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phys124s11-hw07

# phys124s11-hw07 - HW 7 Solutions What Is a Sound Wave a...

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1 HW 7 Solutions What Is a Sound Wave? a) Based on the information presented in the introduction of this problem, what is a sound wave? Propagation of pressure fluctuations in a medium b) Having established that a sound wave corresponds to pressure fluctuations in the medium, what can you conclude about the direction in which such pressure fluctuations travel? Pressure fluctuations travel along the direction of propagation of the sound wave. c) Does air play a role in the propagation of the human voice from one end of a lecture hall to the other? Yes. d) The graphs shown in Fig.1 represent pressure variation versus time recorded by a microphone. Which could correspond to a sound wave? Enter the letters of all the correct answers in alphabetical order. Do not use commas. For example, if you think all three graphs could represent sound waves, enter ABC . BC e) Figure 2 shows a sound wave consisting of a sinusoidal displacement of air particles versus time, as recorded at a fixed location. For sinusoidal waves, it is possible to identify a specific frequency (rate of oscillation) and wavelength (distance in space corresponding to one complete cycle). Taking the speed of sound in air to be 344 m/s, what are the frequency f and the wavelength λ of the sound wave shown in the graph? Express your answers in, respectively, hertz and meters to three significant figures. Separate the two answers with a comma. 1000Hz, 0.344 m f) A certain sound is recorded by a microphone. The same microphone then detects a second sound, which is identical to the first one except that the amplitude of the pressure fluctuations is larger. In addition to the larger amplitude, what distinguishes the second sound from the first one? It is perceived as louder. g) A certain sound is recorded by a microphone. A second sound that has twice the frequency is detected by the same microphone. In addition to the higher frequency, what distinguishes the second sound from the first one? It is perceived as higher in pitch. When we double the frequency of a sound wave, we produce a sound that is said to be an octave above the first.

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2 h) What varies between two tones that are different in timbre, that is, two tones that have the same fundamental frequency but are produced, say, by different musical instruments? Note that Figures 1b and 1c could represent tones with different timbre. The harmonic content. The Decibel Scale. a) What is the sound intensity level β , in decibels, of a sound wave whose intensity is 10 times the reference intensity (i.e., 0 10 I I = )? Express the sound intensity numerically to the nearest integer.
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phys124s11-hw07 - HW 7 Solutions What Is a Sound Wave a...

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