Science notes, unit 6

Sound

Textbook Science Year 4 (DLP), pages 111–122

What this chapter covers

You can explain how sound is produced by vibrations (6.1.1)
{'You can name ways of producing sound': 'blowing, knocking, plucking, bowing and clapping (6.1.1)'}
You can say that sound travels in all directions (6.1.2)
You can explain that sound is reflected by a hard surface, and give examples such as echo, sonar and ultrasound (6.1.3)
You can describe useful and harmful sounds and the unit of loudness, the decibel (6.1.4)
You can suggest ways to reduce sound pollution (6.1.5)

Every day you hear voices, bells, horns and music. In this unit you will find out how sound is made, where it goes, and how we can use it and protect ourselves from too much of it.

6.1Sound

Vibrations and sound

Sound is produced when an object vibrates. To vibrate means to move quickly to and fro.

Fariz and his friends play in the school band. Each instrument makes sound in its own way.

Way of producing sound	Instrument	What vibrates
Blowing	Flute	The air inside the flute
Knocking	Xylophone	The bars
Plucking or bowing	Guitar, cello	The strings
Clapping	Cymbals	The cymbal pieces

School band: a boy blowing a flute, a girl knocking a xylophone, a boy plucking a guitar, a girl clapping cymbals and a boy bowing a cello — Blowing, knocking, plucking, bowing and clapping all make parts of the instrument vibrate.

Figure: Science Year 4 (DLP), p.112.

Try it: the metal hanger (p.113)

Tie a thread to a metal hanger and wrap the ends around your index fingers. Put your fingers near your ears. When a friend taps the hanger with a pencil, you hear a ringing sound. The hanger vibrates and makes the thread vibrate too. This shows that vibrations produce sound.

Touch your throat while you hum. You can feel the vibration that makes your voice.

Sound is not made by "hitting" itself. It is made by the vibration of the object that is hit, blown, plucked, bowed or clapped.

Name the part that vibrates when a guitar is plucked, and the part that vibrates when a flute is blown.

Show answerHide answer

Guitar: the strings. Flute: the air inside the flute.

Sound travels

Sound travels in all directions, away from the object that makes it.

Fariz, Teruni and Mei Lan are at different places, but all three hear the bell of the ice cream seller. In school, pupils on the upper floor can hear the school bell too, because the sound goes in every direction, even upwards.

Three children at different positions around an ice cream seller whose bell sends out rings of sound — Everyone around the bell can hear it.

School building seen from above with the bell sending sound rings to the pupils on the ground and upper floors — The sound of the school bell reaches the upper floor.

Figures: Science Year 4 (DLP), p.114.

Sound can be reflected

When sound hits a hard surface, it bounces back. This is called reflection of sound.

When Fariz speaks in an empty room, the sound hits the hard walls and comes back to him. So he hears his voice again. The same happens when you shout in a cave or an empty hall.

A boy speaking in an empty room; sound waves hit a hard surface and are reflected back, with arrows showing the direction of travel and reflection — The sound travels to the hard surface and is reflected back.

Figure: Science Year 4 (DLP), p.115.

Soft things such as curtains and carpets do not reflect sound well. They absorb it. That is why a furnished room has less echo than an empty one.

Phenomena of reflection of sound

Phenomenon	What it is
Echo	A sound heard again after it is reflected by a hard surface.
Ultrasound	Sound that humans cannot hear. Bats, dolphins and whales use it to find their way and to detect prey. Doctors use ultrasonic machines to see organs in the body or a baby in the womb.
Sonar	Technology that reflects ultrasonic sound to detect objects in water.

Example: a bat at night

A bat sends out ultrasound. The sound hits an insect and is reflected back. The bat hears it and knows where its prey is.

An echo needs a hard surface and some distance. A cushion does not make an echo.

A fishing boat uses sonar. What is it detecting, and how?

Show answerHide answer

It detects objects (such as shoals of fish) in the water. Ultrasonic sound is sent out and reflected back from the objects.

Useful and harmful sounds

Use of sound	Example
Communication	People talk to each other.
Entertainment	Music from a guitar.
Emergency alert	The siren of an ambulance.
Notification	A phone rings for an incoming call.

An ambulance with its siren on, in traffic — The siren warns other drivers of an emergency.

Some sounds are harmful. Loud or unwanted sound is called sound pollution. It can cause:

problems in communication, because we cannot hear each other
disturbance and stress
hearing problems, if loud noise is frequent or too much

A man holding his head with a hand, disturbed by traffic noise — Noise from vehicles disturbs us and causes stress.

Loudness of sound is measured in decibels (dB). Noise above 85 dB can damage our hearing.

A road worker using a loud drill beside a street with cars, people and pigeons — A drill is useful for work but its noise is harmful if it is too loud.

Figures: Science Year 4 (DLP), pp.118-119.

The same sound can be useful or harmful. Music is fun at a party, but not when the neighbour plays it loudly late at night.

Reducing sound pollution

Way	How it helps
Wear ear protectors	Airport ground staff wear them to protect their ears from aircraft noise.
Use sound absorbers	Carpets, curtains and windows reduce the vibration of sound, so less noise enters a room.
Plant trees	Trees reduce the noise from the road reaching homes.

Give two ways to stop traffic noise from entering your room.

Show answerHide answer

Close the windows and hang thick curtains (sound absorbers). Plant trees outside.

Summary

Sound is produced by vibrations.
Sound is made by blowing, knocking, plucking, bowing and clapping.
Sound travels in all directions and is reflected by hard surfaces (echo, sonar, ultrasound).
Sound is used for communication, entertainment, emergency alert and notification.
Sound pollution causes communication problems, disturbance, stress and hearing problems. Reduce it with ear protectors and sound absorbers.