How to find f0 when given highest and lowest frequency

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In summary: I just provided a summary of the conversation.In summary, Dave found that by using the highest frequency, he was able to find f0 by finding the average between the two frequencies. Then, he used the same method to find Vsource. However, when he tried to do the same thing with the lowest frequency, he got two different values. One value was 2000 m/s and the other was 158.6 m/s.
  • #36
Yeah i think the 50 was the sound bouncing off the wall.
mfb said:
Anyway, if you want to find a solution for those values, set up both equations for doppler shift and solve for both variables. Orodruin's suggestion is possible as well, but probably more complicated to understand.
Do you mean solving by substitution?
 
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  • #37
HHH said:
Do you mean solving by substitution?
There are many ways to solve a system of two equations, and I don't like very specific names for simple algebraic operations on them, but the answer is probably "yes".

HHH said:
Yeah i think the 50 was the sound bouncing off the wall.
That should not give 50 Hz.
 
  • #38
mfb said:
That should not give 50 Hz.

any idea on what the 50Hz and 200Hz was? It came up multiple times when the teacher was swinging the device.
 
  • #39
Something oscillating with the frequency of the power grid (depending on where you live)?
Or some random other source of sound. Where does the number 200 come from? If you have even more other frequencies, it just shows the values are not reliable.
 
  • #40
mfb said:
Something oscillating with the frequency of the power grid (depending on where you live)?
Or some random other source of sound. Where does the number 200 come from? If you have even more other frequencies, it just shows the values are not reliable.
We did it in a classroom, how can an app pick up power grid thing. Also, during the lab we kept getting different frequencies ranging between 50 to 640, teacher said use the highest and lowest.
 
  • #41
158 m/s is very obviously wrong. It is about 600 km/h; things moving at such speeds require a lot of energy pumped into them just to keep moving through the atmosphere, and that energy is then dissipated into heat and sound - a lot of sound, making the entire exercise rather pointless.

A reasonable speed would not exceed 20 m/s, then the min/max observed frequencies should differ by about 10%.
 
  • #42
HHH said:
how can an app pick up power grid thing
Various objects can transfer electric currents to mechanical vibrations. All sorts of coils love to do that.
HHH said:
teacher said use the highest and lowest.
That is a bad idea.
 
  • #43
mfb said:
That is a bad idea.

Yeah i agree, i think he just wants to see of our math is correct. Thanks for the help
 

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