Sound 26.4-26.7

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Sound26.4-26.7
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Objectives

1. Factors that affect the speed of sound
2. Loudness vs. sound intensity
3. Examples of forced vibration
4. How a sounding board works
5. Describe natural frequency

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Whats FasterSound or Light?

• Light
• Any examples?
• Whats the speed of sound in normal air temp (_at_
  20C or 68F)?
• 340m/s
• 1 millionth the speed of light

4
How Far Away is the Storm?

• How far away is a storm if you note a 3-second
  delay between a lightning flash and the sound of
  thunder?
• d vt
• 340 m/s x 3s 1020m

5
Is the speed of sound always constant in air?

• Speed of Sound changed by
• Water vapor
• Why?
• H2O molecules (less mass) move faster than O2 or
  N2 shortens collision time
• 2. Increased temperature
• Why?
• Warm air --gt faster moving particles

6
What does the speed of sound depend on when
traveling through materials?

• The speed of sound depends on the elasticity of a
  material. What is elasticity?
• Elasticity
• The ability of a material to change shape and
  then resume its initial shape
• Atoms close together respond quickly to each
  others motion.
• Elasticity is NOT stretchability.
• Is this rubber band elastic?
• Steel is one of the most elastic material around

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Speed of Sound

• 4 xs as fast in water than in air
• 15 xs as fast in steel than in air
• Just to keep things in perspective light (in
  air) is 1million xs faster!!!

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Checkpoint

• Is sound going to travel faster on a foggy day or
  a clear day, given all other conditions are the
  same? Why?
• Foggy day. Moisture in the air, water molecules
  move faster than O2 and N2 molecules

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Intensity vs. Loudness

• Sound Intensity
• Objective
• Measurable data
• measured by instruments (oscilloscope)
• Loudness
• Subjective
• opinion
• sensation sensed by brain

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Sound Intensity Source of Sound Level (dB) Jet
engine 140 Threshold of pain 120 Loud Rock
Music 115 Subway Train 100 Average
Factory 90 Busy Street Traffic 70 Normal
Speech 60 Library 40 Close
Whisper 20 Normal Breathing 10 Hearing
threshold 0
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Forced Vibrations Sound Boards

• Acoustic guitar
• String --gt saddle --gt soundboard--gt ear
• Soundboard
• Larger surface whose vibrations reinforce sound
  of instrument
• Vibration of string sets larger surface in motion
• Acoustic guitar vs. electric guitar (unplugged).
  Which is louder? Why?

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Demos

• Demos
• tuning fork on table
• Music box
• Record player with paper/pin
• Talkie strips How can we make this louder?
• I want you to write down in your notes how each
  of these things work