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Sound 13-3

Sound 13-3. A “physical phenomenon that stimulates the sense of hearing.”. What do you think? . A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different. What is the same about the sound? Are the frequencies produced the same?

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Sound 13-3

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  1. Sound13-3 A “physical phenomenon that stimulates the sense of hearing.”

  2. What do you think? • A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different. • What is the same about the sound? • Are the frequencies produced the same? • Are the wave patterns the same? • Why do the instruments sound different?

  3. Standing Waves • Standing waves are produced when two identical waves travel in opposite directions and interfere. • Interference alternates between constructive and destructive. • Nodes are points where interference is always destructive. • Antinodes are points between the nodes with maximum displacement.

  4. Standing Waves on a String • There is a node at each end because the string is fixed at the ends. • The diagram shows three possible standing wave patterns. • Standing waves are produced by interference as waves travel in opposite directions after plucking or bowing the string. • The lowest frequency (one loop) is called the fundamental frequency (f1).

  5. Standing Waves on a String • To the left is a snapshot of a single loop standing wave on a string of length, L. • What is the wavelength for this wave? • Answer:  = 2L • What is the frequency? • Answer:

  6. Harmonics • n is the number of loops or harmonic number. • v is the speed of the wave on the string. • Depends on tension and density of the string • L is the length of the vibrating portion of the string. • How could you change the frequency (pitch) of a string?

  7. Fundamental Frequency Click below to watch the Visual Concept. Visual Concept

  8. Standing Waves in an Air Column • Wind instruments also use standing waves. • Flutes, trumpets, pipe organs, trombones, etc. • Some instruments have pipes open at both ends while others have one end closed. • Air is free to move at open ends so antinodes occur. • Closed ends are nodes. • The velocity of the wave is now the velocity of sound in air (346 m/s at 25°C).

  9. Both Ends Open

  10. Closed at One End

  11. Wind Instruments • Wind instruments are not as simple as organ pipes. • The shape is not always cylindrical. • The holes change the wave patterns as well. • The size of the “pipe” varies along the length.

  12. Practice Problems • One string on a toy guitar is 34.5 cm long. • What is the wavelength of the first harmonic or the fundamental wavelength? • Answer: 69.0 cm or 0.690 m • The string is plucked and the speed of the waves on the string is 410 m/s. What are the frequencies of the first three harmonics? • 590 Hz, 1200 Hz, 1800 Hz • Note: The use of significant figures causes the multiples of 590 to be 1200 and 1800 because only two significant figures are present in the answer.

  13. Practice Problems • An organ pipe open at both ends is 34.5 cm long. • What is the wavelength of the first harmonic or the fundamental wavelength? • Answer: 69.0 cm or 0.690 m • What are the frequencies of the first three harmonics if the air temperature is 25.0°C? • Answers: 501 Hz, 1000 Hz, 1500 Hz • Answer the same questions if the pipe is closed at one end. • Answers: 251 Hz, 753 Hz, 1250 Hz

  14. Practice Problems • A violin string that is 50.0 cm long has a fundamental frequency of 440 Hz. What is the speed of the waves on this string?

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