DK Science: Sound
What would our world be like without sound? Complete silence might seem peaceful, but there would be no speech, music, or birdsong. Sound is a type of energy that objects produce when they vibrate (move back and forth). The energy travels at high SPEED through air, water, or another substance, in a pattern of sound waves. When the sound waves reach us, they make the eardrums of our inner ears vibrate. Our brains recognize these vibrations as sounds made by different things.
Sound energy travels through air in waves. When an alarm clock rings, nearby air molecules vibrate. The air molecules jiggle about and make neighbouring molecules vibrate too, starting a wave of energy that travels out from the clock. The wave of sound travels in a pattern of compressions (where the air molecules are squeezed together) and rarefactions (where the air molecules are stretched apart).
A person’s voice makes a pattern of sound waves that is called a voiceprint. This pattern can be shown on an oscilloscope screen. Everyone speaks in a slightly different way, so voiceprints are unique, just like fingerprints. Any recording of a voice can be analysed and the speaker later identified by his or her voiceprint.
When an alarm clock goes off, vibrations from the bell inside the clock create sound waves that travel through the air and quickly reach our ears. If there were no air, there would be nothing to carry the sound waves and we would not be able to hear the clock. Sound always needs to travel through some kind of medium, such as air, water, wood, or metal.
It is impossible for astronauts to talk to one another in space as they would on Earth, no matter how loud they shout. There is no air in space, so there is nothing for sound waves to travel through. In this totally silent place, astronauts have to communicate by radio, using microphones and headsets in their space helmets.
Sound travels faster through some substances than through others. When passing through dry air at a temperature of 0°C (32°F), sound travels at a speed of 1,190 kph (740 mph). It travels faster than that in warmer air, and more slowly in colder air. Sound moves about four times faster in water than in air. Dense, heavy substances (made up of molecules closely packed together) allow sound to pass through more quickly than lighter substances do.
An aircraft engine sends out sound waves in all directions. When an aircraft travels at a subsonic speed (slower than the speed of sound), sound waves travel ahead of the plane. If you look up after hearing an aircraft travelling at such a speed, the sound it makes appears to be coming from the aircraft itself, just as you would expect.
When an aircraft reaches the speed of sound (sometimes called the sound barrier), it catches up with its own sound waves. The sound waves are squeezed up in front of it and this produces a loud bang, called a sonic boom. People on the ground hear a sonic boom as a very loud, thundery noise that seems to sweep past them.
Travelling at a supersonic speed (faster than the speed of sound), an aircraft surges ahead of its own sound waves. When you look up into the sky as an aircraft flying at a supersonic speed passes overhead, the noise it makes seems to be coming from some distance behind it. The sound waves only reach you after the plane has passed by.
Whales communicate with one another by making eerie, low-frequency (deep-pitched) moaning noises. Sound waves can carry great distances in water, and the sounds that whales make can travel hundreds or even thousands of miles across entire oceans. In contrast, dolphins communicate over shorter distances by exchanging clicking noises of a higher frequency.
