Adv Physics

1
Adv Physics

• Chapter 13
• Section 2

2
Intensity

• How a wave spreads its power over an area
• - Power divided by the cross-sectional area
  perpendicular to the direction of wave motion
• I P/A where I intensity
• P power
• A cross sectional
  area
• I W/m2

3
Sample Problem

• Suppose 12 x 10-5 W of power pass
  perpendicularly through 2 surfaces having areas
  of 4 m2 and 12 m2. Find the sound intensity at
  each surface.

4
Range of Intensities

• Threshold of hearing smallest intensity the
  human ear can detect
• Io 1 x 10-12 W/m2
• Threshold of pain intensity that is so large
  that it causes the human ear actual pain, 1 W/m2

5
Sample Problem

• During a fireworks display a rocket explodes
  high in the air. Sound spreads out uniformly.
  When the sound reaches a listener 640 m away the
  sound has an intensity of 0.1 W/m2. What is the
  intensity detected by a listener 160 m away?

6
Intensity Level

• Measure of the intensity of sound relative to a
  reference intensity using the logarithmic scale
• ß 10 log (I / Io)
• where ß intensity level
• I intensity
• Io reference intensity (usually
  threshold
• of hearing)
• ß decibels, dB

7
Notes about Decibels

• Threshold of hearing is 0 dB
• Threshold of pain is 120 dB
• 0 dB doesnt mean no sound it means the sound
  has the same intensity as the threshold of
  hearing
• Doubling the intensity doesnt double the
  loudness (takes 10 dB increase to do that)
• 1 dB change is the smallest change in loudness
  detectable

8
Sample Problem

• Two people talk simultaneously. If the
  intensity level is 60 dB when either one speaks
  alone, what is the intensity level when both
  speak at once?

9
Sample Problem

• When a 100 dB sound wave comes through an open
  window of area 0.5 m2, how much acoustic energy
  passes through the window in a 10 min interval?