Conceptual Questions

17.2 Speed of Sound, Frequency, and Wavelength


How do sound vibrations of atoms differ from thermal motion?


When sound passes from one medium to another where its propagation speed is different, does its frequency or wavelength change? Explain your answer briefly.

17.3 Sound Intensity and Sound Level


Six members of a synchronized swim team wear earplugs to protect themselves against water pressure at depths, but they can still hear the music and perform the combinations in the water perfectly. One day, they were asked to leave the pool so the dive team could practice a few dives, and they tried to practice on a mat but seemed to have a lot more difficulty. Why might this be?


A community is concerned about a plan to bring train service to their downtown from the town’s outskirts. The current sound intensity level, even though the rail yard is blocks away, is 70 dB downtown. The mayor assures the public that there will be a difference of only 30 dB in sound in the downtown area. Should the townspeople be concerned? Why?

17.4 Doppler Effect and Sonic Booms


Is the Doppler shift real or just a sensory illusion?


Due to efficiency considerations related to its bow wake, the supersonic transport aircraft must maintain a cruising speed that is a constant ratio to the speed of sound (a constant Mach number). If the aircraft flies from warm air into colder air, should it increase or decrease its speed? Explain your answer.


When you hear a sonic boom, you often cannot see the plane that made it. Why is that?

17.5 Sound Interference and Resonance: Standing Waves in Air Columns


How does an unamplified guitar produce sounds so much more intense than those of a plucked string held taut by a simple stick?


You are given two wind instruments of identical length. One is open at both ends, whereas the other is closed at one end. Which is able to produce the lowest frequency?


What is the difference between an overtone and a harmonic? Are all harmonics overtones? Are all overtones harmonics?

17.6 Hearing


Why can a hearing test show that your threshold of hearing is 0 dB at 250 Hz, when Figure 17.39 implies that no one can hear such a frequency at less than 20 dB?

17.7 Ultrasound


If audible sound follows a rule of thumb similar to that for ultrasound, in terms of its absorption, would you expect the high or low frequencies from your neighbor’s stereo to penetrate into your house? How does this expectation compare with your experience?


Elephants and whales are known to use infrasound to communicate over very large distances. What are the advantages of infrasound for long-distance communication?


It is more difficult to obtain a high-resolution ultrasound image in the abdominal region of someone who is overweight than for someone who has a slight build. Explain why this statement is accurate.


Suppose you read that 210-dB ultrasound is being used to pulverize masses of abnormal cell growth. You calculate the intensity in watts per centimeter squared and find it is unreasonably high (105W/cm2)(105W/cm2) size 12{"10" rSup { size 8{5} } "W/cm" rSup { size 8{2} } } {}). What is a possible explanation?