176 Science and Technology
WHAT IS RESONANCE?
A wine glass has a natural frequency at
which it vibrates. A singer can break the
glass by singing a note at the same
frequency. Because the frequencies
match, energy transfers from the
sound to the glass until its
vibrations become so strong
it shatters. This is called
resonance. Resonance is
used to strengthen the
sound in some types
of musical instrument.Our world is full of sounds. Sound is a form of energy
that travels as. SOUND WAVES. As well as making
and hearing sounds, we also record them. Today, many
recordings are made using. DIGITAL SOUND.
WHY ARE SOUNDS DIFFERENT?
Sounds are different because sound waves have
different frequencies. The frequency is the number of
vibrations or sound waves produced in a second. We
sometimes describe frequency as pitch. High-pitched
sounds, such as from whistles, have a higher
frequency (more waves per second) than low-pitched
sounds, such as from bass drums.HOW ARE SOUNDS MADE?
Sound is made when something moves or vibrates. The
movement sets up a sound wave in the surrounding air.
Continuous sounds, such as drumming, are made when an
object vibrates to and fro. A sudden clap or bang sends out
a single sharp pulse of sound called a shock wave. The shock
wave from an explosion can knock people over.HOW DO MUSICAL INSTRUMENTS MAKE MUSIC?
Stringed instruments (such as violins) have a series of
stretched strings, which vibrate when they are plucked
or stroked. Players of wind instruments (such as flutes)
blow across or into a mouthpiece to force columns of
air to vibrate in tubes or pipes. Percussion instruments
(such as drums) vibrate when they are struck.4 SOUND FREQUENCY
Frequency (the number of vibrations or waves per
second) is measured in hertz (Hz). Low-pitched sounds
have low frequencies, high-pitched sounds have high
frequencies. There is an enormous variety of sounds in
the animal world and different types of animals make,
and hear, sounds of different frequencies.BAT 12,000 – 150,000 HZ
Bats hunt and fly at night.
To help them find their way
in the dark, they make a series
of very high-frequency clicks
(which humans cannot hear).
Then they use their sensitive
ears to listen for the echoes
bouncing back off objects
in their path.HUMAN 85 – 11,000 HZ
Human hearing is not as
sensitive as that of a bat or
a dog, but we do make a wide
range of sounds. Using the air
in our lungs, we vibrate the
vocal chords in our throat
to make complicated sound
patterns. We cry, scream,
laugh, sigh, speak, and sing.FROG 50 – 8,000 HZ
A male frog croaks to attract
a female. The frog puffs up a
pouch of skin under its jaw.
Then it forces air through its
vocal chords to make them
vibrate. The air in the pouch
picks up the vibration and
strengthens it by resonance,
making the sound louder.ELEPHANT 10 – 10,000 HZ
When an elephant trumpets a
warning, humans can hear it.
But elephants also produce
low-frequency sounds that we
cannot hear. Sounds that
happen outside our frequency
range are called ultrasonic
(higher frequencies), or
infrasonic (lower frequencies).HOWLER MONKEY
400 – 6,000 HZ
The loud hooting call of
a howler monkey can travel
for several kilometres
through the dense forest
where it lives. A hollow space
in its neckbones works like an
amplifier to strengthen the
call through resonance.GRASSHOPPER
7,000 – 100,000 HZ
To attract females, male
grasshoppers make a loud
rasping sound by scraping their
hardened wing cases across a
comb-like series of pegs on
their back legs. Grasshoppers
have ears on their abdomens.DEAFENING SOUND 3
The sound of a jet aircraft at
takeoff is millions of times louder
than the quietest sounds we can
hear. Very loud sounds can cause
us pain and damage our ears.Sound
FREQUENCY SCALE
Jet engines work
hardest and make most
noise at takeoffEardrums need
protection from
very loud sounds