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Sound

Sound. Chapter 13. Sounds from Saturn. https://files.oakland.edu/users/blswartz/web/Images/saturn.jpg. But wait…. “But Denzmore, I thought sound couldn’t travel through space” Brief explanation:

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Sound

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  1. Sound Chapter 13

  2. Sounds from Saturn https://files.oakland.edu/users/blswartz/web/Images/saturn.jpg

  3. But wait… • “But Denzmore, I thought sound couldn’t travel through space” • Brief explanation: • Molecules transmit sound waves and although interstellar space isn’t very dense it isn’t a complete vacuum • Light waves don’t need a medium and we can sometimes “convert” them to sound waves • http://www.space.com/scienceastronomy/mystery_monday_030922.html

  4. More eerie space sounds for you • http://www-pw.physics.uiowa.edu/space-audio/ • http://www.nasa.gov/vision/universe/features/halloween_sounds.html

  5. What is a sound wave? • Sound waves are longitudinal (compressional) waves produced by vibrating objects • Air molecules move parallel to the wave motion

  6. Longitudinal (Compressional) Wave Crests: Regions of High Density because The coils are compressed Troughs: Areas of Low Density because The coils are stretched

  7. Longitudinal (Compressional) Wave • Compression: The region of the longitudinal wave where the density is highest • Rarefaction: The region of the longitudinal wave where the density is lowest

  8. Audible Sound Waves • For humans, audible sound waves have frequencies between 20 and 20,000 Hz • Infrasonic Waves: Frequencies below 20 Hz • Ultrasonic Waves: Frequencies above 20,000 Hz

  9. Uses for Ultrasonic Waves • Ultrasonic waves have short wavelengths and consequently are easily reflected off small objects • That’s how ultrasounds are produced!

  10. Frequency and Pitch • Pitch: How high or low we perceive sound to be • The frequency of a sound wave determines the pitch • High frequency = High Pitch • Low frequency= Low Pitch http://phet.colorado.edu/simulations/sims.php?sim=Sound

  11. The Speed of Sound • The speed of sound depends on the medium and the temperature of the medium • Solid- Fastest speed of sound • Gas- Slowest speed of sound • Higher temps mean the speed is higher

  12. Speed of Sound in Various Media(p.482)

  13. Shape of a sound wave • Sound waves travel away from a vibrating source in all three dimensions • Therefore sound waves are spherical waves http://wpcontent.answers.com/wikipedia/commons/1/12/Spherical_wave2.gif

  14. Spherical Waves • Wave front: Center of compression • Ray: Lines Perpendicular to the wave fronts that indicate the direction of motion

  15. What is The Doppler Effect? • http://www.colorado.edu/physics/2000/applets/doppler2.html

  16. Explaining the Doppler Effect • Relative motion between a source and an observer creates a change in frequency • Since pitch depends on frequency, relative motion creates a change in pitch

  17. Illustration of the Doppler Effect

  18. Explaining the Illustration • The car is moving toward person A. Although the frequency of the horn is staying the same, since the car is moving toward person A, the wave fronts “bunch together”. The wavelength decreases, the frequency increases and that increases the pitch.

  19. Doppler Effect for Light

  20. M33: Which way is it rotating?

  21. Sound Intensity (p.487) • As sound waves travel, energy is transferred from one air molecule to the next. • The rate at which energy is transferred through a unit of area is called INTENSITY

  22. Equation for Intensity (p. 487) • Intensity of a spherical wave: Intensity (measured in Watts/m2) decreases as distance from source increases

  23. Sample Problem p. 488 #4 • How much power is radiated as sound from a band whose intensity is 1.6 x 10-3 W/m2 at a distance of 15 m?

  24. Solve the problem • Given: • I= 1.6 x 10-3 W/m2 • r= 15 m

  25. Intensity and Frequency (p.489) • Intensity and frequency determine which sounds are audible. • Sounds below 50 Hz and above 12,000 Hz need to be relatively intense to be heard

  26. Graph on p. 489 of your book

  27. Decibel Level (p. 490) • Decibel Level (measured in dB; also known as relative intensity): Found by relating the intensity of a given sound to the intensity at the threshold of hearing. • A difference in 10 dB means the sound is approx twice as large. • i.e. a 30 dB sound is twice as large as a 20 dB sound

  28. When the intensity of a sound is multiplied by 10, the decibel level increases by 10 Intensity to Decibel Level p. 490

  29. Sample Problem (not in book) • When the decibel level of a crowded room rises from 20 dB to 40 dB, how much louder does the noise seem? If the original intensity of the noise was 1.0 x 10-10 W/m2, what is the new intensity?

  30. Resonance (pp. 491 – 492) • Resonance: the tendency of a system tooscillateat maximumamplitudeat certainfrequencies, known as the system's resonance frequencies • In other words, resonance occurs when the frequency of a force applied to a system matches the natural frequency of the vibration of the system

  31. Tacoma Narrows Bridge

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