16.6 The Speed of Sound

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16.6 The Speed of Sound

• Sound travels through gases, liquids, and solids
  at different speeds.
• Room temperature speed of sound in air is 343
  m/s (767 mi/h) and greater in liquids and solids.
• Sound travels more than four times faster in
  water and more than seventeen times faster in
  steel than it does in air.
• Sound travels slowest in gases, faster in
  liquids, and fastest in solids.

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• Issac Newton was the first to measure sound.
• http//www.youtube.com/watch?vPzUU4G4uW18feature
  related

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Sound in a gas

• Only when molecules collide that the
  condensations and rarefactions of a sound wave
  can move from place to place.
• Speed of sound in a gas to have the same order of
  magnitude as the average molecular speed between
  collisions.

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Speed in Ideal Gas

• Translational rms (root mean square) speed given
  in the following equation
• 14.6
• T Kelvin
• M mass
• k Boltzmanns constant (relation between
  absolute temp. and the KE contained in each
  molecule of an ideal gas)
• Overestimates the speed of sound. It gives the
  correct dependence on Kelvin temperature and
  particle mass.

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Speed of sound in an ideal gas

• (16.5)
• y cp/cv ratio of specific heat capacity at
  constant pressure cp to the specific heat
  capacity at constant volume cv. (adiabatic)
• Ideal monatomic gas y 5/3 (atoms are not
  bonded together)
• Y 7/5 for ideal diatomic gases (composed of 2
  atoms)
• Y is here because the condensations and
  rarefactions of a sound wave are formed by
  adiabatic compressions (impassable) and
  expansions of the gas.

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Example 4 An Ultrasonic Ruler
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Practice Problem 44
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Homework

• Pg. 506
• Ques. 29, 30, 31

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Sonar

• Sound navigation ranging
• Used to determine water depth and locating
  underwater objects, such as reefs, submarines,
  and schools of fish.
• Ultrasonic transmitter and receiver mounted on
  the bottom of a ship.
• Transmitter emits a short pulse of ultrasonic
  sound, later the reflected pulse returns and is
  detected by the receiver.
• Depth determined the time it took for the sound
  to return. http//www.youtube.com/watch?vXc2EZjG0
  b8Ufeaturerelated

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Liquids

• In a liquid, the speed of sound depends on the
  density p and the adiabatic bulk modulus Bad of
  the liquid
• (16.6)

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Liquids

• Adiabatic bulk modulus Bad is used when
  calculating the speed of sound in liquids.
• In seawater sound is 1522m/s (4x greater than in
  air).

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In Medicine

• Ultrasonic probe called an A-scan is used to
  measure the length of the eyeball in front of the
  lens, thickness of the lens, and the length of
  the eyeball between the lens and the retina.
• Needed information is the speed
• of sound in the material in front
• of and behind the lens of the
• eye is 1532m/s and that within
• the lens is 1641m/s.

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Solid Bars

• When sound travels through a long slender solid
  bar, the speed of the sound depends on the
  properties of the medium according to

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16.7 Sound Intensity

• Sound waves carry energy that can be used to do
  work.
• Sonic boom can carry enough energy to cause
  damage to windows and buildings.
• The amount of energy transported per second by a
  sound wave is called the power of the wave and is
  measure in SI units of joules per second (J/s) or
  watts (W).

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Sound Intensity I

• Power spreads out as it leaves the source.
• It spreads out but has the same power even when
  spread out over a greater area.
• Sound Intensity I sound power P that passes
  perpendicularly through a surface divided by the
  area A of that surface

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Example 6 Sound Intensities
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Example 7 Fireworks
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16.8 Decibels
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