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Chapter 15 Sound

Chapter 15 Sound. What is Sound? a pressure disturbance Mechanical longitudinal wave moves away from the source with velocities that are characteristic of the medium Examples of the speed of sound Air 331m/s Water 1493 m/s Iron 5130 m/s. How is sound detected?.

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Chapter 15 Sound

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  1. Chapter 15 Sound • What is Sound? • a pressure disturbance • Mechanical longitudinal wave • moves away from the source with velocities that are characteristic of the medium Examples of the speed of sound Air 331m/s Water 1493 m/s Iron 5130 m/s

  2. How is sound detected? • Sound detectors convert sound energy into another form of energy • The human ear is a sound detector. • It receives pressure waves and converts them into electrical impulses which are then sent to the brain for interpretation. • What are the Frequency Ranges? • 0-20 Hz infrasonic • 20-20,000 Hz audio • 20,000 Hz ultrasonic

  3. Physical Properties of Sound • 1. Wavelength- distance between compressions or rarefactions • 2. Frequency- # of waves/s • V=f • 3. Amplitude- measure of the variation in pressure along a wave

  4. Perceiving Sound Properties • 1. Pitch- highness or lowness of a sound wave • - depends of the frequency of vibration • 2. Loudness- sound intensity as sensed by the ear and interpreted by the brain • - depends on the pressure wave’s amplitude • Measured on a logarithmic scale called the Sound Levelmeasured in decibels

  5. The Doppler Effect • A shift in frequency caused by the relative motion of the source and/or the listener • Example: As an ambulance nears the pitch of the siren gets higher. As it passes and moves away the pitch gets lower

  6. Doppler Effect Fd= fs(v-vd) (v-vs) V= velocity of sound wave Vd= velocity of the detector Vs= velocity of the source fs= frequency of source wave fd= frequency received by the detector Remember to define your coordinate system so that the positive direction is from the source to the detector

  7. Example Problem 1page 409 • A trumpet player sounds C above middle C(524 Hz) while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume that the temperature is 20° C. • What if the car is moving away from you at the same speed?

  8. Practice problem #10 p. 409 • A sound source plays middle C(262 Hz). How fast would the source have to go to raise the pitch to C sharp(271 Hz)? Use 343 m/s as the speed of sound.

  9. The Physics of Music • Resonance- the frequency at which sound waves selectively absorb energy • Is different for each type of instrument( brass, reed, string) • Let’s look at stringed instruments

  10. Resonance on Strings • -has 2 nodes and an antinode in the center • -the first mode of vibration is ½ of a wavelength longFundamental or the 1st Harmonic • 2nd Harmonic- 1 wavelength • 3rd Harmonic- 1 ½ wavelength

  11. Cont. • The length(L) of the fundamental is ½ wavelength • L=1/2  • So • =2L • Look at the equation: • V=f  if we substitute in 2L we get • V=f2L

  12. Cont. • 2nd Harmonic would be 1 wavelength • So • L= • 3rd Harmonic would be 1 ½ wavelength • L=3/2  or =2/3 L

  13. Beats • 2 waves close in frequency but out of step • - The frequency of the beat is the magnitude of difference between the frequencies of the two waves • Fbeat= |fA - fB |

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