15 Are You Listening The Sense of Hearing

CHAPTER 1 What Is Hearing? SOUNDS MAKE WAVES IN THE AIR. THE WAVES TRAVEL TO YOUR EAR, AND YOU HEAR THE SOUND. Some sounds are high. These come in short, quick waves. Low sounds come in long, slower waves. Some sounds are so high or so low that people can't hear them. But, some animals can! Dogs, for example, can hear very high sounds. When we want to say how high or low a sound is we use hertz (Hz) and kilohertz (kHz). Younger people can hear higher sounds than older people. This machine is called a Mosquito. It makes a very high sound. Teenagers can hear it, but people over 25 can't. Teenagers don't like the sound of the Mosquito. If you want teenagers to stay away from a store, or another place, you can put one there. A good idea? Well, maybe, but teenagers came back with something new. They use the Mosquito's sound as a ringtone for their cell phones. Most teachers can't hear this, so teenagers can use their phones in class, and the teacher doesn't know!

CHAPTER 2 How Loud Is Loud? WATER, ANIMALS, DRUMS, AND MANY OTHER THINGS MAKE LOUD NOISES. High waves in the sea are louder than low waves. Why? Because the waves in the water make sound waves in the air, and high waves make higher, louder sound waves. The sea is often a noisy place, but some animals in the sea are very loud, too. The blue whale can whistle louder than the noise of a plane. Other whales can hear it thousands of kilometers away. Blue whales are the largest and loudest animals in the world. Some other animals are loud, too. You can hear a lion eight kilometers away, so you have time to start running. Howler monkeys live in groups. They shout really loud so that other groups stay away. We use hertz to say how high or low a sound is, but we use decibels (dB) to say how loud a sound is. A decibel meter is red above 80 dB because sounds louder than this can damage a person's ears. Loud music can be up to 150 dB. What was the loudest noise of all time? In 1883, on the island of Krakatoa, in Indonesia, a volcano exploded. Most of the island flew up into the sky. People heard the sound in Perth, Australia— 3,110 kilometers from Krakatoa! People near the volcano went deaf. It was probably 180 dB. The loudest possible sound is 194 dB.

Think about a band of musicians going down the street. When they walk away from you, what instrument do you hear last? The big drum. Its low sound travels in long waves. Long, low sound waves are very strong and go farther than short, high waves. A ship's foghorn sends out very low sounds to tell other ships where it is in a fog. This low sound is very loud, about 120 dB, and can travel 6 kilometers. Robert Foulis made the first foghorn in 1853. He heard his daughter playing the piano when he walked home one night, but at first he only heard the low notes. He understood that low notes travel farther. Loud noises wake us up... usually. Some people live near airports, but the planes don't wake them up. But if you don't live near an airport and a plane flies near your house, you wake up. Why? Because this isn't a normal sound where you live. The job of alarm clocks is to wake us up. Some alarm clocks are very loud. They can go up to 113 dB. The problem is that they can also wake up other people nearby. You can hear someone else's alarm clock as far away as 100 meters!

CHAPTER 3 How Soft Is Soft? WHERE IS THE QUIETEST PLACE IN THE WORLD? IS IT SOMEWHERE WITH NO PEOPLE AND NO CARS? THE ARCTIC? A LONG WAY UNDER WATER? There is a special room at Orfield Laboratories in Minnesota, USA. Sounds go in special spaces on the walls and the floor, so there are no echoes. The walls are very strong, so no sounds come in from outside. It's the quietest place in the world. You feel different in this special room. First, you may have to sit down. When there's no sound, it's hard to stand up. Then you start to hear your heart. After some time you may see things that aren't there. Nobody has stayed in this room for more than 45 minutes! A whisper is usually about 15 dB. Normally you can only whisper to people near you. But in St. Paul's Cathedral in London, you can whisper to a friend more than 30 meters away. In the famous Whispering Gallery, the sound goes around the walls to your friend on the other side.

Today, we have electric cars. They are very quiet. Are they too quiet? Most people use their ears when they walk across a street— they don't always look left and right. Electric cars can be a big problem for blind people. When you can't see, you have to listen for cars coming. Some countries say that electric cars must make a noise. So the makers of the cars need to know the best noise, and how loud it needs to be. Is the country quieter than the city? Birds wake up early, and they can be noisy— above 70 dB. But, yes, the country is quieter than a busy city. When some people go and live in the country, they find it's too quiet. Sometimes they can't sleep because the noises are different, or because there's not enough noise. Listening is different from hearing. Sometimes you hear noises without listening to them. There's a noise outside maybe people working. You're not listening to it, but you know it's there. When the noise stops, you look up. Why? Because you want to know why it stopped.

CHAPTER 4 Do You Need Ears to Hear? WE HAVE ONE MOUTH AND ONE NOSE. WHY DO WE HAVE TWO EARS? You're standing next to a road. You hear a car. You know it's coming from the left because the sound gets to your left ear first. If you are deaf in one ear, it's more difficult to know where a sound is coming from. Do all animals hear in the same way? Bats hear with their ears, but they also send out a high sound with their mouth or nose. When this sound hits something, echoes come back. The bat now knows where the thing is and how big it is. We call this echolocation. Some people use echolocation, too. Daniel Kish is blind. He takes other blind people for walks. He knows where to go because he makes noises and listens to the echoes. Because of these echoes, he knows what's near him. For example, he knows if there are trees close to him. Daniel teaches other blind people how to use echolocation to do things. With this skill, blind people can walk more easily and can also play some games.