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13.2 Sound Intensity and Resonance

13.2 Sound Intensity and Resonance. pp. 487 - 493 Mr. Richter. Agenda. Podcast Check Questions about 13.1 Sound Waves Upcoming Schedule Notes: Intensity and Volume Decibel levels Forced Vibrations and Resonance. Upcoming Schedule. Today 13.2 Intensity and Resonance

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13.2 Sound Intensity and Resonance

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  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 • 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 (4πr2) • Units of intensity: W/m2

  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. Decibel Levels • 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. • 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. • 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.

  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

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