Chapter 17Mechanical Waves & 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. • 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 a rope 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?

• Answer: They are opposite/perpendicular!

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:

CompressionRarefaction

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 both up-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

17.2: Properties of Mechanical Waves

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

Period & Frequency

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

How to measure wavelength…

Wavelength & Frequency

• Question: When wavelengths are shorter, the crests are closer together, is the frequency higher or lower?

• Answer: HIGHER! – 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, wavelength is 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 WavesWhat 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 reflects when it hits the wall

Wave Reflection

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

Interference (2 Types)

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.

17.4: Sound & Hearing

Sound Waves

• QUESTION: What type of waves are sound waves?– Hint: compressions and rarefactions travel

through a medium in sound waves

• ANSWER: Longitudinal 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

Sonar

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

Doppler Effect

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

Hearing & the Ear

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)

Speakers

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

Resonance