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Chapter 16

Chapter 16. Waves and Sound. The distance between adjacent condensations is equal to the wavelength of the sound wave. Individual air molecules are not carried along with the wave. Sound waves are spherical.

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Chapter 16

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  1. Chapter 16 Waves and Sound

  2. The distance between adjacent condensations is equal to the wavelength of the sound wave.

  3. Individual air molecules are not carried along with the wave.

  4. Sound waves are spherical As the distance from the source increases, wave fronts can be treated as planes waves (the radius of curvature becomes great)

  5. Audible range (average) 20 Hz – 20 KHz Infrasonic Ultrasonic

  6. THE FREQUENCY OF A SOUND WAVE The frequency is the number of cycles per second. A sound with a single frequency is called a pure tone. The brain interprets the frequency in terms of the subjective quality called pitch.

  7. A 512Hz sound waves is travelling through: Air (at 20o C) Sea water (at 20o C) Glass Steel Determine: 1) the wave length 2) How long it will take to travel 1 km in each medium (let’s pretend there won’t be any energy loss and the sound will be heard).

  8. Conceptual Example 5 Lightning, Thunder, and a Rule of Thumb There is a rule of thumb for estimating how far away a thunderstorm is. After you see a flash of lighting, count off the seconds until the thunder is heard. Divide the number of seconds by five. The result gives the approximate distance (in miles) to the thunderstorm. Why does this rule work?

  9. Sound can be pleasant and destructive Sound waves carry energy that can be used to do work. The amount of energy transported per second is called the power of the wave. The sound intensity is defined as the power that passes perpendicularly through a surface divided by the area of that surface.

  10. Example 6 Sound Intensities 12x10-5W of sound power is emitted from the speaker. What will be the intensity of the sound heard by a person standing 3.0 m away from the speaker? By what factor will the intensity change if the person double the distance from the speaker?

  11. THE PRESSURE AMPLITUDE OF A SOUND WAVE Loudness is an attribute of a sound that depends primarily on the pressure amplitude of the wave.

  12. Determine the intensity and decibel level of each speaker 15.0 m away from it.

  13. The decibel (dB) is a measurement unit used when comparing two sound intensities. Because of the way in which the human hearing mechanism responds to intensity, it is appropriate to use a logarithmic scale called the intensity level: Note that log(1)=0, so when the intensity of the sound is equal to the threshold of hearing, the intensity level is zero.

  14. Convert intensity to dB Find the dB level of the following intensities: 1x10-4 W/m2, 1x10-9 W/m2 2x10-4dB, 6x10-9 W/m2 Determine the power of sound source is the perceived loudness 15 m away from it is 93 dB

  15. 16.11 The Sensitivity of the Human Ear

  16. Listen!

  17. Doppler on water

  18. The Doppler effect is the change in frequency or pitch of the sound detected by an observer because the sound source and the observer have different velocities with respect to the medium of sound propagation.

  19. MOVING SOURCE

  20. A high-speed train is traveling at a speed of 44.7 m/s when the engineer sounds the 415-Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at the crossing, when the train is (a) approaching and (b) leaving the crossing?

  21. MOVING OBSERVER

  22. GENERAL CASE

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