Question

I don't want an answer to this question, I was just hoping someone could explain it and walk me through it.
A whistle you use to call your hunting dog has a frequency of 21 kHz, but your dog is ignoring it. You suspect the whistle may not be working, but you can't hear sounds above 20 kHz. To test it, you ask a friend to blow the whistle and hop on their moped. In which direction should they ride (toward or away from you) to know if the whistle is working? Explain in words or with a diagram.

Answer 1

Hi! Do you know about the Doplar Effect?

Answer 2

Well, sound is the vibration of a medium, like air or wood. We usually sense sound through the air that touches our eardrums. That's our medium in this example, air!

Answer 3

A speaker is really a material that pushes away air, and the pulls it back. The air it pushes pushes other air before being drawn back, which draws back other air too.

But the air doesn't all move at once. When the speaker pushes the air, the air gets compressed together at first.

Answer 4

The air moves as it is pushed by the speaker and other air, and will go back when the speaker does. The air passes this along as vibrations. Like when you push water.

But the waves travel in the air, and so that determines how fast the sound waves move.

As it propagates, there are compressed air areas (black) and expanded air areas (white). These pressure differences cause our ear drums to be moved towards the low pressure and then away from the high pressure, back and forth. The frequency determines what we think of as pitch. High frequency is high pitch. The frequency is the number oscillations in an interval of time.

But the "waves" move relative to the air. And each new push and pull from the speaker produces another compression and expansion. But the sound travels at pretty much the same speed as the air. So if your speaker moves relative to the air, the speaker is getting closer to it's last compression or expansion.What those illustrations show are the compressions, and the wavelengths between them. Greater wavelength means lesser frequency.

In the direction of motion, the speaker gets closer to its last compression or expansion, so the compressions and expansions have a higher frequency there.

In opposite the direction of motion, the speaker gets farther away from its last compression or expansion, so there is lower frequency behind it.

Answer 5

So, you want the whistle to be lower frequency, which means a greater wavelength. Which means behind the moving whistle.

Answer 6

I bet YouTube has lots of information on this!

I assumed you were familiar with waves, but feel free to ask if anything confuses you.

Answer 7

ah... so nice of theEric-san to be willing to help to this extent! A Like to him anyone? :D

Answer 8

Haha, thanks @chenping . I wonder if I'm even close to competing with some YouTube videos on the Doppler Effect. I spelled that wrong earlier...

Haha, look at the bee (animated)!
http://www.youtube.com/watch?v=76X1LtWkUE8

By the way, the object doesn't have to move relative to the air for to notice. YOU could be the one moving.

It's not relevant to the whistle thing, the way they chose to work it out, but I'll mention it anyway.

Without an animation, you'll have to continue using your imagination.As the person moves toward the source, he or she will quickly experience the oscillations coming towards him or her (and they keep on coming). Afterwards, that person will be going in the same direction as the oscillations are propagating. So that person will run into them less frequently. Imagine you're on a highway, going slow. You're you. And all the passing cars are oscillations. Less frequent than, say, if you were driving in the opposite direction.

So, this means you can blow the whistle while your friend drives the scooter and listens. But it'd probably be hard to hear with wind in your ear, so that's probably why they didn't do that.