Waves and Sound. What is a Wave? Wave- a repeating disturbance or movement that transfers energy through matter or space For example, during earthquakes,

Waves and Sound

What is a Wave? • Wave- a repeating disturbance or movement

that transfers energy through matter or space

• For example, during earthquakes, energy is transferred by powerful waves that travel through Earth

Waves and Energy

• Because it is moving, the falling pebble has energy

• As it splashes into the pool, the pebble transfers some of its energy to nearby water molecules, causing them to move

• What you see is energy traveling in the form of a wave on the surface of the water

• A pebble falls into a pool of water and ripples form

Waves and Matter

• Only the energy carried by the waves is transferred

• All waves have this propertythey transfer energy without transferring matter from place to place.

• Imagine you’re in a boat on a lake, and approaching waves bump against your boat

• The waves don’t carry the boat along with them as they pass, or even the water.

Making Waves

• Suppose you are holding a rope at one end, and you give it a shake

• You would create a pulse that travels along the rope to the other end, and then the rope would be still again

Making Waves

• It is the up-and-down motion of your hand that creates the wave

• Anything that moves up and down or back and forth in a rhythmic way is vibrating

• The vibrating movement of your hand at the end of the rope created the wave (in fact, all waves are produced by something that vibrates)

Mechanical Waves

• The matter the waves travel through is called a medium

• The medium can be a solid, a liquid, a gas, or a combination of these

• Not all waves need a medium

• Some waves, such as light and radio waves, can travel through space

Mechanical Waves

• Mechanical waves- waves that are caused by a disturbance or vibration in matter that can travel only through matter

• The two types of mechanical waves are transverse waves (i.e. water) and compressional waves (i.e. sound) NOTE: THESE ARE ALSO CALLED LONGITUDINAL

Transverse Waves

• Transverse wave- matter in the medium moves back and forth at right angles to the direction that the wave travels

• For example, a wave on a rope travels horizontally as the rope moves vertically up and down

• Animation

Compressional Waves • Compressional wave- matter in the medium

moves back and forth along the same direction that the wave travels

• You can model these waves with a coiled spring toy

• Squeeze several coils together at one end of the spring and then let go of the coils

• Animation

Compressional Waves• As the wave moves, it looks as if the whole

spring is moving toward one end• The wave carries energy, but not matter,

forward along the spring

Which type of mechanical wave is represented here?

A. compressionalB. electromagneticC. seismicD. transverse

Check for Understanding

The answer is A. The wave travels horizontally along the spring, in the same direction that the coils move


Sound Waves

• Sound waves are compressional waves• When a noise is made, such as when a locker

door slams shut and vibrates, nearby air particles are pushed together by the vibrations

• The air particles are squeezed together like the coils in a coiled spring

• The compressions travel through the air to make a wave

Sound in Other Materials

• Sound waves also can travel through other mediums, such as water and wood

• When a sound wave reaches your ear, it causes your eardrum to vibrate

• Your inner ear then sends signals to your brain, and your brain interprets the signals as sound

Guitar strings Eardrum

Soundand

Nascar

Water Waves • A water wave causes water to move back and

forth, as well as up and down• Water is pushed back and forth to form the

crests and troughs

• The low point of a water wave is formed when water is pushed aside and up to the high point of the wave

Water Waves

• Ocean waves are formed most often by wind blowing across the ocean surface

• The size of the waves that are formed depend on the wind speed, the distance over which the wind blows, and how long the wind blows

Seismic Waves

• Forces in Earth’s crust can cause tectonic plates to suddenly slide past each other

• This sudden sliding between tectonic plates produces seismic waves that carry energy outward

• Tsunami

Seismic Waves

• Seismic waves are a combination of compressional and transverse waves that can travel through Earth and along Earth’s surface

• The more the crust moves during an earthquake, the more energy is released

Click image to view movie

The Parts of a Wave

• Waves can differ in how much energy they carry and in how fast they travel

• Waves also have other characteristics that make them different from each other

• A transverse wave has alternating high points, called crests, and low points, called troughs

• On the other hand, a compressional wave has no crests and troughs

• When you make compressional waves with a coiled spring, a compression is a region where the coils are close together

The Parts of a Wave

• The coils in the region next to a compression are spread apart, or less dense which is called a rarefaction

Which is represented by area A?

A. compressionB. crestC. rarefactionD. trough


The answer is C. A rarefaction is the less-dense region of a compressional wave


Wavelength • A wavelength (λ) is the distance between one

point on a wave and the nearest point just like it

• For transverse waves the wavelength is the distance from crest to crest or trough to trough

Wavelength

• A wavelength in a compressional wave is the distance between two neighboring compressions or two neighboring rarefactions

• The wavelengths of sound waves that you can hear range from a few centimeters for the highest-pitched sounds to about 15 m for the deepest sounds

Frequency and Period

• The frequency (f) of a wave is the number of wavelengths that pass a fixed point each second

• You can find the frequency of a transverse wave by counting the number of crests or troughs that pass by a point each second

• Frequency is expressed in hertz (Hz)=1/sec

• For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second

Frequency and Period

• The period of a wave is the amount of time it takes one wavelength to pass a point

• As the frequency of a wave increases, the period decreases

• Period has units of seconds

Wavelength is Related to Frequency

• As frequency increases, wavelength decreases

• The frequency of a wave is always equal to the rate of vibration of the source that creates it

• If you move the rope down, up, and back down in 1 s, the frequency of the wave you generate is 1 Hz (1 wave/second)

• The speed of a wave depends on the medium it is traveling through

• Sound waves usually travel faster through liquids and solids than they do through gases

• However, light waves travel more slowly through liquids and solids than they do through gases or through a vacuum

• Sound waves usually travel faster through warmer materials than through cooler materials

Wavelength is Related to Frequency

Calculating Wave Speed

• You can calculate the speed of a wave represented by v by multiplying its frequency times its wavelength

Amplitude and Energy

• Amplitude is related to the amount of disturbance from a wave

• The greater the wave’s amplitude is, the greater the disturbance that the wave produces

• Amplitude is measured differently for compressional and transverse waves

Amplitude of Compressional Waves

• The amplitude of a compressional wave is related to how tightly the medium is pushed together at the compressions

• The denser the medium is at the compressions, the larger the wave’s amplitude is and the more disturbance the wave creates

Amplitude of Compressional Waves

• The closer the coils are in a compression, the farther apart they are in a rarefaction

• So the less dense the medium is at the rarefactions, the more energy the wave carries

Amplitude of Transverse Waves

• The amplitude of any transverse wave is the distance from the crest or trough of the wave to the rest position of the medium

What is the distance from the rest position to the crest in the diagram called?

A. compressionB. wavelengthC. amplitudeD. frequency


The answer is C. The distance from the rest position to the crest or from the trough to the rest position is the amplitude


What is the speed of a wave that has a wavelength of 5.0 m and a frequency of 2 Hz?

A. 5.5 m/sB. 5.0 m/sC. 10 m/sD. 2.5 m/s


The answer is C. Wave speed is equal to the frequency multiplied by wavelength

= 5 m X 2 Hz


Reflection • Reflection occurs when a wave strikes an

object and bounces off of it • All types of wavesincluding sound, water and

light wavescan be reflected• How does the reflection of light allow you to see yourself in the mirror?

• First, light strikes your face and bounces off

• Then, the light reflected off your face strikes the mirror and is reflected into your eyes

Echoes

• A similar thing happens to sound waves when your footsteps echo

• Sound waves form when your foot hits the floor and the waves travel through the air to both your ears and other objects

• Sometimes when the sound waves hit another object, they reflect off that object and come back to you

• Your ears hear the sound again, a few seconds after you first heard your footstep

The Law of Reflection

Ken Karp for McGraw-Hill Companies

• The line perpendicular to the surface of the mirror is called the normal

• The angle formed by the incoming beam and the normal is the angle of incidence (i)

• The angle formed by the reflected beam and the normal is the angle of reflection (r)

• According to the law of reflection, the angle of incidence is equal to the angle of reflection

• All reflected waves obey this law

Refraction

• When a wave passes from one medium to another (such as when a light wave passes from air to water) it changes speed

• Refraction the bending of a wave caused by a change in its speed as it moves from one medium to another

• Light waves travel slower in water than in air. This causes light waves to change direction when they move from water to air or air to water

• When light waves travel from air to water, they slow down and bend toward the normal

Refraction of Light in Water

Which would you predict will happen if the flashlight is submerged and the beam directed upward?


A. The light waves would slow down and bend away from the normal

B.The light waves would speed up and bend away from the normal

C. The light waves would slow down and bend toward the normal

D. The light waves would speed up and bend toward the normal


• When light waves travel from water to air, they speed up and bend away from the normal


The answer is B. Light waves travel slower in water than in air, so if the flashlight were submerged and directed upward, the light waves speed up and bend away from the normal



• You may have noticed that objects underwater seem closer to the surface than they really are

• In the figure, the light waves reflected from the swimmer’s foot are refracted away from the normal and enter your eyes

• The light waves that enter your eyes seem to have come from a foot that was higher in the water

Diffraction

• When waves strike an object, several things can happen:• The waves can bounce off, or be reflected• If the object is transparent, light waves can

be refracted as they pass through it• Waves also can behave another way when

they strike an object…they can bend around the object

Diffraction • Diffraction- when an object causes a wave to

change direction and bend around it• Diffraction and refraction both cause waves to

bend

• The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object

Diffraction

• Waves also can be diffracted when they pass through a narrow opening

• After they pass through the opening, the waves spread out

Diffraction and Wavelength

• The amount of diffraction that occurs depends on how big the obstacle or opening is compared to the wavelength

• When an obstacle is smaller than the wavelength, the waves bend around it

• If the obstacle is larger than the wavelength, the waves do not diffract as much. In fact, if the obstacle is much larger than the wavelength, almost no diffraction occurs

Hearing Around Corners

• You’re walking down the hallway and you can hear sounds coming from the lunchroom before you reach the open lunchroom door

• The wavelengths of sound waves are similar in size to a door opening. Sound waves diffract around the door and spread out down the hallway

Interference

• Interference- when two or more waves overlap and combine to form a new wave

• Interference can either be constructive or destructive

Constructive Interference • With constructive interference, the waves add

together• This happens when the crests of two or more

transverse waves arrive at the same place at the same time and overlap

• The amplitude of the new wave that forms is equal to the sum of the amplitudes of the original waves

Destructive Interference • With destructive interference, the waves subtract

from each other as they overlap

• This happens when the crests of one transverse wave meet the troughs of another transverse wave

• The amplitude of the new wave is the difference between the amplitudes of the waves that overlapped (out of phase)

• Every sound is produced by an object that vibrates and causes compressional waves

• Your friends’ voices are produced by the vibrations of their vocal cords, and music from a speaker are produced by vibrating speakers

What produces sound?

• Compressions and rarefactions move away from the speaker as particles of air collide with their neighbors

Sound Traveling as a Wave

• A series of compressions and rarefactions forms that travels from the speaker to your ear and transfers energy to your eardrum

• This sound wave is what you hear

• Most sounds you hear travel through air to reach your ears

Sound Traveling Through Materials

• If you’ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water

• Sound waves can travel through any type of matter, or mediumsolid, liquid, or gas

• Sound waves cannot travel through a vacuum

In which environment would sound waves not travel?

A. at altitudes of 10,000 – 15,000 m

B. in solid aluminum

C. on the Moon

D. under water


The answer is C. Sound waves require a medium through which to travel. Sound waves cannot travel through a vacuum.


The Speed of Sound through Different Materials

• The speed of a sound depends on the substance the medium is made of, its temperature and whether the medium is solid, liquid, or gas

• In general, sound travels the slowest through gases, faster through liquids, and even faster through solids

The Speed of Sound through Different Materials

• Sound travels faster in liquids and solids than in gases because the molecules in a liquid or solid are closer together than the molecules in a gas

• However, the speed of sound doesn’t depend on the loudness of the sound

• Loud sounds travel through a medium at the same speed as soft sounds

A Model for Transmitting Sound • A line of people passing a bucket is a model for

molecules transferring the energy of a sound wave

• When the people are far away from each other, like the molecules in gas, it takes longer to transfer the bucket of water from person to person

• The closer the particles, the faster they can transfer energy from particle to particle

Temperature and the Speed of Sound

• As the temperature of a substance increases, its molecules move faster

• This makes them more likely to collide with each other and transfer energy

A. 5 kmB. 10 kmC. 20 kmD. 30 km

Use the table to estimate how far a sound will travel through a steel rail in 5s


The answer is D. Remember that distance equals speed multiplied by time


• The amount of energy a wave carries corresponds to its amplitude

• For a compressional wave, amplitude is related to how close together the particles are that make up the compressions and rarefactions

• The difference is that quieter sound waves do not carry as much energy as louder sound waves do

• What happens to the sound waves from your radio when you adjust the volume? The notes sound the same as when the volume was higher, but something about the sound changes

Intensity and Loudness

• When an object vibrates strongly with a lot of energy, it makes sound waves with tight, dense compressions

• When an object vibrates with low energy, it makes sound waves with loose, less dense compressions


• The density of particles that make up the rarefactions behaves in the opposite way

• It is important to remember that matter is not transported during the compression and rarefaction of a compressional waveonly energy is transported

• The matter compresses and expands as the wave passes through that medium


Intensity

• Intensity- is the amount of energy that is transferred across a certain area in a specific amount of time

• When you turn down the volume of your radio, you reduce the energy carried by the sound waves, so you also reduce their intensity

• Intensity influences how far away a sound can be heard

Intensity Decreases with Distance

• Sound intensity decreases with distance for two reasons

• Second, some of a sound wave’s energy converts to other forms of energy, usually thermal energy, as the sound travels through matter

• First, the energy that a sound wave carries spreads out as the sound wave spreads out

Loudness

• Loudness is the human perception of sound volume and depends primarily on sound intensity

• When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do

A Scale for Loudness

• The intensity of sound can be described using a measurement scale

• Each unit on the scale for sound intensity is called a decibel, abbreviated dB

• On this scale, the faintest sound that most people can hear is 0 dB

• The pitch of a sound is related to the frequency of the sound waves—high pitch = high frequency

• Mosquitos

Pitch

• If you were to sing a scale, your voice would start low and become higher with each note

• Pitch is how high or low a sound seems to be

The Doppler Effect

• The change in pitch or wave frequency due to a moving wave source is called the Doppler effect

• The Doppler effect occurs when the source of a sound wave is moving relative to a listener OR if a listener is moving relative to a sound

• Doppler effect sounds

Moving Sound • As a race car moves, it sends out sound waves in

the form of compressions and rarefactions

• The race car creates a compression, labeled A that moves through the air toward the flagger

• By the time compression B leaves the race car, the car has moved

Moving Sound

• Because the car have moved since compression A, compressions A and B are closer together than if the car had stood still

• As a result, the flagger hears a higher pitch because of the increased frequency in the beginning and a lower one in the end

Using the Doppler Effect

• The Doppler effect also occurs for other waves besides sound waves • The frequency of electromagnetic waves, such as

waves from a radar gun, change when they reflect off a moving car depending on how fast the car is going.

• Weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado

What is music?

• Music and noise are caused by vibrationswith some important differences

• Noise has random patterns and pitches

• Music is made of sounds that are deliberately used in a regular pattern

Echolocation • At night, bats swoop around in darkness without

bumping into anything • Their senses of sight and smell help them navigate

• Many species of bats also depend on echolocation-the process of locating objects by emitting sounds and interpreting the sound waves that are reflected back

Sonar

• Sonar is a system that uses the reflection of underwater sound waves to detect objects

Sonar is used to locate an underwater object. At approximately what depth would you expect to find the object if the speed of sound in water is 1,439 m/s and the total time taken for the sonar pulse was 1.6 s? Tricky!!!!


A. 600 mB. 1,200 mC. 1,800 mD. 2,400 m

The answer is B. Remember that the time it takes for the sonar pulse to reach the bottom of the ocean is half the total time


Ultrasound in Medicine • One of the important uses of ultrasonic waves

is in medicine• Using special instruments, medical

professionals can send ultrasonic waves into a specific part of a patient’s body

• Reflected ultrasonic waves are used to detect and monitor conditions such as pregnancy, certain types of heart disease, and cancer

Documents

Waves and Sound. What is a Wave? Wave- a repeating disturbance or movement that transfers energy through matter or space For example, during earthquakes,