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Sound and Hearing

Sound and Hearing Problems

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1. A guitar string of length, L, is fixed at both ends. Which of the following wavelengths for standing waves is not possible on this string?
a. L/2 b. 2L/3 c. L d. 2L e. 4L

2. Pipe A is 30.0 cm long and is closed at one end, open at the other. Pipe B is open at both ends. How long will pipe B have to be to have the same fundamental frequency as pipe A?

3. Two speakers receive the same 256 Hz signal from an amplifier. When one speaker is placed directly behind the other, what is the smallest distance between them that produces destructive interference at any point front of them? (speed of sound = 342 m/s)

4. Signal m is 6.8 dB louder than signal w. What is the ratio of the sound intensity of signal m to signal w?

5. A rock is dropped from a cliff is heard striking the ground 3.4s later. If the speed of sound is 340 m/s, how high is the cliff?

6. Why is the loudness directly proportional to the logarithm of intensity?

7. The noise from a grinder in a factory is 32000 times more powerful than the noise from a dishwasher. By how many decibels is the factory noise greater than the dishwasher noise?

8. Pat stands between two high walls, at a point one-third the distance between the walls. Pat fires a starter pistol, and hears one echo one-tenth of a second after the first echo. If the speed of sound is 340 m/s, how far apart are the walls?

9. A loud sound is 90 dB. When the sound is reduced to 5% of its original intensity, what is the decibel level?

Answers to Sound and Hearing Problems

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2.

60.0 cm

At closed end of a pipe, the fundamental will have a node, at the open end a maximum.

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7. A decibel is one tenth of a bel, where a bel is logarithmic ratio of two intensities.

Intensity = 10log(I1 / I0) (dB)

In this case,

I = 10log(32000 / 1 ) = 45

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