Question

Chapter 20

Questions

:1. Noise-cancelling headphones are a common application of destructive interference, and even some cell

phones have a built-in app for noise cancellation. Find an explanation of how noise cancellation works.

Cite the reference and then explain it

in your own words

. (Here’s an explanation you can use if you

want to: www.youtube.com/watch?v=VTx4JgYsW5s .)

2. You hear the siren of a police car. At first the car is sitting still, but then it begins to move toward

you. As it does, considering the Doppler effect, how do the following change:

(a) the frequency of the sound you hear;

(b) the wavelength of the sound you hear;

(c) the speed of the wave you hear?

3. What two physics mistakes occur in a science fiction movie that shows a distant explosion in outer

space, where you see and hear the explosion at the same time?

4. If the handle of a tuning fork is held solidly against a table, the sound from the tuning fork becomes

louder. Why? How will this affect the length of time the fork keeps vibrating? Explain.

5. The picture below shows a standing wave on a string, which is 150 cm long. What is the wavelength of

the standing wave? Explain.

Answer 1

1) We know that each sound wave consists of troughs and crests. The noise cancelling headphones use active sound waves to counteract the incoming sound waves. When the headphones detect an outside noise signal, a wave is produced in such a way that its crest meets with trough of noise signal and trough meets with crest , this will result in a plane and straight wave means noise is cancelled.

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2)  the frequency is higher than it would be if the vehicle was stationary. the wavelength will be shorter.

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3) There are two mistakes:

Mistake 1 - sound waves need a medium to travel. They cannot travel in vacuum. so, we will not be able to hear any explosion

Mistake 2 - sound waves and light travel at different speeds. We cannot hear and see an explosion at the same time.

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4) The sound becomes louder because the table vibrates at the same frequency as the natural frequency of the tuning fork. The tuning fork will vibrate for a shorter time since it takes more energy to move a larger surface.

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5) wavelength = (2/3)L

wavelength = (2/3)150

wavelength = 100 cm