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Sound Waves

The ear has a protective overload mechanism called the "acoustic reflex". A sound in excess of about 85 dB causes muscles attached to the eardrum and ossicles to engage, which provides a safety margin of about 20 dB or 30 dB (the equivalent of ear plugs). This reflex takes about 30 ms to 40 ms to cut in, and maximum effect only occurs after about 150 ms. Why do many hunters show loss of hearing (especially in the right ear)?

2. Suppose you sing or yell into an open piano with its dampers lifted off the strings. You are likely to hear the instrument "speak" back in response. Explain.
Imagine that you have two identical tuning forks and you tap one and then place it near the other. What, if anything, will you hear if you now silence the first fork?
"If one bows the bass string on a viola rather smartly", wrote Galileo, "and brings near it a goblet of fine, thin glass having the same tone as that of the string, this goblet will vibrate and audibly resound". Explain

Multiple Choice

1. In order to cause the loudness of a sound to appear half its original value it must be: (a)increased by 50 dB (b) decreased by 50 dB (c) decreased by 10 dB (d) decreased by 3 dB (e) none of these. Explain your choice.

2. The flute and piccolo are in teh same family of instruments, and each functions as a pipe open at both ends. Both have the same fingering but different lengths. The flute is about 66-cm long and is an octave lower than the piccolo - that is, the piccolo plays at twice the frequency of the flute. It follows that the piccolo, in comparison tot he flute is about: (a) twice as long (b) half as long (c) eight times as long (d) the same length (e) none of these. Explain your choice.

What will happen to the fundamental frequency of an organ pipe open at both ends if a cap is placed over one end sealing it? The frequency will: (a) go up 20% (b) be halved (c) stay the same (d) double (e) none of these. Explain your choice.

Problems

1. Imagine a hypothetical piano with all strings made of the same material and all under the same tension. The piano extends from 27.5 Hz to 4186 Hz which is over seven octaves - that is, seven doublings of frequency. If the highest note corresponds to a string 15-cm long, how long will the lowest string have to be? What can you conclude about this approach to piano design?

2. A narrow glass tube is 0.50-m long and sealed at its bottom end is held vertically just below a loudspeaker that is connected to an audio generator and amplifier. A tone with a gradually increasing frequency is fed into the tube, and a loud resonance is first observed at 170 Hz. What is the speed of sound in the room?

3. A train whistle is blown by an engineer who hears it sound at 650 Hz. If the train is heading toward a station at 20 m/s, what will the whistle sound like to a waiting commuter? Take the speed of sound to be 340 m/s.

4. An ultrasonic wave at 80 000 Hz is emitted into a vein where the speed of sound is about 1.5 km/s. The wave reflects off the red blood cells moving toward the stationary receiver. If the frequency of the returning signal is 80 020 Hz, what is the speed of the blood flow?

5. A point source of sound on top of a police car emits a signal at 1000 Hz. If the car is traveling in a straight line at 30 m/s, what will be the wavelength percieved by people standing on the road both directly in front of and behind the car? Take the speed of sound to be 335 m/s. What is the wavelength as measured in the car?

Solution Preview

Sound Waves
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<br>The ear has a protective overload mechanism called the "acoustic reflex". A sound in excess of about 85 dB causes muscles attached to the eardrum and ossicles to engage, which provides a safety margin of about 20 dB or 30 dB (the equivalent of ear plugs). This reflex takes about 30 ms to 40 ms to cut in, and maximum effect only occurs after about 150 ms. Why do many hunters show loss of hearing (especially in the right ear)?
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<br>A: The only reason is that the sound exceeds 85dB and is very abrupt. The acoustic reflex works well if the sound intenstiy increases slowly (such as soft music changing into loud music).
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<br>2. Suppose you sing or yell into an open piano with its dampers lifted off the strings. You are likely to hear the instrument "speak" back in response. Explain.
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<br>A: Suppose the main harmonic of my voice is f Hz. Strings with resonant frequencies of 1f, 1/2f, 1/3f, 1/4f etc will resonate at my fundamental voice harmonic. Strings with frequencies of 2f, 3f, 4f, etc will resonate at their frequencies (because of the higher order harmonics in my voice). Other responses are possible, such as
<br>(1 1/2)f, but their effect would be dominated by other strings.
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<br>Imagine that you have two identical tuning forks and you tap one and then place it near the other. What, if anything, will you hear if you now silence the first fork?
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<br>A: Since the forks are identical and vibrate under the same thermal conditions, I will hear the resonating second fork.
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<br>"If one bows the bass string on a viola rather smartly", wrote ...

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