Physics 101: Lecture 33 Sound

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Physics 101 Lecture 33Sound

• Todays lecture will cover Textbook Sections 16.5
  - 16.10

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From Last Time
Speed of a wave is a constant, i.e. it only
depends on the medium through which it travels
v ? / T ? f
Speed of a transverse wave traveling along a
string
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Sound

• Sound is a longitudinal wave, created by a
  vibrating object (e.g. , vocal cords, guitar
  string).
• Sound needs a medium, it does not exist in
  vacuum.
• Sound waves in air are traveling disturbances of
  varying air pressure slightly less
  (rarefication) and higher than normal
  (condensation).
• Each cycle of a sound wave consists of one
  rarefication and condensation. The frequency is
  the number of cycles per second that passes by a
  given location.
• Human ear frequency 20 20,000 Hz
• Loudness amplitude of the pressure wave (air
  pressure over distance). The larger the amplitude
  the larger the sound.

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Speed of Sound

• Speed of a sound wave depends on the medium
• Ideal gas (adiabatic compression and
  expansion of
• gas)
• v (g k T/m)1/2
  gcp/cv
• Liquid (BadBulk modulus, volume
  change under
• 
  adiabatic conditions)
• v (Bad/r)1/2
• Solid Bars (YYoungs modulus)
• v (Y/r)1/2

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Concept Question

• A sound wave having frequency f0, speed v0 and
  wavelength l0, is traveling through air when in
  encounters a large helium-filled balloon. Inside
  the balloon the frequency of the wave is f1, its
  speed is v1, and its wavelength is l1Compare the
  frequency of the sound wave inside and outside
  the balloon
• 1. f1 lt f0
• 2. f1 f0
• 3. f1 gt f0

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Concept Question

• A sound wave having frequency f0, speed v0 and
  wavelength l0, is traveling through air when in
  encounters a large helium-filled balloon. Inside
  the balloon the frequency of the wave is f1, its
  speed is v1, and its wavelength is l1Compare the
  speed of the sound wave inside and outside the
  balloon
• 1. v1 lt v0
• 2. v1 v0
• 3. v1 gt v0

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Concept Question

• A sound wave having frequency f0, speed v0 and
  wavelength l0, is traveling through air when in
  encounters a large helium-filled balloon. Inside
  the balloon the frequency of the wave is f1, its
  speed is v1, and its wavelength is l1Compare the
  wavelength of the sound wave inside and outside
  the balloon
• 1. l1 lt l0
• 2. l1 l0
• 3. l1 gt l0

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Sound Intensity

• Amount of energy transported by a sound wave per
  second is called power of the wave
• Power energy/time SI unit
  J/s W
• Sound intensity measures the sound power that
  passes
• Perpendicular trough a surface A per surface
  area
• I P/A
  SI unit W/m2
• Decibels (dB) are used to compare two sound
  intensities
• b 10 dB Log (I/I0)
• I0 threshold of hearing

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Concept Question

• Suppose you are standing a distance D away from a
  speaker that is radiating sound in a spherically
  uniform way. You walk away from the speaker until
  the loudness of the sound is reduced by a factor
  of two. About how far from the speaker are you
  now (neglecting any reflections from the ground)?
  
• 1. 10D
• 2. 4D
• 3. 3D
• 4. 2D

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Speaker radiating power P
Changing I by 10 will changeloudness by 2 (10 dB)
We want I1/I2 10
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Doppler Effect

• The wavelength of a sound wave of a source moving
• with speed vs is shortened in front of the source
  by
• ll-vs T
• and lengthened behind the source by
• llvs T
• Thus, the frequency of the sound perceived by a
• stationary observer in front of the source is
• f0 v/l fs / (1-vs/v)
  gt f0 gt fs
• and behind the source
• f0 v/l fs / (1vs/v)
  gt f0 lt fs
• v speed of sound
• vs speed of source
• fs frequency emitted by the source
• f0 frequency heard by the observer
• Links http//library.thinkquest.org/19537/
• http//antwrp.gsfc.nasa.gov/apod/ap01022
  1.html

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Concept Question

• A You are driving along the highway at 65 mph,
  and behind you a police car, also traveling at 65
  mph, has its siren turned on.
• B You and the police car have both pulled over
  to the side of the road, but the siren is still
  turned on.
• In which case does the frequency of the siren
  seem higher to you?
• 1. Case A
• 2. Case B
• 3. same