2. Sound Waves

3. Sound... n...a longitudinal wave in air caused by a vibrating object. n Produced by tiny fluctuations of air pressure n Carried through air at 345 m/s (770 m.p.h) as compressions and rarefactions.

4. wavelength compressed gas rarefied gas

5. n Waves in air can’t be transverse, because the molecules are not bound to each other. n Air molecules can only bump into one another. Why is Sound Longitudinal?

6. 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

7. n The frequency of an audible sound determines how high or low we perceive the sound to be. (pitch) n greater frequency = greater the pitch. Origin of Sound (Cont.)

8. Nature of Sound in Air and Solids n Speed of sound in air is related to the frantic motions of molecules as they jostle and collide. n Solids have faster sound speeds due to close molecule proximity & molecular bonding. –don’t have to rely on atoms to traverse gap –spring compression can (and does) travel faster than actual atom motion.

9. Medium sound speed (m/s) air (25  C) 345 water1490 gold3240 brick3650 wood 3800–4600 glass5100 steel5790 aluminum5100 Example Sound Speeds

10. 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? 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 meters/second = 1020 meters Sample Problem

11. DOPPLER EFFECT n …a frequency shift that is the result of relative motion between the source of the waves and the observer. n Doppler with Sound

12. Some applications of the Doppler effect Doppler flow meter measures speed of blood flow in blood vessels. A transmitter generates high frequency (~5MHz) “sounds” that move through the body and bounce off red blood cells. The motion of the blood cells cause a Doppler shift in the sound waves (in the hundreds of Hz range), that can be measured by a receiver.

13. Ultrasound, sonar, echolocation and seismic exploration are all ways in which machines or animals can image the surroundings using sound waves. A sound wave is emitted, bounces off reflective surfaces in the environment, and the reflected signals are observed. The time delay between transmission and reception of the wave is related to the distance to the reflecting surface, transmission and reflection properties of the media, etc., and this information can be used to reconstruct an image of the surroundings. ultrasound image of a fetussonar image of a shipwreck in Lake Ontario Some applications of Doppler effect

14. Doppler radar used in weather forecasting to detect local velocities in a storm system: same principle as with the Doppler effect in sound waves, only here the frequency shift is in radio waves bouncing off moving water droplets in a storm. Some applications of Doppler effect

15. Radar True Velocity Tangential Velocity Radial Velocity Some applications of Doppler effect