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CH 17 - MECHANICAL WAVES & SOUND

Sec. 17.2 - Mechanical Waves

Mechanical Wave - disturbance in matter that carries energy from one place to another.

• Mechanical waves require matter called a MEDIUM to travel through• A medium can be solid, liquid or gas• A mechanical wave is created when a source of energy causes a vibration to travel through a medium.

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3 TYPES OF MECHANICAL WAVES:

1) TRANSVERSE - a wave that causes the medium to vibrate at right angles (perpendicular) to the direction in which the wave travels.

o Shaking a rope up and downo Crest = highest point from rest positiono Trough = lowest point below rest position

http://www.phy.hk/wiki/englishhtm/TwaveA.htm

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2) LONGITUDINAL WAVE- a wave in which the vibration of the medium is PARALLEL to the direction the wave travels; sound waves are this type

§ Created by pushing and pulling in one direction§ Compression = area where particles are spaced close together§ Rarefaction = area where particles are spread out§ P waves ( primary waves ) caused by earthquakeshttp://www.acoustics.salford.ac.uk/feschools/waves/wavetypes.htmhttp://en.wikipedia.org/wiki/File:Longitudinalwave.ogg

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3) SURFACE WAVE- a wave that travels along a surface separating two media.• Ocean waves• Combination of transverse and longitudinal causes a bobber on the surface to move in a circle in deep water• When ocean waves enter shore they topple over themselves because friction with the shore slows down the bottom of the wave.

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Sec. 17.2 - PROPERTIES OF MECHANICAL WAVES

Periodic motion - any motion that repeats at regular intervals

Period - the time required for one cycle (time between 2 successive crests or compressions)

Frequency - the number of complete wave cycles in a given time (cycles per second = hertz (Hz)

Frequency = frequency of the vibration source producing the wave

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Wavelength- the distance between a point on one wave and the same point on the next cycle of the wave (crest to crest or compression to compression.)

Increasing the frequency of a wave decreases its wavelength.

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Amplitude - maximum displacement of the medium from its rest position (height of wave)o The more energy a wave has the greater its amplitude!o In longitudinal waves the amplitude is the maximum displacement of a point from its rest position

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Speed of wave = wavelength x frequency

V = λ f Speed = wavelength x frequency

The speed of a wave can change if it enters a new medium or if pressure and temperature change.If not told otherwise, assume waves are traveling at a constant speed. Therefore, wavelength is inversely proportional to frequency.

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Try these:1) The waves in a pool have a wavelength of 0.20 m and a frequency of 2.8 Hz. What is the speed of these waves?

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2) A student moves the end of a soft spring back and forth to make waves. The waves travel at 1.8 m/s and have a wavelength of 1.2 m. What is the frequency of these waves?

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Sec. 17.3 - BEHAVIOR OF WAVES

1) REFLECTION - occurs when a wave bounces off a surface that it cannot pass through.

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.

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2) REFRACTION- the BENDING of a wave as it enters a new medium at an angle; occurs because one side of the wave moves more slowly than the other side.

If ocean wave fronts approach the shore at an angle they will refract because one side of the wave moves more slowly than the other side.

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3) DIFFRACTION - Bending of a wave as it moves around an obstacle or passes through a narrow opening. A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle

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4) INTERFERENCE - Occurs when two or more waves OVERLAP and combine together

1. CONSTRUCTIVE INTERFERENCE - When two or more waves combine to produce a wave with a larger displacement (amplitude.)

2. DESTRUCTIVE INTERFERENCE - When two or more waves combine to produce a wave with smaller displacements (amplitude.)

http://id.mind.net/~zona/mstm/physics/waves/interference/intrfrnc.html

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Chromatic interference is seen in sea foam, which is made out of Plankton. It is an example of the naturally occurring interference.

http://en.wikipedia.org/wiki/File:Two_sources_interference.gif

http://www.acoustics.salford.ac.uk/feschools/waves/super2.htm

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STANDING WAVES - A wave that appears to stay in one place, not travel through the medium

v When a wave is created and its reflected wave interferes with it “perfectly.”v Plucking a guitar string produces a standing wavev NODE - point on standing wave that has NO displacement (no movement) from resting position due to complete destructive interference.

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v ANTINODE - point on standing wave were a crest or a trough occurs midway between two nodes. Maximum displacement due to complete constructive interference.

A standing wave forms ONLY if half of a wavelength or a multiple of half a wavelength fits EXACTLY into the length of a vibrating cord.

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How many wavelengths does this standing wave have?How many nodes? antinodes?

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Sec. 17.4 - Sound & Hearing

Sounds waves --> longitudinal waves that travel through a medium

Properties of sound waves:

1) Speed --> 342 m/s in dry, 20oC air; speed varies depending on the medium.

Sound waves tend to travel fastest in solids, slower in liquids and slowest in gases because the distance between particles is greatest in gases. Both density and elasticity of the particles affect speed.

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2) Intensity --> rate at which a wave‛s energy flows through a given area; depends on both the wave‛s amplitude & distance from the sound source.

Intensity is measured in decibels. For every 10decibel increase, the intensity increases tenfold

Sound Intensity level(decibels)

Human Hearing threshold 0

Whisper 15­20

Normal conversation 40­50

Street noise 60­70

Inside a bus 90­100

Operating heavy machinery 80­120

Rock concert 110­120

Threshold of pain 120

Jet plane taking off 120­160

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3) Loudness --> physical response to the intensity of sound; is subjective as it depends on ear health and brain interpretation

4) Pitch --> how the frequency of sound is perceived; remember that frequency is how fast the wave is vibrating.

High pitch sounds have a high frequency. Low pitch sounds have a low frequency.

Humans typically hear between 20-20,000 hertz.

Ultrasound --> frequencies greater than 20,000 hertz; beyond range of human hearing.

Sonar = SOund NAvigation & Ranging; uses thespeed of sound in water and the time that thesound takes to reach an object and the bounceback from the object (echo); also calledecholocation; uses ultrasound frequencies.

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Ex: A submarine uses SONAR to measure the distance to the bottom of the ocean. If an ultrasound signal is sent and it takes 7 seconds to receive the echo, how far away is the bottom of the ocean? (speed of sound in water = 1546 m/s)

speed = distance x time 1546 m/s = (d)(7 s.)

d = 271 m. / 2 (because its an echo)

so: d = 136 m. to the bottom

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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 observerhears a higher frequency. When the sound moves away, the observer hears a lower freqency.

Observer B will hear a higher pitch because the waves are bunched together while Observer A will have a lower pitch because the waves are spread apart.

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The Human Ear --> the outer ear gathers and focuses sound into the middle ear; the middle ear receives and amplifies the vibrations; the inner ear uses nerve endings to sense vibrations and send signals to the brain.

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How human hearing works:

1) The pinna (outer sound-collecting part) funnels sound waves down the ear canal.

2) The ear canal (about 2.5 cm. long) carries the sound waves to the tympanic membrane.

3) The tympanic membrane (a tightly stretched membrane) vibrates at the same frequency as the sound waves striking it.

4) The tympanic membrane is touching the malleus (hammer) which then begins to vibrate. The malleus strikes the incus (anvil) which then begins to vibrate.The incus then strikes the stapes (stirrup) which is set into motion.

5) These bones act to amplify the motion of the eardrum.

6) The stirrup is in contact with the cochlea, a spiral-shaped canal filled with fluid. The inside of the cochlea is lined with thousands of nerve cells with tiny hair-like projections. As the projections sway back and forth, they send electrical impulses to the brain.

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How sound is reproduced:1) Sound is recorded by converting sound waves into electronic signals that can be processed and stored.2) Sound is then reproduced by converting these electronic signals back into sound waves.

No matter how sound is stored, it must be converted back into sound waves by speakers. In a speaker, an electronic signal causes a magnet to vibrate. The magnet is attached to a membrane. The membrane vibrates and sends sound wavelengths through the air.

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Large-diameter speakers are better at reproducing lower frequencies of sound. Smaller-diameter speakers are better at reproducing higher frequencies of sound.

Singing into a microphone does the opposite - sound waves vibrate a membrane inside the microphone, the membrane causes a magnet to vibrate which produces an electronic signal in the microphone wires. This electronic signal can then be processed and stored.

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Music --> most musical instruments vary pitch by changing the frequency of standing waves.

Resonance --> the response of a standing wave to another wave of the same frequency; can be used to amplify sound; is a form of constructive interference; pianos use the sound board to create resonance and increase the amplitude

Acoustics --> study of sound transmissions; veryimportant in designing concert halls to prevent“dead spots” where sound waves can be cancelledout by destructive interference.

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