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Chapter 17: Q. 77 (page 487)
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The sound speed is 170 Hz.
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A manufacturing firm has hired your company, Acoustical Consulting, to help with a problem. Their employees are complaining about the annoying hum from a piece of machinery. Using a frequency meter, you quickly determine that the machine emits a rather loud sound at 1200 Hz. After investigating, you tell the owner that you cannot solve the problem entirely, but you can at least improve the situation by eliminating reflections of this sound from the walls. You propose to do this by installing mesh screens in front of the walls. A portion of the sound will reflect from the mesh; the rest will pass through the mesh and reflect from the wall. How far should the mesh be placed in front of the wall for this scheme to work?
Noise-canceling headphones are an application of destructive
interference. Each side of the headphones uses a microphone to
pick up noise, delays it slightly, then rebroadcasts the noise next
to your ear where it can interfere with the incoming sound wave
of the noise. Suppose you are sitting 1.8 m from an annoying,
110 Hz buzzing sound. What is the minimum headphone delay, in
ms, that will cancel this noise?
A steel wire is used to stretch the spring of FIGURE P17.42. An oscillating magnetic field drives the steel wire back and forth. A standing wave with three antinodes is created when the spring is stretched 8.0 cm. What stretch of the spring produces a standing wave with two antinodes?
An open-open organ pipe is 78.0 cm long. An open-closed pipe has a fundamental frequency equal to the third harmonic of the open-open pipe. How long is the open-closed pipe?
As the captain of the scientific team sent to Planet Physics, one
of your tasks is to measure g. You have a long, thin wire labeled
1.00 g/m and a 1.25 kg weight. You have your accurate space cadet
chronometer but, unfortunately, you seem to have forgotten a
meter stick. Undeterred, you first find the midpoint of the wire by
folding it in half. You then attach one end of the wire to the wall
of your laboratory, stretch it horizontally to pass over a pulley at
the midpoint of the wire, then tie the 1.25 kg weight to the end
hanging over the pulley. By vibrating the wire, and measuring
time with your chronometer, you find that the wire鈥檚 second harmonic
frequency is 100 Hz. Next, with the 1.25 kg weight still
tied to one end of the wire, you attach the other end to the ceiling
to make a pendulum. You find that the pendulum requires 314 s to
complete 100 oscillations. Pulling out your trusty calculator, you
get to work. What value of g will you report back to headquarters?
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