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Science and Technology^177

HOW CAN ONE PERSON PRODUCE THE

SOUND OF AN ENTIRE ORCHESTRA?

Different instruments make sounds with a mixture of

different frequencies. By making sounds with the right

mix of frequencies, an electronic synthesizer can

imitate any instrument in an orchestra. A computer

helps the player to arrange the sounds into music.

A vibrating object moves to and fro. As it moves forwards it

pushes against the air around it, compressing or squashing

it. As it moves backwards it lets the air spread out. These

squashing, expanding movements create a sound wave.

HOW DOES SOUND TRAVEL?

The energy in a sound wave moves

outwards from its source, passing from air

molecule to air molecule in a series of pulses

called compressions (air is squashed) and

rarefactions (air is spread out). Sound travels

easily through air.

Sound can be recorded or transmitted as

a digital signal. A digital signal holds the

pattern of a sound wave as a series of

numbers that can be stored on a CD, digital

tape, or in a computer. When the signal is

played, it changes back into sound waves.

WHY IS DIGITAL SOUND BETTER THAN ANALOGUE?

Analogue recordings rely on making an exact copy

of the sound wave pattern. But making more copies of

the original recording can distort the pattern and add

extra noises (hiss, for example). Because a digital

recording is just numbers, it can be copied and

corrected, if necessary, over and over again.

HOW IS SOUND RECORDED?

A microphone changes sound waves into electrical

signals that rise and fall in the same pattern as the

sound. Analogue recordings store the pattern as a

wavy groove cut into a plastic disc (record), or as a

magnetic pattern on a plastic tape.

SOUND WAVES

4 RECORDING NUMBERS

The numbers in a digital

recording are stored as binary

code. This is made up of just two

digits, 0 and 1. Each binary

number is pressed onto a

compact disc (CD) as a series of

pits (dents) and no pits.

COUNTING WAVES 1

In a digital recording, the

pulsing pattern of each

sound wave is mapped as a

series of numbers.

DIGITAL SOUND

SOUND BARRIER 1

A jet travelling faster than sound

creates a loud shock wave. This is

called breaking the sound barrier.

4 SOUND WAVE

In a sound wave, a series of

compressions and rarefactions

carries the sound energy along.

3 5 6 6 4 2 1 2

The low points

of a sound wave have

the lowest numbers

The high points

of a sound wave

have the highest

numbers

Aircraft in flight
give out sound
waves in all
directions

Binary numbers

are shown as pits

or no pits

Wave height

is turned into

binary numbers

COMPRESSION RAREFACTION

3 5 6 6 4 2 1 2

0 1 1 1 0 1 1 1 0 1 1 0 1 0 0 0 1 0 0 0 1 0 1 0

THOMAS EDISON

American, 1847-1931

In 1877, Thomas Edison

and his team invented the

phonograph, the first practical

device for recording and

playing back sounds. This used

a vibrating needle to scratch

a groove into a wax cylinder.

sound