1. 13.2 Sound Intensity and Resonance pp. 487 - 493 Mr. Richter

2. Agenda  Podcast Check  Questions about 13.1 Sound Waves  Upcoming Schedule  Notes:  Intensity and Volume  Decibel levels  Forced Vibrations and Resonance

3. Upcoming Schedule  Today 13.2 Intensity and Resonance  Tomorrow 13.3 Harmonics  Friday: Wrap Up and Begin Review  Mr. Richter Out Monday and Tuesday  Monday: In class review (on your own)  Tuesday: Ch. 13 Quizam

4. Objectives: We Will Be Able To…  Calculate the intensity of sound waves.  Relate intensity, decibel level, and perceived loudness.  Explain why resonance occurs.

5. Sound Intensity

6.  Sound intensity is the power of a sound wave for a given area.  It’s how much energy a plane wave carries through an area  Since sound waves are spherical, the area is the surface area of a sphere (4 π r 2 )  Units of intensity: W/m 2

7. Sound Intensity  The intensity of a sound wave decreases with the square of the distance from the source.  The same amount of energy is spread over a larger area.

8. Decibel Levels

9.  Humans interpret the intensity of a sound wave as volume.  The more intense a wave is, the louder it sounds.  However, volume (human perception of intensity) is not directly proportional to intensity.  Humans measure volume (relative intensity) on a logarithmic scale: decibels (dB).  If the intensity is 10 times louder, the decibel level increases by 10 dB.  An increase in 10 dB is perceived to be approximately twice as loud.

10. Decibel Levels  A vacuum cleaner is has 10 times the intensity of an air conditioner.  Which means it is 10 dB louder.  Which means the vacuum cleaner sounds twice as loud as an air conditioner to us.

11. Forced Vibrations and Resonance

12. Forced Vibrations  If a vibrating object is attached to another object, the other object will begin to vibrate as well.  Think of you kicking your legs sitting on a swing.  You vibrate, and the swing will vibrate as well.  This is called forced vibration. The other object vibrates with sympathetic vibrations.  The vibrating object transfers energy to the other objects.vibrating object  Often this causes a change in intensity (volume).

13. Resonance  Every object has a natural frequency of vibration.  When the forced vibration (input) is equal to the natural frequency of vibration of the object, the system is said to be in resonance.  Systems in resonance will transfer energy much more quickly from one object to another.resonance  To swing on the swings, you have to kick your feet at the correct intervals. Otherwise, chaos.

14. Resonance  Resonance is not always a good thing.  Just ask the Tacoma Narrows Bridge.Tacoma Narrows Bridge

15. Wrap-Up: Did we meet our objectives?  Calculate the intensity of sound waves.  Relate intensity, decibel level, and perceived loudness.  Explain why resonance occurs.

16. Homework  p. 486 #1, 3, 4, 6, 7  p. 493 #1-5