13. In a student laboratory exercise, the wavelength of a 40,000-Hz ultrasound wave is measured to be 0.868 cm. Find the air temperature.

15. A person stands directly in front of two speakers that are emitting the same pure tone. The person then moves to one side until no sound is heard. At that point, the person is 7 m from one of the speakers and 7.2 m from the other. What is the frequency of the tone being emitted?

17. A sonic depth gauge is placed 5 m above the ground. An ultrasound pulse sent downward reflects off snow and reaches the device 0.03 seconds after it was emitted. The air temperature is -20celsius. (a) How far is the surface of the snow from the device? (b) How deep is the snow?

21. A person stands at a point 300 m in front of the face of a sheer cliff. If the person shouts, how much time will elapse before an echo is heard?

23. The sound level measured in a room by a person watching a movie on a home theater system varies from 65 dB during a quiet part to 95 dB during a loud part. Approximately how many times louder is the latter sound?

25. What are the frequencies of the first four harmonics of middle C (261.6 Hz)?