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Take-home investigation: feeling sound

Find a CD player and a CD that has rock music. Place the player on a light table, insert the CD into the player, and start playing the CD. Place your hand gently on the table next to the speakers. Increase the volume and note the level when the table just begins to vibrate as the rock music plays. Increase the reading on the volume control until it doubles. What has happened to the vibrations?

Describe how amplitude is related to the loudness of a sound.

Amplitude is directly proportional to the experience of loudness. As amplitude increases, loudness increases.

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Identify common sounds at the levels of 10 dB, 50 dB, and 100 dB.

10 dB: Running fingers through your hair.

50 dB: Inside a quiet home with no television or radio.

100 dB: Take-off of a jet plane.

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Test prep for ap courses

In order to waken a sleeping child, the volume on an alarm clock is tripled. Under this new scenario, how much more energy will be striking the child’s ear drums each second?

  1. twice as much
  2. three times as much
  3. approximately 4.8 times as much
  4. six times as much
  5. nine times as much

(e)

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A musician strikes the strings of a guitar such that they vibrate with twice the amplitude.

  1. Explain why this requires an energy input greater than twice the original value.
  2. Explain why the sound leaving the string will not result in a decibel level that is twice as great.
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Section summary

  • Intensity is the same for a sound wave as was defined for all waves; it is

    I = P A , size 12{I= { {P} over {A} } } {}
    where P is the power crossing area A . The SI unit for I is watts per meter squared. The intensity of a sound wave is also related to the pressure amplitude Δ p
    I = ( Δ p ) 2 2 ρv w , size 12{I= { { left (Δp right )} over {2 ital "pv" size 8{m}} } rSup {2} } {}
    where ρ size 12{p} {} is the density of the medium in which the sound wave travels and v w size 12{p} {} is the speed of sound in the medium.

  • Sound intensity level in units of decibels (dB) is

    β dB = 10 log 10 I I 0 , size 12{β left ("dB" right )="10""log" rSub { size 8{"10"} } left ( { {I} over {I rSub { size 8{0} } } } right )} {}
    where I 0 = 10 –12 W/ m 2 is the threshold intensity of hearing.

Conceptual questions

Six members of a synchronized swim team wear earplugs to protect themselves against water pressure at depths, but they can still hear the music and perform the combinations in the water perfectly. One day, they were asked to leave the pool so the dive team could practice a few dives, and they tried to practice on a mat, but seemed to have a lot more difficulty. Why might this be?

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A community is concerned about a plan to bring train service to their downtown from the town’s outskirts. The current sound intensity level, even though the rail yard is blocks away, is 70 dB downtown. The mayor assures the public that there will be a difference of only 30 dB in sound in the downtown area. Should the townspeople be concerned? Why?

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Problems&Exercises

What is the intensity in watts per meter squared of 85.0-dB sound?

3.16 × 10 –4 W/m 2

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The warning tag on a lawn mower states that it produces noise at a level of 91.0 dB. What is this in watts per meter squared?

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A sound wave traveling in 20ºC air has a pressure amplitude of 0.5 Pa. What is the intensity of the wave?

3.04 × 10 –4 W/m 2

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What intensity level does the sound in the preceding problem correspond to?

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Questions & Answers

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Communication is effective because it allows individuals to share ideas, thoughts, and information with others.
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Every time someone flushes a toilet in the apartment building, the person begins to jumb back automatically after hearing the flush, before the water temperature changes. Identify the types of learning, if it is classical conditioning identify the NS, UCS, CS and CR. If it is operant conditioning, identify the type of consequence positive reinforcement, negative reinforcement or punishment
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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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