UNIT 5: VIBRATIONS, WAVES

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UNIT 5 VIBRATIONS, WAVES SOUND
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Waves and Oscillations

• Pendulum swinging back and forth shows how
  oscillations can create waves.

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WAVES

• Energy transfer by doing work, by heat, or by
  waves!
• Wave a disturbance (vibration) that travels
• mechanical waves require a material medium
  (solid, liquid, or gas) particles vibrate in
  simple harmonic motion (water, sound, earthquake
  waves)
• 2 types transverse and longitudinal
• electromagnetic waves travel through a material
  or a vacuum vibrating electric and magnetic
  fields (radio, microwave, infrared, visible
  light, ultraviolet, x-ray, gamma rays)

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WAVES

• Transverse waves vibrations are perpendicular to
  wave direction

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WAVES

• Longitudinal waves vibrations parallel to wave
  direction

rarefaction
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WAVES

• Frequency, f number of waves each second, unit
  Hertz (Hz) 1 Hz 1 wave/sec
• Period, T time for one wave to pass, unit s
  f1/T
• Wavelength, l distance between identical points
  on two waves, unit m

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WAVES

• Amplitude, A maximum displacement from
  equilibrium, unit m

• Wave speed, v speed of the wave, not the
  particles, unit m/s vfl
• use difference in wave speeds to find distance
• ex lightning thunder

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WAVE INTERACTIONS

• Reflection waves "bounce back" at boundary

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WAVE INTERACTIONS

• Law of Reflection qi qr i incidence, r
  reflection

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WAVE INTERACTIONS

• Refraction wave path bends as wave crosses
  boundary. Note that speed wavelength change as
  wave moves into new medium, but frequency remains
  constant.

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Refraction Examples

• 1. Dish filled with water 2. Light
  through glass

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Refraction Rules

• When a wave goes from fast to slow mediums the
  wave will bend toward the normal.
• When a wave goes from slow to fast mediums the
  wave will bend away from the normal.
• Vi Sin qr Vr Sin qi

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WAVE INTERACTIONS

• Diffraction wave spreads out or bends beyond
  edge of barrier

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WAVE INTERACTIONS

• Diffraction greatest when l is greater than or
  equal to the size of opening or object

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SOUND INTERACTIONS

• Resonance (sympathetic vibration)
• objects have natural vibrating frequency
• sending waves to an object at at its natural
  frequency will make it vibrate
• pushing a child on a swing
• using microwaves to heat up water

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SOUND WAVES

• Source a vibrating object (vocal cord, string,
  reed, etc.)

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SOUND INTERACTIONS
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WAVE INTERACTIONS

• Interference waves pass through each other
  without changing each other, but their
  displacements add together

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WAVE INTERACTIONS

• constructive interference combined wave
  displacement is greater than individual waves

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WAVE INTERACTIONS

• destructive interference combined wave
  displacement is less than individual waves

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WAVE INTERACTIONS

• Standing Waves interference of two identical
  waves goingopposite directions makes waves
  appear to vibrate in place

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WAVE INTERACTIONS

• Standing Waves
• nodes no displacement
• antinodes maximum displacement
• Harmonic number is how many crests are trapped

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SOUND WAVES

• Pitch musical tone or note frequency of a wave
• sonic spectrum

• musical scale specific proportional frequencies

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MUSICAL INSTRUMENTS

• l2L/nL length of string, and n is 1,2,3
• fv/lv wave speed in string
• vvTL/mT tension, m mass of string

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MUSICAL INSTRUMENTS

• Stringed Instruments
• quality mixture of fundamental and harmonics
  (makes different instruments sound different)
• sound boards boxes more air surface contact -
  amplifiers

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MUSICAL INSTRUMENTS
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MUSICAL INSTRUMENTS

• Wind Instruments
• pitch frequency of vibration of column of air
• f v/l v sound speed in air 340 m/s
  l wavelength, depends on length of air column

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MUSICAL INSTRUMENTS

• open-end tube each end of tube is antinode
• l 2L/n L length of tube and n is 1,2,3
• Examples flutes, saxophones, some organ pipes

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MUSICAL INSTRUMENTS

• closed-end tube closed end of tube is node
• l4L/n L length of tube and n is 1,3,5
• Examples clarinets, some pipe organs

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PHYSICS

• UNIT 5 VIBRATIONS, WAVES SOUND

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SOUND INTERACTIONS

• The Doppler Effect apparent change in frequency
  due to motion of source or listener

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SOUND INTERACTIONS

• Wave speed stays constant, remember wave speed
  depends on medium.
• Frequency changes and wavelength changes
• When source or observer moves toward each other
  wavelength decreases and frequency increases.
• When source or observer moves away from each
  other wavelength increases and frequency decreases

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SOUND INTERACTIONS

• Radar uses Doppler Effect in radio waves
  reflected off an object to determine its speed
  (speed traps, locating enemy aircraft)
• Red shift (decreased frequency) and Blue shift
  (increased frequency) of light tells astronomers
  whether a star or galaxy is moving toward or away
  from Earth.

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SOUND INTERACTIONS
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SOUND INTERACTIONS

• The Doppler Effect
• sound barrier pile-up of sound waves
  (pressure) in front of object traveling Mach 1
• sonic boom cone-shaped pressure pulse following
  an object traveling at supersonic speeds (bow
  wave, also called water wake, following a
  speedboat)

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SOUND INTERACTIONS
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PHYSICS

• UNIT 5 VIBRATIONS, WAVES SOUND

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QUIZ 5.4

• The speed of sound in earth is 3500 m/s. An
  earthquake wave, frequency 5 Hz, travels from its
  source to a distant mountain range and returns in
  3.4 minutes.
• (a) How far away is the mountain range?
• (b) What is the wavelength of the earthquake
  wave?
• (c) If the mountain range was moving away at 0.50
  m/s. what would be the frequency of the reflected
  wave?

357,000 m
700 m
5.00 Hz
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UNIT 5 REVIEW
f 1/T v fl qi qr
visinqr vrsinqi node dist l/2 loop height 4A v 331 0.6T
I P/4pr2 b 10log(I/I0) I0 1 10-12 W/m2
open pipe l 2L closed pipe l 4L x vt