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Sound

Sound. Sound Waves. Mechanical Waves (require a medium) Longitudinal waves Formed by a series of compressions and rarefactions. Frequencies of Sounds. Increasing Frequency. Pitch. How high or low we perceive a sound to be, depending on the frequency of the sound wave.

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Sound

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  1. Sound

  2. Sound Waves • Mechanical Waves (require a medium) • Longitudinal waves • Formed by a series of compressions and rarefactions.

  3. Frequencies of Sounds Increasing Frequency

  4. Pitch • How high or low we perceive a sound to be, depending on the frequency of the sound wave. • As the frequency of a sound increases, the pitch of that sound increases. Which graph represent the sound with the highest pitch? C A B C What is wrong with these graphs representing sound waves? Sound is longitudinal, not transverse.

  5. Ultrasound • Images produced by ultrasonic sound show more detail then those produced by lower frequencies. • Ultrasonic sound has many applications in the field of medicine. • Ultrasound images, such as the one shown here, are formed with reflected sound waves.

  6. Amplitude • The amplitude of a sound wave corresponds with how loud the sound is. • A large amplitude is a loud sound. • A small amplitude is a quiet sound.

  7. Practice • Draw a loud and high pitched wave. • Draw a loud and low pitched wave. • Draw a quiet sound wave with medium pitch

  8. Speed of sound depends on medium and temperature. Source: Serway/Faughn, p. 461 (Table 14.1)

  9. To calculate the speed of sound through air at different temperatures… 331 m/s is the speed of sound at 0oC T = temperature in Kelvin Remember: Kelvin = oC + 273

  10. Sound waves propagate in 3D • Sound waves travel away from a vibrating source in all directions. • In these spherical waves, the circles represent compressions (wave fronts). Source Wave front l

  11. Intensity • Intensity (I) of a wave is the rate at which energy flows through a unit area (A) perpendicular to the direction of travel of the wave.

  12. However, power is also the rate at which energy is transferred (W = J/sec) • And sound waves are spherical, so the power is distributed over the surface area of a sphere (4pr) I = Intensity (W/m2) P = Power (W) R = Distance from source (m)

  13. What is the intensity of the sound waves produced by a trumpet at a distance of 3.2 m when the power output of the trumpet is 0.20 W?

  14. Human Hearing - Frequency • The range of human hearing is generally considered to be from about 10 Hz to about 20,000 Hz. • In reality, it’s much worse. Few people can hear above 14-15 thousand Hz, and it gets worse as you grow older.

  15. Human Hearing - Intensity • Hearing also depends on the intensity of the sound. • The softest sound that can be heard by the human ear has an intensity of 1x10-12 W/m2. This intensity is said to be the Threshold of Hearing. • The loudest sound the human ear can tolerate has an intensity of 1.0 W/m2. This is known as the Threshold of Pain.

  16. Human Hearing - Decibles • When dealing with human hearing, the intensity range is very large (1x10-12W/m2 to 1 W/m2). • A sound with twice the intensity, isn’t heard as twice as loud. • The ear works on a logarithmic scale. • Sound loudness is measured in decibels (dB) which compare the sound’s intensity to the intensity at the threshold of hearing.

  17. Conversion of intensity to decibel level

  18. Decibels and Intensity • When the intensity is doubled (one person talking vs two people talking) there is a three decibel increase. • When the intensity is ten times as large there is a ten decibel increase and the noise sounds twice as loud.

  19. Example:A rather noisy typewriter produces a sound intensity of 1 x 10-5 watts/m2 which is 70 dB. Find the decibel level when a second identical machine is added to the office. Two machines would be 73 dB

  20. Calculating Decibel Level: • = 10 log (I/Io) = 10 log (I/1x10-12) Where: Io is the threshold of hearing (1x10-12 W/m2) and b is the decibel level Thus… Threshold of hearing 0dB Threshold of pain 120 dB Doubling the sound intensity is a 3 dB increase.

  21. 80 dB

  22. 100 dB

  23. 120 dB

  24. 140 dB

  25. Example:Michael wants to install a 100. W stereo amp in his sweet new VW. What will the dB level be, at his ears, approximately 1.50 m away from the speakers? = 125 dB Note this is three and a half times the threshold of pain.

  26. Waves & Movement the Doppler Effect

  27. The Doppler Effect • Relative motion between the source of waves and the observer creates a change in frequency. See: http://www.lon-capa.org/~mmp/applist/doppler/d.htm Non-Java applet: http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/doppler.htm

  28. Doppler Effect Equation: fdPerceived frequency heard by the detector fsFrequency being created by the source. * Define the + direction to be from the source to the detector vdvelocity of the detector vsvelocity of the source. V velocity of sound

  29. Highest frequency Lowest frequency + + + Vs- Vd + Vs + Vd + + - Vd- Vs + Vd - Vs Doppler effect possibilities:

  30. Imagine sitting inside a car. The car’s horn has a frequency of 500 Hz. What frequency would you hear inside the car, moving at 25 mi/hr? 500 Hz

  31. Imagine yourself outside the car • As the car approaches you, is the frequency higher or lower then 500 Hz? • As the car passes and leaves you behind, is the frequency higher or lower then 500 Hz?

  32. HEAR DOPPLER CAR

  33. Example: An ambulance moving at 25 m/s drives towards a physics student sitting on the side of the road. The EMTs in the ambulance hear the siren sounding at 650 Hz. What is the frequency heard by the student? (assume speed of sound is 343 m/s) + fs = 650 Hz fd = ? Vs = + 25m/s Vd = 0 m/s

  34. Example:At rest a car’s horn sounds the note A (440 Hz). While the car is moving down the street, the horn is sounded. A bicyclist moving in the same direction with 1/3 the car’s speed hears a lower pitched sound.(A) Is the cyclist ahead of or behind the car? The observed frequency is lower than the actual frequency, therefore they must be moving apart from one another. This means the cyclist is behind the car because he is moving slower than the car. Wow, the bike rider is invisible!

  35. (B) If the car is moving at 33 m/s (with a horn frequency of 440 Hz) and the bike is following the car at 11 m/s, what is the frequency detected by the bicyclist? (assume speed of sound is 343 m/s) + fd = ? fs = 440 Hz Vd = - 11m/s Vs = - 33 m/s

  36. Supersonic Movement

  37. Beats

  38. Beats

  39. Guitars can be tuned using beats -- tune to “zero beat frequency”

  40. Beats • The frequency of the resulting beats can be calculated by:

  41. A certain piano key is suppose of vibrate at 440 Hz. To tune it, a musician rings a 440 Hz tuning fork at the same time as he plays the piano note and hears 4 beats per second. What frequency is the piano emitting if the note the piano plays is too high? 4 Hz = f1 - 440 f1 = 444 Hz 4 Hz = 440 - f2 f2 = 436 Hz

  42. Beats can also occur from two sources playing the same frequency

  43. Along the yellow lines there is destructive interference. There is no wave disturbance there. Constructive Interference Destructive Interference

  44. Constructive Interference (n=0)

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