Chapter 20 Sound
Exercises
6. Bats! v = λ* f ; both have the same speed of sound in air @ 340 m/s.
 | Cats have lower frequency and λ = .00486 m |
| Bats have higher frequency and λ = .00283 m |
7. Carrier frequency of the electromagnetic waves is 101.1 MHz.
9. Electromagnetic. Sound travels 340 m/s and electromagnetic travels at 300,000 km/s. If they both have the same frequency then using v = λ * f :
| Sound at 1 Hz would have a λ = 340 m |
| Electromagnetic at 1 Hz would have a λ = 300,000 km |
12. This eliminates a delay due to the speed of sound in the air; otherwise, the closest runner would hear the gun first.
13. Yes, because of the compression (more dense) and rarefaction (less dense) of the waves.
15. Snow is a good absorber of sound, so it is quieter.
16. See because light passes through a vacuum. Not sound because sound needs a material medium to vibrate and transmit.
20. v will stay the same (340 m/s if in air); f doubles then λ will compress to half.
24. Sound travels faster in the solid ground than air.
26. The short λ of ultrasound can image smaller objects; the short λ of blue light in the microscope will image smaller objects than ultrasound; the even smaller ultra short λ of the electron microscope will image smaller objects than the blue light microscope.
33. The break in step prevents forced vibrations or resonance of the bridge.
47. The beat is 2/min. Will be in step 2 times per minute or every 30 seconds. Friend takes 25 strides and you will take 24 strides when both are in step.
50. Hear the beat of 2 kHz. 2,000 Hz is within the audible frequency range.
Problems
1. (a) f = 340 Hz and v = 340 m/s
| λ = v/f = 340 m/s / 340 Hz = 1 m |
(b) f = 34,000 Hz
| λ = v/f = 340 m/s / 34,000 Hz = .01 m = 1 cm |
3. Speed = distance/time v = 1530 m/s Echo is 6 seconds; one-way is 3 seconds
| d = speed * time = 1530 m/s * 3 seconds = 4,590 m |
4. v = 340 m/s Echo is 0.1 second; one-way is 0.05 second
| d = 340 m/s * 0.05 second = 17.0 m |
5. 340 m away from you. The first strike was silent. Her second strike was heard synchronized. When she stopped it took 1 second for the last strike to reach you.
7. (a) Constructive – in phase. (b) Constructive – in phase. (c) Destructive – out of phase.
8. f = v/λ = 340 m/s / 6 m = 56.67 vibs/s = 56.67 Hz. This is a low pitch within the hearing range of 20 – 20,000 Hz.
9. λ = v/f = 1500 m/s / 56.67 vibs/s = 26.47 m/vib. Note that due to the change in speed between water and air the λ in water would be 26 m , but the λ in air would be 6 m at the same frequency.
Chapter 21 Musical Sounds
Problems
1. 10 dB is 10X more intense than 0 dB; 30 dB is 1,000X more intense than 0 dB; 60 dB is 1,000,000X more intense than 0 dB.
2. 40 dB is 10X more intense than 30 dB.
Chapter 28 Reflection and Refraction
Problems
5. 92%. 96% is transmitted through the first face, and 96% of the transmitted 96% will pass through the second face. (96% * .96 = 92%)