night), they rely on sound to help them navigate and find food.

For further study, do this project to make a simple water xylophone using household items.