Answers to Readers' Questions on Sound and Doppler Effect. Also refer to Equations, frequency, wavelength, velocity, speed, angle, cosine, Physical Science, physics, Ron Kurtus, School for Champions. Copyright © Restrictions
Answers to Readers' Questions on Sound and Doppler Effect
The following 2 questions have been answered. They are listed according to date.
List of first 10 items
Who hears the exact frequency?
Question
May 19, 2008
A source S produces sound waves of frequency f and is moving along a straight line as shown
I S-> IV
II III
Which observer I, II, III or IV could hear a sound of frequency f when the source is in the position shown?
A. I
B. II
C. III
D. IV
Ruby - Australia
15972
Answer
I can't tell by your drawing. The person who is exactly perpendicular to the motion of the source should hear the exact frequency.
Experiment talking into a fan
Question
April 24, 2008
The question is why do you sound funny when talking into a fan? We understand it has to do with the Doppler effect, but how do you calculate or prove this? If we count the rotations per second, will that give us the frequency of the fan? And how would we determine velocity for each setting of the fan (low, medium, high)?
This is for a fifth grade science fair project.
Thank you for your time.
- USA
15789
Answer
When you talk into a fan, your voice sounds funny--sort of choppy and higher pitched. You are hearing you voice reflected off the blades of the fan when they are in the correct position. Other times, the sound goes right through the fan.
Because the blades of the fan are at an angle, their surface is moving toward you. They act like a source of sound moving toward you, and thus there is a slight Doppler effect, which should increase the pitch of the reflected sound you hear. If you talk into the fan from behind it, the pitch should be slightly lower, because the angle makes the blade surface move away from you.
Someone on the other side of the fan will hear a similar effect should simply hear a choppy voice from the blades interfering with the sound.
If the fan goes too fast, the effect can't be heard very well. A slow fan in the best, because you can distinguish between the echoes. It would be great to use a fan with a variable speed control that you could adjust for peak performance.
If you know the rotations per second and divided by the number of blades, you would get the frequency in Hz of the choppy sound waveform.
The big problem is to determine the rotations of the fan. I don't think many would have it written on the fan. Often the rotations could be found using an adjustable strobe light. Some school science labs have such a strobe available.
The big thing is: What do you want to show or prove with your experiment? You could simply demonstrate the sound in the front, back and through the fan at the different speed. Do the experiment and record the results with a tape or digital recorder. Then you can also play it back. If you have a recorder where you can adjust the playback speed, you can slow down the playback and really hear the choppy sound.
I hope this gives you some ideas. Good luck and let me know how your project turns out.
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Answers to Readers' Questions on Sound and Doppler Effect