Waves

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Waves Sound
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Objectives

• Describe the measurable properties of waves and
  explain the relationships among them and how
  these properties change when the wave moves from
  one medium to another.
• Qualitatively describe the shift in frequency in
  sound or electromagnetic waves due to the
  relative motion of a source or a receiver.

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Chapters 2526

• Waves are periodic disturbances that propagate
  through a medium or space
• a medium does not travel with the wave
• Mechanical waves require a medium
• Electromagnetic waves do not require a medium

4
More on waves

• Longitudinal waves particles of the medium move
  parallel to the direction of the wave

Sound is an example of this type of wave.
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Transverse Waves

• Transverse waves displacement of the particles
  of the medium are perpendicular to the direction
  of propagation of the wave.

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Phase of a wave
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Frequency
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Properties of Waves

• 1. Rectilinear propagation advancement of a
  wave is perpendicular to the wave front
• 2. Reflection waves bounce off barriers and
  rebound in opposite direction
• Law of reflection
• Incident angle reflected angle (i r)
• 3. Refraction bending (changing direction) of a
  wave as it travels from one medium into another

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More properties

• 4. Diffraction spreading of a wave as it passes
  beyond the edge of a barrier
• http//www.physicsclassroom.com/Class/waves/U10L3b
  .html
• 5. Interference result of 2 or more waves
  passing through the same medium at the same time

http//www.physicsclassroom.com/Class/waves/U10L3c
.html
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Math associated with waves

• Frequency 1 / Period (f 1/T)
• Period 1 / Freqency
• i.e., f 60cycles per second 60 hz 60/s
• Also v f l
• where velocity frequency x wavelength

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Sound Waves
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Math re waves

• I P/A Intensity Power/Area
• Units watt / cm2
• Intensity relates to loudness

You are 1 km away from a radio tower, what is the
intensity of the signal of a 100,000 kw radio
station? 1 km 105 cm 100,000 kw 100,000,000w
108 w I P/A 100,000 kw / 4 p (105)2 7.96 x
10-7 w/cm2
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More math

• b 10 log I
• Io 10-16 w/cm 2
• b, Intensity level
• I, Intensity
• Io, threshhold of hearing
• We all dont hear the same, so this is a
  comparative measurement, in decibels

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Flow chart for b (intensity) problems

• If I 4.7 x 10-10 w/cm2
• 10xlog(4.7 2nd EE -10 / 1 2nd EE -16) 66.7
  dB
• If I 2.9 x 10-3 w/cm2
• 10xlog(2.9 2nd EE -3 / 1 2nd EE -16) 135
  dB

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Now we are going backwards from intensity level
(dB) to intensity (w/cm2)

• If the intensity level is 83 dB, convert that to
  intensity in w/cm2.
• B 10 log I / Io get to a working eqtn
• B /10 log I / Io
• Log-1(B/10) Log-1(log I/Io)
• Log-1(B/10) I/Io

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More to come

• Doppler Effect is the apparent change in
  frequency as a result of relative motion between
  the source of a sound and an observer.

F o f s v v o v v
s
fo, frequency heard by observer fs, frequency of
source v, velocity of sound in air vo, velocity
of observer vs, velocity of source
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More on Doppler

• Stationary observer
• Observer moving away v-vs on bottom
• Observer moving toward vvs on bottom
• Stationary source
• Observer moving toward vvo on top
• Observer moving away v-vo on top