Childrens Illustrated Encyclopedia

473

Sound

We live in a noisy World. The roar of city traffic, the

music from a piano, the bark of a dog, all come to our ears

as sound waves traveling through the air. sound is generated

when a disturbance sets air moving—for example, when

someone plucks a guitar string. We hear sounds when

sound waves—tiny vibrations in the air—strike our

eardrums. sound waves need a substance to travel through.

This substance may be a liquid, such as water; a solid, such

as brick and stone; or a gas, such as air.

sounds such as musical notes have a certain

pitch. a high-pitched sound makes the air vibrate

backward and forward more times each second

than a low-pitched sound. The

number of vibrations per

second is called the

frequency of the sound

and is measured in hertz

(cycles per second).

Humans cannot hear

sounds with frequencies

above about 20,000 hertz

or below about 30 hertz.

loudness and decibels

The sound of a train is louder

than the sound

of a whisper

because the train

produces larger

vibrations in the air.

The loudness of

sound also depends

on how close you are

to its source. loudness

is measured in decibels

(db). a jet airliner

taking off is rated at

about 120 db; the rustling

of leaves is about 33 db.

resonance

an object such as a glass gives out

a musical note when struck, because it has

its own natural frequency of vibration. if

you sing a musical note of this frequency,

the object vibrates at its natural frequency,

pushed by the sound waves that hit

it. This is called resonance. a

very loud sound can make a glass

resonate so strongly that it shatters.

sound Waves

a sound wave consists of air molecules

vibrating backward and forward. at each

moment, the molecules are crowded together

in some places, producing regions of high pressure,

and spaced out in others, producing regions of low

pressure. Waves of alternately high pressure and

low pressure move through the air, spreading

out from the source of the sound. These

sound waves carry the sound to your ears.

Region of high-

pressure air

Region of low-

pressure air

The distance from one region of

highest pressure to the next is called

the wavelength of the sound.

The higher the pitch, or frequency,

of the sound, the shorter

the wavelength.

The noise of the boat’s engine sends

sound waves through the water.

Harmonics

in a musical note secondary frequencies,

called harmonics, are mixed with the main

frequency. Harmonics are characteristic of

different instruments, which is why a note

played on a piano sounds different from the

same note played on a violin. Harmonics bring

life to the sound of musical instruments: an

electronically produced sound of a single pure

frequency sounds artificial and dull.

speed of sound

sound travels in

air at a speed of

about 760 mph

(1,224 km/h).

it travels more

slowly when the

temperature and

pressure of the air

are lower. in the

thin, cold air 7 miles

(11 km) up, the speed

of sound is about 620 mph

(1,000 km/h). in water,

sound travels at 3,350 mph

(5,400 km/h), much faster

than in air.

Find out more

ears

music

musical instruments

radio

ecHoes

if you shout in a large hall or near mountains, you can

hear your voice echo back to you. an echo occurs when

a sound bounces off a surface such as a cliff face,

and reaches you shortly after the direct sound.

The clarity of speech and music in a

room or concert hall depends

on the way sounds echo

inside it.

US_473_Sound.indd 473 22/01/16 4:48 pm