You hear sound when vibrations enter your ears and send signals through your nerves to your brain. These vibrations are caused by disturbances in the air. For example, when you hit a drum, the top of it vibrates, causing a disturbance in the molecules in the air. This vibration, or sound wave, travels through the air in all directions, eventually reaching your ears.

If you could see sound waves, they would look much like the waves you see when you drop a stone onto a calm water surface.

How Do We Hear?

Sound waves travel through air at about 1,088 feet (332 meters) per second. When the sound waves or vibrations reach your ears, they push on your eardrums and cause them to vibrate. Each eardrum pushes against a series of three tiny bones in your middle ear. These tiny bones push against another membrane, which causes waves in a fluid inside your inner ear. Here, special cells pick up the differences in pressure from the waves and transform them into electrical signals that travel along nerves to your brain. When these signals reach the brain, you hear the sound and usually recognize its source.

How Is Sound Measured?

Sound waves are usually described with two measurements: frequency and amplitude. Frequency means the number of waves passing a given point in a given period of time. This is usually measured in hertz, abbreviated Hz. One Hz equals 1 cycle per second. Humans can usually hear sounds with frequencies from 20 Hz to 20,000 Hz. The faster a wave vibrates, the higher its frequency and the higher a sound it produces. The highness or lowness of a sound is its pitch. A high-frequency sound has a high pitch.

The amplitude of the sound is its power or loudness. The taller the sound wave, the higher its amplitude and the louder the sound it produces. We usually measure amplitude in decibels. For example, leaves rustling in the wind might produce a sound of about 20 decibels, while a jet taking off creates a sound of at least 140 decibels, loud enough to damage your hearing. Listening to very loud sounds for a long time, including loud music, will damage the tiny nerves in your ears and can lead to permanent hearing loss. Many rock musicians have discovered that they already have hearing problems.

How Long Have People Wondered About Sound?

People have been experimenting with sound for a long time. Pythagoras (572–497 B.C.E.) experimented with strings to determine how sounds changed with changes in the lengths of the strings. Historians credit him with the development of the musical scale.

In about 1700, French physicist Joseph Sauveur first used the word acoustics to describe music and the way sound works. He worked on the mathematics of sound and studied how strings made different sounds depending on their length. Hermann von Helmholtz (1821–1894) discovered much about sound in the 1800s, especially the connections between mathematics and music. He also built one of the first sirens.

Article 3: The Three Things You Need to Make Sound

By Rachel J. Dickinson, Christian Science Monitor, adapted by Newsela staff

Published:10/27/2019

https://newsela.com/read/lib-overview-making-sound/id/55099/?collection_id=339 (You do not need to go to this source, it's already cut and pasted below.)

Writing Prompt: Write a short paragraph that explains the central idea of the article. Use at least two details from the article to support your response.

When a tree falls in the forest and no one's there to hear it, does it make a sound? Science teachers and philosophy professors have asked this question of students for years, and when you know more about sound, you can try to answer this question for yourself.

Sound is the result of three things. First, something must vibrate. Your vocal cords vibrate, for example, and so does a drumhead when you hit it with a drumstick. A tree hitting the ground also causes a lot of vibrations.

Second, you need something through which the vibrations (sound waves) can travel. Vibrations travel easily through many substances, like air, for instance, but they travel even better through water and such dense, rigid things as iron.

Third, these traveling vibrations must be received somehow — your ears, for example, detect the vibrations, and once the vibrations are detected, they are called "sounds."

Try this: Get a metal fork, and hold it in your hand. With the other hand, gently squeeze two tines together a little and then let the tines spring back. What happens? The tines wiggle back and forth, or vibrate. The vibrations travel through the air to your ear, and you detect the vibrations as a high-pitched sound. Try it again, but this time, hold the fork upright, with the end resting on a table. Does it sound louder? The vibrations are traveling through the air and the table, now. Try holding the fork on different surfaces, like wood, metal, rubber — which substance makes it sound loudest? That's the substance through which the vibrations travel the best.

When is sound noise, and when is it music? Noise is a mixture of random sound, and music is controlled sound.

A musical instrument is simply a producer of controlled sound, and all instruments have something that vibrates: a string on a guitar, for instance, a drumhead, a reed in a clarinet or saxophone. In a trumpet, the player's lips vibrate to create the sound. Musicians control the vibration with frets, keys, slides and more.

You can make musical instruments, too — all you need is a source of vibration that you can control. A musical instrument can be as simple as a tin-can drum or as complex as a piano.

Now, about that tree falling in the forest — do you have an answer? The crashing tree creates lots of vibrations, which travel through the air and ground. But by definition, vibrations don't become sound until they are detected, so the tree creates vibrations, but you can't really call those vibrations "sound" unless they are heard.

Physical Science Scavenger Hunt

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