Chapter 16 Sound Waves

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Chapter 16 - Sound Waves

• Speed of Sound
• Sound Characteristics
• Intensity
• Instruments Strings and Pipes
• 2 Dimensional Interference
• Beats
• Doppler Effect
• Sonic Boom and shock waves

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Sound Speed
Variation with Temperature
Air
Seawater
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Pitch is frequency
Middle C on the piano has a frequency of 262
Hz. What is the wavelength (in air)?
1.3 m
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Intensity of sound

• Loudness intensity of the wave. Energy
  transported by a wave per unit time across a unit
  area perpendicular to the energy flow.

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Sound Level - Decibel
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Stringed instruments
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Question 1

• A steel wire in a piano has a length of 0.9 m and
  a mass of 5.4 g. To what tension must this wire
  be stretched so that its fundamental vibration
  corresponds to middle C i.e., the vibration
  possess a frequency 261.6.

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Wind instruments Double open ended pipes
Frequencies are identical to waves on a string
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Wind instruments Single open ended pipes
Only odd harmonics are present
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Question 2 Pepsi Bottle

• What is the fundamental frequency of a pepsi
  bottle 32 cm tall when you blow over it. Assume
  the speed of sound in air is 343 m/s.
• 5 cm of water are added to the bottle. What is
  the new resonant frequency.

32 cm
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Waves on the surface of a liquid
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Two dimensional wave reflection
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Interference in Space
When the path lengths from source to receiver
differ by l/2 destructive interference results.
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Interference in Time - Beats

• Two sounds of different frequency

• Superposition

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Interference in Time - Beats

• Trig identity again

Amplitude varies in time at a frequency equal to
the difference in the two frequencies
Beat Frequency
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Beats
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Doppler Effect
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Doppler Effect 4 cases

• Source moving toward receiver
• Source moving away from receiver
• Receiver (observer) moving towards source
• Receiver (observer) moving away from source.

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Source moving case
Towards
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Receiver (observer) moving case
Towards
Away
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Source and receiver moving

• Numerator Receiver (observer)
• Toward
• Away
• Denominator Source
• Toward
• Away

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Doppler Example

• Intelligence tells you that a particular piece of
  machinery in the engine room of a Soviet Victor
  III submarine emits a frequency of 320 Hz. Your
  sonar operator hears the machinery but reports
  the frequency is 325 Hz. Assume you have slowed
  to a negligible speed in order to better hear the
  Russian.
• Is the VIII coming toward you or moving away from
  you?
• Assuming the Victor is either moving directly
  toward or away from you, what is his speed in
  m/s?

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Shock waves and the sonic boom
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Sometimes you hear 2 booms