Science High School Reviewer Physical Science: Waves and Sound

Waves

When a duck lands in a pond, it disturbs the
water. This disturbance produces a series of circles
that grow larger as they move away from the duck.
These circles are waves. A wave is a disturbance that
transfers energy from one point to another. Some
waves, such as light waves, can travel through empty
space. Other kinds, such as sound waves, must
travel through a medium, or substance, in order to
transfer energy. A medium can be a solid, a liquid,
or a gas. The movement of particles by a wave is
called vibration. Waves are classified by the type of
vibration they cause in a medium.

Types of waves

Transverse wave

When a transverse wave travels through a
medium, matter moves up and down as the wave
travels through it. A wave that you might make by
moving in the water is a transverse wave.

Compressional wave

When a compressional wave travels, matter
moves back and forth as the wave travels through it.
Think of a long coiled spring. If you move one part
back and forth, it hits the next coil on the spring. The
coil then returns in the other direction. A wave travels
down the coils, moving each coil back and forth.
The vibration of the coils produces
a compression, an area where
particles are pushed together. Behind
the compression is a rarefaction
(rayr•uh•FAK•shuhn), an area where
particles are spread apart. The wave
moves through its medium as a series
of compressions and rarefactions.

Features of Waves
The crest of a transverse wave is
its highest point. In a compressional
wave, the crest is the point of greatest
compression—the area where particles
are closest together. Waves also have
troughs (trawfs). The trough of a
transverse wave is its lowest point. In a
compressional wave, the trough is the
point of greatest rarefaction—the area
where particles are farthest apart.

How can you measure waves?
Wavelength is the distance between wave crests or
troughs. Frequency is a measure of how many wave crests
or troughs pass a given point in one unit of time, such as a
second. High-frequency waves have shorter wavelengths and
transfer greater energy. If you vibrated a clothesline rapidly,
the waves would move at a high frequency. If you vibrated it
more slowly, the waves would have a lower frequency. The
period of a wave is the amount of time it takes for a wave
to complete one full cycle. Period is the inverse of frequency.
Amplitude, the height of the wave from its trough or crest
to its midpoint, is a measure of the wave’s intensity. In the
ocean, a wave’s amplitude increases as it nears the shore.

Frequency and Speed of Waves
Frequency is measured in hertz (Hz), the number of waves
per second. Hertz means “cycles per second” with respect to
frequency.

Speed describes how fast something
travels in a specific amount of time.
The distance a wave travels per second
determines its speed. For example, a
boat passes within 65 meters of a buoy.
The waves from the boat take 5 seconds
to reach the buoy. The wave’s speed is
65 meters divided by 5 seconds, or 13
meters per second. A high-frequency
wave can travel slowly. Likewise, a fastmoving
wave can have a low frequency.

Factors That Affect Speed
The medium through which a wave
travels affects its speed. The depth of
the water affects the speed of ocean
waves. The deeper the water, the faster
the wave travels. In sound waves, the
distance between particles affects wave
speed. Sound waves, as you might infer,

move fastest through solids, slower
through liquids, and slowest through
gases. Think of two springs of equal
length, one with more coils than the
other. The coils represent particles of
matter in the medium. The spring with
more coils conducts sound waves more
rapidly, because the particles are in
closer contact and will spring back
and transmit energy faster.

How does sound travel?
A sound wave is a compressional
wave produced by vibrations in
matter.

Reflected Sound
When you look in a mirror, light
waves bounce off the mirror into
your eyes and enable you to see your
image. Sounds also reflect off objects.
Reflection refers to how waves bounce
off an object and change their direction
of travel. An echo is a reflected sound
wave.
The technology called sonar uses
reflected sound waves, or echoes, to
locate unseen objects and to make maps
of the ocean floor.
Refracted Sound
Refraction occurs when the direction
of a wave changes because of a change
in medium. This happens because
waves move at different speeds through
different media. Sound waves travel
faster through deep water than through
shallow water. Sound waves traveling
through air are refracted as they enter
water. If the type of medium through
which waves travel changes, the waves
generally change speed and direction.
Absorbing Sound Waves
The material a sound wave strikes
affects how the sound wave moves.
Hard surfaces easily reflect sound
waves. Soft surfaces absorb sound
waves. A material that absorbs sound
well does not reflect sound waves.
Ceiling tiles that absorb sound waves
are used in concert halls, offices, and
libraries.
Properties of sounds
The pitch,
or the highness or lowness of a sound,
depends on the frequency of the sound
waves. High-pitched sounds have a
high frequency. Low-pitched sounds
have a low frequency. The teakettle
makes a sound with a higher pitch. The
tuba makes a sound with a lower pitch.

Most people can hear sound waves
in the range of about 20 hertz to about
20,000 hertz. These are sound waves
that cause matter to vibrate between
20 times and 20,000 times per second.

The Doppler Effect
The pitch of a sound can seem to
change if its source or listener is in
motion. This is called the Doppler
effect. Think of what happens when
the driver of an approaching car blows
the car’s horn. The motion of the car
toward you causes the sound waves in
front of the vehicle to arrive closer and
closer to one another. This increases
the frequency of the sound waves.
Therefore, you hear a sound wave with
a higher pitch than the wave produced
by the car’s horn. The opposite occurs
as the car passes. Sound waves behind
the receding car arrive farther and
farther apart. Your ears hear a sound
wave with a lower pitch.

The change in pitch you
heard only had to do with the way in
which you and the source of the sound
were moving in relation to each other.

Volume of a Sound
The difference in the loudness of a
sound is called volume. The amount of
energy, or intensity, of the sound wave
determines the volume of a sound.
Volume is determined by amplitude.
The larger the amplitude of a wave,
the greater its energy. Loud sounds are
produced by high-intensity waves with
large amplitudes.

The volume of a sound is measured
in units called decibels (dB). A whisper
has a volume of about 30 decibels, and
regular speech has a volume of about
60 decibels. Sounds of greater than 90
decibels can damage people’s hearing.
Interference
If you play a song on a stereo,
sound waves move through the air.
Listeners can hear the sound produced.
Suppose another set of speakers played
the same song in the same room. This
motion of two or more waves passing
through the same medium at the same
time is called interference.

Interference can be either positive
or negative. If the crests or troughs of
the waves meet, as they would if both
stereos were side by side and playing
the same song, the amplitudes of the
waves combine. The combined sound
waves of the stereos would produce a
louder sound than that from one stereo
alone. This is constructive interference.
However, this does not occur if the
crest of one wave meets the trough of
another. In this case the sound waves
together have a lower amplitude than
the sound made by one source alone.
This is destructive interference.

How do we hear music?
The sounds made by musical
instruments differ according to the
types of vibrations they produce.
Stringed instruments, such as guitars
or violins, produce vibrations of wires
or strings. Wind and brass instruments,
such as trombones or flutes, produce
vibrations in columns of air. The
difference among sounds of the
same pitch and amplitude in various
instruments is called sound quality.
As sound waves travel, they transfer
energy. When sound waves reach your
ear, they pass through the ear canal
to your eardrum. The sound waves
strike the eardrum and cause it to
vibrate. The vibrations stimulate nerve
cells located deep inside the ear. These
vibrations are then converted to nerve
impulses that your brain recognizes. By
interpreting pitch and sound quality,
the brain identifies different sounds.

“Music” is a combination of sounds
that a listener finds pleasing, and
“noise” is a combination of sounds
that a listener finds unpleasant. Music
usually blends instrumental or vocal
tones in a structured and continuous
manner. However, any agreeable and
harmonious sounds can be interpreted
as musical. Sound that is interpreted
as musical has a mathematical
structure of both tones and silence.
This structure is often referred to as
rhythm. Sounds that lack harmony,
rhythm, and mathematical structure
are interpreted as noise.

Diagram: sound as perceived by the human ear