Transcript The Nature of Sound (ppt)

18 – 2 The Nature of Sound
What is Sound?

Sound is a form of energy caused by
vibrations.

To vibrate means to move back and forth
rapidly.

What vibrates are the molecules that
make up matter.
 Can there be sound without vibrations?

NO
Tuning Fork Demo

If you hear a sound is the fork vibrating?
___________

How come the ball moves when the fork is held near
it?

__________________________________________

If you were to put a tuning fork in water would
anything happen? ______________________________

Strike the tuning fork first, now put it in water what
happens? _____________________________________
Sound and Longitudinal
Waves

Sound waves carry
energy through a
medium without
particles of the
medium traveling
along.
 Sound is a disturbance
that travels through a
medium as a
longitudinal wave.
How is sound made?

Creating vibrations.
 Example: Drum head.
 It moves and pushes the molecules together
creating a compression. When the
drumhead moves again the molecules move
apart creating rarefaction.
Sound

Sound can travel only
if there is a medium to
transmit the
rarefactions and
compressions.
Sounds Bend

Diffraction: sound waves spread out as they
go through small holes.
 Diffraction also makes it possible to hear
around corners.
Speed of sound

The speed of sound
depends on properties
of the medium it
travels through.
 Speed of sound is 750
mph
 Light is 186,000 miles
per second.

Properties include:
 Elasticity
 Density
 Temperature
Elasticity

The ability of a
material to bounce
back after being
disturbed.

Solids are more elastic
then liquids and gases.
Density

The speed of sound
also depends on how
close together particles
of the substance are.

Particles of a dense
material do not move
as quickly as less
dense.
Temperature

Sounds travel
slowly at low
temperatures and
faster at high
temperatures.
Pitch

Pitch how high or low a sound is on a
musical scale. Pitch depends on how fast an
object vibrates in one second.
 We call this frequency of vibration.

Faster or more frequently an object
vibrates the higher the pitch.

The slower or less frequently an object
vibrates the lower the pitch.
Hertz

Frequency of vibration is measured with a
unit called hertz.
 One hertz is one vibration per second.
UltraSonic Waves


Sounds with a frequency above 20,000 Hz
which people cannot hear.
Ultrasonic waves are used in sonar as well
as in medical diagnosis and treatment.
 Sonar is used to estimate size, depth, shape
of under water objects.
Infrasonic

These waves have frequencies below 20
Hz.
 Produced by sources such as heavy
machinery or thunder. You can’t hear them
you sense the disturbing rumble in your
body.
What is loudness?
 Loudness
is the
amount of
energy a sound
has. The greater
the energy the
greater the
loudness.
Decibels

Loudness is measured
in decibels. The
higher the decibel
number the louder the
sound. They start at 0.
 A sound of 120
decibels or greater
could hurt or damage
your ears permanently.
What is intensity?

The intensity of a sound wave depends on
the amount of energy in each wave. As
intensity increases the waves amplitude
increases.
Doppler Effect

The Doppler effect : as the source of sound moves toward
the listener, the waves reach the listener with a higher
frequency. The pitch appears to increase because of the
Doppler effect.
 Example: as a police car with its siren on moves toward
you the pitch becomes higher. As the car goes by and
moves away the pitch drops. The frequency of the siren is
not really changing. The apparent change in frequency as
the wave source moves in relation to the listener is called
the Doppler effect. If the waves are sound waves, the
change in frequency is heard as a change in pitch.
What is an echo

An echo is a reflected sound.
 Sound does not bounce off all surfaces. It
will bounce or reflect off hard surfaces but
not off soft surfaces.
 Sound that is bounced off of a hard surface
produces an echo.
 Sound that is bounced off a soft surface is
absorbed
Resonance

Resonance is the ability of an object to pick
up energy waves of its own natural
frequency.

Every object has its own frequency of
vibration – the frequency any object
vibrates. This is called natural frequency.

Example: the frequency of an object may be 300
hz, another object may be 325 hz. A sound of a
certain frequency will cause an object whose
natural frequency is the same to vibrate.
 The ability of an object to pick up energy of its
own natural frequency is called resonance. ( can
be annoying – crack windows and other glass
objects).
Demo


Energy of Sound
Resonance: Strike the tuning fork against rubber
stopper. Hold it near two tuning forks – one of
same frequency and one of a different frequency.
What
happens?_________________________________
 Doppler effect: Swing tuning fork over head, or
in circle in front of you. What happens to the
sound?
 ______________________________________