Chapter 17 Mechanical Waves & Sound

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# Chapter 17 Mechanical Waves & Sound - PowerPoint PPT Presentation

Chapter 17 Mechanical Waves &amp; Sound. WHY IS THIS???. 17.1: Mechanical Waves. What types of waves can you identify in a wave pool @ at amusement park?. In a wave pool, energy is being carried across the pool in the form of waves.

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What types of waves can you identify in a wave pool @ at amusement park?
• In a wave pool, energy is being carried across the pool in the form of waves.
• The sounds of laughter and talking in the pool are also being carried by waves.
Mechanical Waves
• Mechanical wave- a disturbance in matter that carries energy from one place to another
• You can see the effects of a wave’s energy in the wave pool when it lifts people in the water.
• Mechanical waves require matter to travel through.
Medium
• Medium- material through which a wave travels
• solids, liquids, and gases can all act as mediums
• Waves travel through a rope when you shake one end of it. Rope = medium
How are mechanical waves created?
• Vibration- a repeating back-and-forth motion
• A mechanical wave is created when a source of energy causes a vibration to travel through a medium.
3 Types of Mechanical Waves
• There are three (3) main types of mechanical waves:
• Transverse
• Longitudinal
• Surface
Transverse Waves
• Transverse wave- wave that causes the medium to vibrate at right angles to the direction in which the wave travels
• These waves carry energy from left to right in a direction perpendicular to the up and down motion of the medium
• Example: shaking one end of arope up and down
Parts of a Transverse Wave
• Crest- the highest point of the wave above the rest position
• Trough- the lowest below the rest position
Parts of a Transverse Wave
• Identify points B, D, F, & H as either crests or troughs.
QUESTION
• How does the direction of a transverse wave compare with the direction of the medium?
Longitudinal Waves
• Longitudinal wave- wave in which the vibration of the medium is parallel to the direction the wave travels
• Example: Sound
• Wave in spring moving BACK and FORTH (not up and down)
• P-Waves - longitudinal waves produced by earthquakes
Parts of a Longitudinal Wave
• Compression- the area where the particles in a medium are spaced close together
• Rarefaction- the area where the particles in a medium are spread out
Parts of a Longitudinal Wave
• Label at least one compression and one rarefaction on the longitudinal wave below:

Compression

Rarefaction

Question
• How does the direction of a longitudinal wave compare with the direction of the medium?
• Answer: They are the same/parallel!
Surface Waves
• Surface wave- wave that travels along a surface separating two media
• Example- ocean waves
• They occur at the surface between water and air
• A bobber floating in the waves will travel in a circle because the motion from these surface waves is bothup-and-down and back-and-forth like in the transverse and longitudinal waves
• Most waves do not move matter from one place to another
Most waves do not move matter from one place to another…however
• When the waves approach the shore they behave differently
• What causes a wave to break?
• As a wave enters shallow water the bottom of the wave has too much friction acting on it from the seafloor, while the top of the wave continues at its original speed
• As a result, the wave carries the medium and anything in it toward the shore
Surfing
• Question: How do surfers know when the next wave is coming?
• Answer: They can count the time between crests, and the next crest will usually follow this pattern.
• They can do this because waves follow periodic motion.
Periodic Motion
• Periodic motion- any motion that repeats at regular time intervals
• Many things display periodic motion
The surfers were timing the PERIOD
• Period- time required for one cycle, a complete motion that returns to its starting point
• For an ocean wave, the period is the time between 2 successive crests
Frequency
• To find how many crests pass in a given time, you need to know the frequency
• Frequency- number of complete cycles in a given time
• The frequency of a wave is how many wave cycles pass a point in a given time
• Frequency is measured in cycles per second, or hertz  (Hz)
• A wave’s frequency equals the frequency of the vibrating source producing the wave
Wavelength
• Wavelength- distance between a point on one wave and the same point on the next cycle of the waves.
• For a transverse wave, the wavelength is measured between adjacent crests or between adjacent troughs
• For longitudinal waves, the wavelength is the distance between the center of a compression to the center of the next compression
Wavelength & Frequency
• Question: When wavelengths are shorter, the crests are closer together, is the frequency  higher or lower?
• Increasing the frequency of a wave decreases its wavelength!

Which has the highest frequency? _______Which has the lowest frequency? _______ Which has the shortest wavelength? _______ Which has the longest wavelength? _______

A

B

Wave Speed
• To determine how fast a wave is traveling, remember how we find speed:distance/time
• In waves, distance is like wavelength, and time is period.
• So we can solve for the speed of a wave using the following formula:

Speed = WAVELENGTH  x FREQUENCY

Solving for Wave Speed
• The units for wave speed are the same as the units for speed we covered earlier:
• meters per second (m/s)
• If we keep speed constant, wavelengthis inversely proportional to frequency
• What does this mean if you have 2 waves with different frequencies?
• The wave with the lower frequency has a longer wavelength
• Refer to Math Skills on page 506 for practice
Amplitude…Energy…
• Consider the difference in the waves you would see if you dropped a pebble into the water compared to a cannonball….
• Which waves would be higher?
• The cannonball
Amplitude & Energy
• Amplitude- the maximum displacement of the medium from its rest position
• Considered the “height” of the wave
• The more energy a wave has, the greater its amplitude

### 17.3: Behavior of Waves

What do you think of when you hear the word reflection?

Now…relate that thought to waves!!!

Wave Reflection
• Reflection- occurs when a wave bounces off a surface that it cannot pass through, ex: echo
• The reflection of a wave is like the reflection of a ball thrown at a wall. The ball cannot go through the wall so it bounces back.
• If you send a transverse wave down a rope attached to a wall, the wave reflectswhen it hits the wall
Wave Reflection
• Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down.
• If the reflection occurs at a fixed boundary (wall), then the reflected wave will be upside-down compared to the original wave (see picture on previous slide)
Wave Refraction
• Refraction- the bending of a wave as it enters a new medium at an angle
• As an ocean wave approaches the shore at an angle, the wave bends, or refracts toward shore because one side of each wave front hits before the other side does as it gets into shallow water.
• Refraction of the wave occurs only when the two sides of a wave travel at different speeds.
Wave Refraction
• When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side.
Wave Diffraction
• Diffraction- the bending of a wave as it moves around an obstacle or passes through a narrow opening.
• The pattern is very similar to the circular ripples you see when a pebble is tossed into a pond.
• A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle
Examples of Wave Diffraction

A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle

Wave Diffraction
• The pattern is very similar to the circular ripples you see when a pebble is tossed into a pond.
Interference
• Interference occurs when two or more waves overlap and combine together
• There are two types of interference:
• constructive interference
• destructive interference
Constructive Interference
• Constructive interference- occurs when two or more waves combine to produce a wave with a larger displacement (taller), ex: crest meets crest
Destructive Interference
• Destructive interference- occurs when two or more waves combine to produce a wave with a smaller displacement, ex: crest meets trough
Standing Waves
• Standing waves- wave that appears to stay in one place – it does not seem to move through the medium
• Interference from reflected wave
• You can observe one if you pluck a guitar
• Only certain points are stationary- called nodes.
Sound Waves
• QUESTION: What type of waves are sound waves?
• Hint: compressions and rarefactions travel through a medium in sound waves
Sound Waves

Here's what sound waves look like. The caption reads, "A visible pattern of sound waves. This new technique of studying sound demonstrates the focusing effect of an acoustical lens on sound waves issuing from the horn at extreme left.

Properties of Sound
• Some properties of sound:
• Speed
• Intensity
• Loudness
• Frequency
• Pitch
Speed of a Wave
• It takes time for sound to travel
• travels at 342 m/s
Speed of Sound Varies…
• Refer to Figure 14 on page 514 for the speeds of sound in different media!
• Generally, sound waves travel fastest in solids, slower in liquids, and slowest in gases
• This is partly due to the spacing of the particles in each phase
Intensity
• Intensity- rate at which a wave’s energy flows through a given area
• Sound intensity depends on both wave’s amplitude and the distance from the sound source
• Measured in decibel (dB)
• See Figure 15: Sound Intensity Level
• Whisper: 15-20 dB
• Rock Concert: 110-120 dB
• Jet Plane (taking off): 120-160 dB

Intensity depends on wave’s amplitude (energy) and distance from the sound source

• If someone whispers in your ear, the sound intensity may be greater than when someone shouts at you from the other end of a field
Loudness
• Loudness- physical response to the intensity of sound, modified by physical factors
• Subjective, open to interpretation
• As intensity increases, loudness increases, however:
• Loudness also depends on factors such as the health of your ears and how your brain interprets the information in sound waves!
Frequency of a Sound Wave
• Vibration becomes faster as sound frequency becomes higher
• The frequency of a sound wave depends on how fast the source of the sound is vibrating
• In musical instruments, the size of the tubing through which air moves produces different frequencies
• Longer tubing = longer wavelength = lower frequency
Pitch of a Sound Wave
• frequency of a sound as you perceive it
• High-frequency sounds have a high pitch
• Low-frequency sounds have a low pitch
• BUT pitch, like loudness, depends on other factors such as your age and health of your ears!
Ultrasound
• Most people hear sounds between 20 Hz and 20,000 Hz.
• Infrasound: sound at frequencies lower than most people hear
• Ultrasound: sound at frequencies higher than most people hear
• Ultrasound is used in a variety of applications, sonar & ultrasound imaging
Sonar
• Sonar: a technique for determining the distance to an object under water
• Stands for SOund NAvigation and Ranging
• Distance calculated by using the speed of sound in water and the time that the sound wave takes to reach and object and the echo takes to return
Doppler Effect
• Doppler Effect - a change in sound frequency caused by motion of the sound source, motion of the listener, or both
• As a source of sound approaches, an observer hears a higher frequency
• When the sound source moves away, the observer hears a lower frequency
• This is due to the sound waves being close together and spread out
Hearing & the Ear
• You are able to pick up slight vibrations in the air from sound because your ear has a membrane that vibrates when a sound wave strikes it.
• The outer ear gathers, and focuses sound into the middle ear like a funnel
• The middle ear receives and amplifies the vibrations
• The inner ear uses nerve endings to sense vibrations and send signals to the brain to decode
Recording Sound
• Sound is recorded by converting sound waves into electronic signals that can be processed and stored
• Sound is reproduced by converting electronic signals back into sound waves
• Larger-diameter speakers are better at reproducing lower frequencies of sound (like a bass drum)
• Smaller-diameter speakers are better for reproducing higher frequencies of sound (like a small bongo drum)
Music
• Most instruments vary pitch by changing the frequency of standing waves
• Resonance- the response of a standing wave to another wave of the same frequency
• Think of a child being pushed on a swing…if the pushes are timed at the right frequency, the child can swing higher and higher.
• In the same way, one wave can “push” another wave to a higher amplitude