Sound is a kind of energy created when something vibrates. When this vibration reaches an ear, it is translated into what we recognize as a sound. Sound vibration must travel through matter. This is typically air. When you say, "Hello," to a friend, the air parts (called molecules) vibrate in small waves which travel to the friend and they hear the word "Hello."
Sound can also travel through other matter. Tap on the table. Do you hear that? Your tapping caused waves to travel through the material of the table and then through the air to your ears.
Ask a friend to touch the top of the table while you tap. They can also feel the waves.
Sound cannot travel through a vacuum. A vacuum is an area without any air, like space. So sound cannot travel through space because there is no matter for the vibrations to work in.
Sound travels in waves!
Sound waves usually travel through air or water, but they can also travel through solids too, like walls or furniture. Sound waves use the matter to move the vibrations.
How do vibrations work?
All matter is made of small particles called molecules.
When a sound is created, the molecules bump into one another in a pattern. Those molecules bump into the next set of molecules, which in turn bump into the next molecules. This continues until the energy runs out. If you have ever thrown a rock into a pond, you have seen the rings of water waves that move out from the place where the rock landed. This is much the same way that sound waves travel.
All sound waves move much the same as a wave in water. There are high spots known as crests and low spots called troughs. The distance between a crest and the next crest is called the frequency. The number of crests that move past a given point in a second is called the frequency. To the human ear, we perceive this as pitch. A child screaming, for example, has a high pitch because the waves are moving quickly.
A big drum would have a low sound because the waves are moving slowly. Notes on a piano each sound different because they each vibrate at a different frequency.
Because sound waves are a kind of energy, they also put out a certain amount of pressure. This pressure can be measured as volume or amplitude. If you could look at a sound wave, you would see that the crests get taller as the amplitude increases.
An echo is the reflection of sound waves bouncing off of a surface and then returning to the sender. Echoes can often be heard in a gym, in a canyon or a concert hall. The sound waves must have some object to bounce off of, the bigger the better. So the walls of a canyon make a great surface for the waves to hit and then return a few minutes later to be heard as an echo.
Have you heard the story that the quack of a duck doesn't echo? Learn more about that here.
The loudness of sound is measured in decibels. Take a peek at this chart to compare the amplitude of common, everyday sounds.
People and animals use sound for communication and as a tool.
Electrocardiogram — Doctors use sound waves to identify the health of a person's heart. Click here for more.
Sonar — Geologists use sound waves to identify geologic features under the surface of the earth. Waves are sent through the ground and they bounce back to special sensing devicesthat measure the materials under the earth. Click here for more.
Ultrasound — Doctors use ultrasound to look into the body using sound waves. It is totally painless and can give experts an opportunity to see if a baby is developing correctly as well as for other purposes. Click here for more.
Ultrasonic cleaners — Dentists use high frequency sound to clean people's teeth. The sound literally shatters the plaque right off the teeth without hurting the gums.
Extracorporeal Shock Waves — Sound waves have been used to shatter kidney and gall bladder stones so that surgery can be avoided. Click here for more.
Doppler — The National Weather Service uses Doppler sound waves to measure weather conditions. Click here for more.
When sound is traveling, a curious effect can take place. All of us have experienced hearing the sound of a train going by or a fire engine with its siren screaming. When the sound is in the distance it has one pitch, but as it gets closer the pitch goes up. Does the sound of the moving object actually change? No, sound waves created by the train or fire engine do not change for people riding in the vehicles. They only change for outside observers as the vehicle moves closer and then moves on past. This change is called the Doppler Effect. It was named after the Austrian physicist Christian Johann Doppler who discovered it.
The Doppler Effect happens when the sound waves from the moving object are moving toward the observer. As the object moves toward the observer, the distance between them gets shorter. Because this distance is decreasing, the sound waves are being compressed between the two. As the object moves past the observer, the distance increases and it takes longer for the sound to reach them. The sound then seems lower. The actual frequency of the sound wave never actually changes; it just seems that way to the observer.
Check out this animation to understand the Doppler Effect better.
Lightning and Thunder
Lightning is the light created by a static charge — a light wave. Thunder is the sound created by the quick movement of the heated air — a sound wave.
Light travels at 186,000 miles per second (299792.458 km/s). The speed of sound can vary depending upon many properties, including temperature and humidity, but 760 miles per hour (340 m/s) on a normal spring day is widely accepted.
This basically means that light can travel faster than sound or that the flash of the lightning will be observed first and the sound will be heard after the flash. To find out how far away the lighting is from you, count the seconds from the flash to the sound. Then divide the number of seconds by 5 to determine how far away in miles the lightning hit. Check out the National Weather Service site for more information.
How do we hear?
Hearing is all about the vibrations of sound as they hit our ears. Inside the outer ear — or that part we all see — is a complex series of ear parts that also vibrate when sound hits them. The eardrum is a drum shaped part that vibrates with the sound waves as they hit it. Behind the eardrum is the snail shaped piece that also regulates balance called the cochlea and three small bones: the hammer, the anvil, and the stirrup. As sound vibrations travel this route of ear parts, it is transferred from piece to piece until it sends signals to the nerves that take the message to the brain.
Learn more about sound and our ears at this KidsHealth site. And check out the Hearing section of our Five Senses site.