CHAPTER 17 MECHANICAL
WAVES AND SOUND
Section 17.1 Mechanical Waves
17.1 Mechanical Waves
• Interest Grabber (journal entry)
• Vibrations
• A wave is a vibration that carries energy from one place to
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another. But not all vibrations are waves.
Hold a pen lightly between your thumb and index finger.
Shake your hand back and forth to make the pen seem to
bend like it’s made of rubber.
Next, watch as the instructor holds a length of string and
shakes the string back and forth. Observe the vibrations.
Questions:
1. Describe the motion of the pen and the motion of the
string.
2. In which case did the vibration move from one place to
another? In which case did the vibration stay in one place?
17.1 Mechanical Waves
• Review Energy- the ability to do work.
• 1. Potential Energy- stored energy as a
result of position or shape.
• 2. Kinetic Energy- energy of motion
• Mechanical Waves- A disturbance in
matter that carries energy from one place
to another.
• Needs matter to travel through
• The material through which the wave
travels is called the medium.
17.1 Mechanical Waves
• Medium- can be a solid, liquid or gas.
• Ex: water- travel on surface, Rope- rope is
the medium
A mechanical wave is created when a
source of energy causes a vibration to travel
through a medium (a vibration is a repeating
back and forth motion)
Ex: Shaking a rope- energy added to one
end, the wave that results is a vibration that
carries energy along the rope.
17.1 Mechanical Waves
• 3 Main Types of Mechanical Waves
• 1. Transverse waves
• 2. Longitudinal waves
• 3. Surface waves
17.1 Mechanical Waves
• 1. Transverse waves- a wave that causes
the medium to vibrate at right angles to the
direction in which the wave travels.
• Crest- the highest point of the wave, above
the rest position.
• Trough- the lowest point below the rest
position.
17.1 Mechanical Waves
• Picture of Transverse Waves
17.1 Mechanical Waves
• 2. Longitudinal waves- a wave in which
the vibration of the medium is parallel to
the direction the wave travels.
• Ex: Coils of a spring
• Compression- area where the particles
in a medium are spaced close together.
• Rarefaction- area where the particles in a
medium are spread out.
17.1 Mechanical Waves
17.1 Mechanical Waves
• Surface waves- a wave that travels
along a surface separating two media.
• Ex: An ocean wave is a surface wave
between the water and the air.
• Looking at a bobber in the water- at the
crest the bobber moves up and at the
trough the bobber moves down.
17.1 Mechanical Waves
• Surface Wave
17.2 Properties of Mechanical Waves
• Periodic motion- any motion that repeats
at regular time intervals.
• Ex: Surfers counting the time between 2
successive crests.
• Properties used to describe waves
• 1. Period
4. Speed
• 2. Frequency
5. Amplitude
• 3. Wavelength
17.2 Properties of Mechanical Waves
• 1.
Period- the time required for one cycle,
a complete motion that returns to its
starting point.
• Ex: An ocean wave- time between 2 crests
• 2.
Frequency- the number of complete
cycles in a given time.
• Ex: For a wave- the number of wave cycles
that pass a point in a given time.
17.2 Properties of Mechanical Waves
• Measured in cycles per second or hertz
• Look at Figures (A and B) P. 504
• A waves frequency equals the frequency of
the vibrating source producing the wave.
• 3. Wavelength- the distance between a
point on one wave and the same point on
the next cycle of the wave.
• Transverse wave- the wavelength is
measured between crests or troughs
• Longitudinal wave- the distance between
adjacent compressions and rarefactions.
17.2 Properties of Mechanical Waves
• 4. Wave Speed- calculated by dividing its
wavelength by its period or by multiplying
wavelength by frequency.
• Speed = distance
•
Time
• Speed = Wavelength × Frequency
• The speed of a wave can change if it enters
a new medium or if pressure or temperature
change.
17.2 Properties of Mechanical Waves
For most waves the speed is constant for a
range of different frequencies.
If waves are traveling at a constant speed,
then wavelength is inversely proportional to
frequency.
Ex: lower frequency- longer wavelength
higher frequency- shorter wavelength
5. Amplitude- maximum displacement of the
medium from its rest position.
The more energy a wave has the greater the
amplitude.
17.3 Behavior of Waves
• Reflection- when a wave bounces off a
surface that it cannot pass through.
• Ex: Like a ball bouncing off a wall
• Does not change the speed or frequency of
a wave, but the wave can be flipped upside
down.
• Refraction- is the bending of a wave as it
enters a new medium at an angle.
17.3 Behavior of Waves
• Ex: Pushing a lawnmower- going from
grass to gravel. Speed difference between
the two wheels causes the lawnmower to
change direction.
• When a wave enters a medium at an
angle, refraction occurs because one side
of the wave moves more slowly than the
other side.
• Ex: ocean waves that enter shore on an
angle refract, parallel to shore do not.
17.3 Behavior of Waves
• Diffraction- the bending of a wave as it
moves around an obstacle or passes
through a narrow opening.
• Ex: like the ripples made when a
pebble is tossed into a pond.
• A wave diffracts more if its wavelengths
this large compared to the size of an
opening or obstacle.
17.3 Behavior of Waves
• Interference- occurs when 2 or more waves
overlap and combine together.
• There are 2 types of interference.
• Constructive interference- this is when the
amplitude is increased. Waves line up crest
to crest.
• Destructive interference- the amplitude
decreases. The waves do not line up
correctly. Crest of one wave hits the trough
of another wave.
17.4 Sound and Hearing
• Sound waves are longitudinal waves-
compressions and rarefactions that travel
through a medium.
• Behaviors of sound can be explained using
the following properties: speed, intensity,
loudness, frequency, and pitch.
• 1. Speed- the speed of sound varies in
different media.
17.4 Sound and Hearing
• Sound travels the fastest in solids and
slowest in gases.
• The reason why is that the particles are
closer together in a solid than in liquids and
gases.
• Speed of sound depends on many factors
such as the density of the medium and how
elastic the medium is.
17.4 Sound and Hearing
• Intensity and Loudness
• 2. Intensity is the rate at which a waves
energy flows through a given area.
• Sound intensity depends on the waves
amplitude and the distance from the sound
source.
• When someone whispers in your ear can
have greater sound intensity than someone
shouting across a field.
• Measured in decibels (dB)
17.4 Sound and Hearing
• 3. Loudness- is more subjective than
intensity because it is how somebody
physically responds to the intensity of
sound.
• As intensity increases so does loudness.
• However, the health of your ears and how
your brain interprets the information can
determine loudness too.
17.4 Sound and Hearing
• Frequency and Pitch
• 3. Frequency of a sound wave
depends on how fast the source of
sound is vibrating.
• The faster the vibration the higher the
frequency.
• Ex: instruments- A trumpet produces
higher frequency than a french horn.
17.4 Sound and Hearing
• 5. Pitch is how you perceive the frequency
of sound.
• High frequency sounds have a high pitch.
• Low frequency sounds have a low pitch.
• Depends on age and health of ear. Older
people have a harder time hearing a high
pitch.
• Most people hear between 20 Hz and
20,000 Hz.
17.4 Sound and Hearing
• Ultrasound- is sound at frequencies
higher than most people hear.
• Used in sonar and ultrasound imaging.
• Sonar stands for sound navigation and
ranging.
• Used to determine the distance to an
object under water.
• Ultrasound shows images by sending
ultrasound pulses into a patient.
17.4 Sound and Hearing
• Doppler Effect- a change in sound
frequency caused by motion of the sound
source, motion of the listener, or both.
• Discovered by Christian Doppler an
Austrian scientist.
• As a source of sound approaches, an
observer hears a higher frequency. When
the sound source moves away, the
observer hears a lower frequency.