MECHANICAL WAVES AND SOUND

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CHAPTER 12 MECHANICAL WAVES AND SOUND
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Goals for Chapter 12

• To describe mechanical waves.
• To study superposition, standing waves and sound.
• To present sound as a standing longitudinal wave.
• To study sound intensity and beats.
• To examine applications of acoustics and musical
  tones.

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A disturbance that propagates from one place to
another is referred to as a wave. Mechanical
waves propagate with well-defined speeds
determined by the properties of the material
(medium) through which they travel. Waves carry
energy, not matter.
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In a transverse wave individual particles move at
right angles to the direction of wave
propagation. In a longitudinal wave individual
particles move in the same direction as the wave
propagation.
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A wave on a string
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As a wave on a string moves horizontally, all
points on the string vibrate in the vertical
direction.
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Water waves from a disturbance.
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Wavelength, Frequency, and Speed
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vwave ? /T ? f vwave
Speed of a wave
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REFLECTIONS AND SUPERPOSITION
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A reflected wave pulse fixed end
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A reflected wave pulse free end
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• The Principle of supperposition
• Whenever two waves overlap, the actual
• displacement of any point on the string,
• at any time, is obtained by vector addition
• of the following two displacements
• The displacement the point would have
• if ONLY the first wave were present
• 2) The displacement the point would have
• if ONLY the second wave were present

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Constructive Interference
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Destructive Interference
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Figure 14-22Interference with Two Sources
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Waves become coherent

• Depending on the shape and size of the medium
  transmitting the wave, different standing wave
  patterns are established as a function of energy.

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Normal modes for a linear resonator

• The resonator is fixed at both ends.
• Wave energy increases as you go down the y axis
  below.

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Fundamental frequencies

• The fundamental frequency depends on the
  properties of the resonant medium.
• If the resonator is a string, cord, or wire, the
  standing wave pattern is a function of tension,
  linear mass density, and length.