2. Chapter 19 Sound Web Page

3. Sound... n...a longitudinal wave in air caused by a vibrating object. n Demo: Vortex Box

4. Origin of Sound n infrasonic –frequencies < 20 Hz n ultrasonic –frequencies > 20,000 Hz n human hearing range –frequencies between 20 Hz and 20,000 Hz

5. Nature of Sound in Air n Sound requires a medium. –solid, liquid or gas –Demo: Bell in a evacuated Bell Jar n Sound waves have compression and rarefaction regions.

6. Speed of Sound in Air n 340 meters/second n 760 miles/hour n Mach 1 n Video Segments –A lumberjack, lightning, and an explosion at a distance.

7. What is the approximate distance of a thunderstorm when you note a 3 second delay between the flash of the lightning and the sound of the thunder? Answer: 3 seconds  340 m/s = 1020 meters

8. Acoustics... n...the study of sound properties. n When a sound wave strikes a surface it can be.… (a) reflected. (b) transmitted. (c) absorbed. (d) all of these. 

9. Reflection of Sound n e.g. an echo n Reverberation - re-echoed sound, multiple reflections of sound waves from walls n Compare reflections from a hard wall with that from a carpet wall. Demo: Whip

10. Refraction of Sound n Refraction - the bending of a wave n Sound waves bend toward cooler air. Desert and Lake Example

11. Forced Vibrations... n …the setting up of vibrations in an object by a vibrating force. n Examples of Forced Vibration: –A tuning fork touching a wood surface –Sounding boards for stringed instruments –Matching tuning fork boxes Desert and Lake Example

12. During forced vibration sound is intensified because a larger surface area is available to vibrate air molecules.

13. Natural Frequency... n …the frequency at which an elastic object naturally tends to vibrate. n At this frequency, a minimum energy is required to produce a forced vibration. n The natural frequency of a body depends on its elasticity and its shape.

14. Natural Frequency Examples n Dropping Aluminum Rods n Ringing Small and Large Bells n Xylophone n Rubbing a Wine Glass n Mass on a Spring

15. Resonance... n …is the result of forced vibrations in a body when the applied frequency matches the natural frequency of the body. n The resulting vibration has a high amplitude and can destroy the body that is vibrating.

16. Examples of Resonance n breaking a wine glass using sound n mass on a spring at resonance n a singing rod caused by forced vibration n a tuning fork exciting a guitar string n In 1940, the Tacoma Narrows Bridge was destroyed by wind-generated resonance.

17. Resonance allows energy to be transferred to a vibrating object efficiently if the energy is delivered at the natural frequency of vibration.

18. Sound Interference n Overlapping crests of a wave will result in an increased amplitude. n Overlapping a crest and a trough results in a decrease in amplitude.

19. n Beats - the periodic variation in loudness of two sounds played together n The beat frequency is equal to the difference in the frequency of the two sounds. n What is the beat frequency when a 262 Hz and a 266 Hz tuning fork are sounded together?

20. Radio Broadcasts n AM - Amplitude Modulation »535 kHz to 1605 kHz n FM - Frequency Modulation »88 MHz to 108 MHz n Modulation - an impression of the sound wave on a higher frequency radio wave