Hey

I've doing a project which i've started to build.
The title of my project is Doppler Demostrator for a physics class.
The outcome is to build a small battery powered transmitter embedded in a sponge ball. An audio receiver will receive this signal, measure its frequency and display the result on a digital display.

I've gone about doing it this way but i think i'm totally on the wrong path.

 

I do not think you will see doppler effect with what you describe.
Your sponge which could be accelerating will transmit but that transmission occurs at close to speed of light. The receiver will receive an un-doppler-distorted(regular) signal and simply demodulate it to recover the frequency. All will look normal unless your sponge was travelling at a significant fraction of the speed of light. lol!

The sound waves would show doppler effect of your moving sponge but only if your sponge sent them into the air (and not the Em spectrum)

Can you do something like the following?

Get one of those hi pitch siren devices like Sonalert etc... and connect this to a portable battery powered circuit that simply keeps the siren running when turned on.

Make another circuit with a microphone, amplifier and filter with an output that will go to an oscilloscope.

Mayby put them both on a rail or track of some sorts where the siren circuit can slide and the microphone circuit remains fixed hooked up the o'scope.

Then when you turn on the siren circuit, you will see a waveform on the scope which is what your amplifier is "hearing" tuned (via the filter) to the frequency of the siren. You can measure this frequency on the scope.

Then slide your siren circuit very fast away from the mic circuit and the scope should show you an expanded waveform indicating that it (and you) are hearing the siren at a lower frequency. Sliding back towards the microphone and the wave gets compressed.

Depending on how fast you can move your sound source, the effects might be barely noticable on the scope but it is measurable and a high speed scope should have no problem showing you that.

 

Thanks very much for info Optikon very helpful. I see how off track I am trying to build this project. Your suggestions has really opened my eyes to doppler.
Does this mean u can not use ultrasonics to measure doppler?
This means, the best way to measure or demonstate doppler is to use audible frequency i.e less than 20KHz.
If i replace ultrasonics with an audio transmitter, and audio receiver and keep suggestions will i be able to detect/demonstrate doppler?
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I don't if block diagram showed in previous post but here it is again.

Attached Images

block_system.gif (4.3 KB, 351 views)

 

If the transmitter in the ball emits a steady tone of known frequency and you threw it toward or away from a microphone there would be a difference in the frequency between what the ball emitted and the microphone heard thanks to Doppler shift. As ball slows down the sound the microphone hears will shift toward the frequency being emitted.

If you are planning on using some kind of digital meter you would need to understand or possibly control when it takes a sample or the gate time. If the time between samples were very short the meter will be changing so fast you might not be able to read anything.

There are a bunch of other ways to do this but it would take forever to detail them in this forum. You might want to start with some understanding of what you can measure then back your way into the required velocity.

An audible way to demonstrate this might be to plug a microphone into a recorder then throw the ball with tone emitter so that it passes by the mic. If the ball is thrown fast enough the downward shift in frequency should be apparent. Another way might be to have a mic and recorder set up on a roadway where you can drive past at 50 mph or more sounding the horn as you approach and pass - the doppler shift will be obvious, especially if you maintain a constant speed. I suppose that if the mic were sensitive enough you'd be able to see an approaching and departing reading on a digital meter.

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stevez

 

just a quick though, if you used a PC based scope, you could measure the frequency, and subtract the known frequency, to give a plus or minus number to show the difference between what the mic is hearing, and what the sound actually is.

Tim

 

Ultrasonic waves in air are subject to the same doppler effects as any other frequency range.

I think 20kHz sounds fine. What I would strive for in the design would be to get the greatest sound magnitude possible though so that the microphone and amplifiers job is easier and you can stay away from the noise floor easier(especially as the sound source becomes distant from the mic)

Along with the others suggestions, I think that you can indeed demonstrate a doppler effect that will be clear to your audience. But you'll have to do some non trivial design work for your system.

 

i am not sure how good this is, but if you try using a lower frequency, probably in an audiable range, like 1KHz, or smaller and throw the ball fast, hit it or something, you could notice the effect just with your ears.
hope i am not mistaking.
i can see this effect with cars passing by that make the horn sound, even when they do not moove fast.

 

Optikon's thinking of radio waves when he said speed of light. Since you're using audio waves the freq shift is easy to calculate.