1. Sound Intensity and Resonance
1986 world cup in mexico

2. Goal of the class
To understand intensity, decibel level and resonance.
Question of the Day: What are the dangers of resonance?
Previous Answer: Sound with lower frequency than humans can hear f < 20 Hz
Previous question: What is infrasonic sound?

3. Sound Intensity
We discussed last time that sound waves spread out in a spherical pattern from the source.
Intensity is given by the equation:
I – the rate at which energy is transferred through an area.

4. Sound Intensity
For a spherical wave the intensity decreases by the inverse square of the distance
Show compression and rarefractions.

5. Decibel Level Show siren to drive off youths from shopping mall.
Ask if they know any animals that can hear outside of this range.
Elephants can hear lower frequencies, dogs higher.
Mention dog whistles.

6. Decibel Level
Scale is logarithmic
120 dB is the threshold of pain

7. Resonance
Every object has a natural frequency, which is the frequency it would like to vibrate at.
If a driving force has the same frequency as the objects natural frequency you can get large oscillations. This is called resonance.
Pendulum example
Talk about kids on swings

8. Dangers of Resonance

9. Homework Read chapter 12 – Section 2
Questions on P432 Q14, 17, 20, 22, 23