The New Childrens Encyclopedia

Science of sound

Sound is a form of energy. It passes

through air, water, and solid objects

as invisible waves. We can hear sound

because the waves make the delicate

skin of our eardrums vibrate. The

vibrations are converted to nerve

signals in our brains.

238

TAKE A LOOK: DECIBEL SCALE

 0 dB

The tiny sound of a

finger brushing skin.

We measure the loudness of sound

using the Decibel scale. This is what

mathematicians call a “log scale”,

meaning the quietest sounds

measure 0 dB, a sound ten times

louder is 10 dB, a sound 100 times

louder is 20 dB, and a sound 1,000

times louder is 30 dB.

 15 dB

A whispered

conversation.

 60 dB

A normal

speaking voice.

 90 dB

The sound of a high-

speed train passing by.

SCIENCE

SOUND WAVES

We can’t see sound waves,

but we can get a good idea

how they work by watching

the way a wave travels along

a glowing string. The end of

the string is vibrated by a

machine called an oscillator.

 FREQUENCY 0.5 HERTZ The string

is vibrating with a long wavelength.

 FREQUENCY 1 HZ Vibrations in

double time give shorter wavelengths.

 FREQUENCY 1.5 HZ At this higher

frequency the wavelength shortens again.

 FREQUENCY 2.5 HZ High frequency

sound waves make high-pitched sounds.

Good vibrations Objects can give

out sound energy when they vibrate.

This vibrating guitar string causes

molecules of air to bump into each

other. The collisions between

molecules spread like ripples in a

pool, carrying the sound outwards

in all directions.

Trough

Sound waves travel through air

at about 1,190 km/h (740 mph).

This is slower than light waves, which

is why we hear the sound of a distant

jet aircraft or explosion after we see it.

Sound travels faster under water,

about 5,000 km/h (3,125 mph),

though the exact speed varies

with temperature.

7

/^7 

 PEAKS AND

TROUGHS

The height, or

“amplitude”, of

peaks and troughs

in a sound wave

dictates loudness.

 TUNING

FORKS vibrate

at a particular

frequency, so it

always gives out

sound at the

same pitch.

Peak

(c) 2012 Dorling Kindersley. All Rights Reserved.(c) 2012 Dorling Kindersley. All Rights Reserved.