Doppler effect in air

Answers to Questions

1. Why do we observe a the change in the selected frequency?.
   We observe the change because the air stream is carrying the sounds waves faster than when the fan is off. The waves are arriving at the sensor faster and the observed frequency is greater.
2. Why the shift in frequency is positive?.
   We have in this case the air stream moving from the frequency source to the sensor(DrDAQ) and the Doppler effect in this case is positive.
3. What happens if we change the air stream direction 180 degrees?.
   We will observe a negative change in frequency because the frequency coming from the source will arrive to the detector (DrDAQ) later than when the fan is off.
4. What happens if a boy is hitting the pipe and running to the sound detector?.
   In this case the speed of the boy make the same effect than the air stream coming from the frequency source to detector. We will observe a positive shift in frequency.
5. What happens if a boy is hitting the pipe and running away from the detector?. In this case the speed of the boy make the same effect than the air stream coming from the detector. We will observe a negative shift in frequency.
6. How can we calculate the speed of the air using the Doppler effect theory?.
   We can use the theory to estimate the speed of air in the experiment using the following equation (Doppler Effect equation):

   Observed frequency = Source frequency/(1-Vair/Vsound).

   Observed frequency= frequency observed when the fan is on and the air stream comes from the source frequency to the detector.

   Source frequency= frequency detected in the range 1800-2500 Hz when the fan is off.

   Vair= Speed of air (m/s).

   Vsound= Speed of sound in air. 340 m/s.

Results of the experiment

We obtain a spectrum in step 1 as shown in Figure 1. We mark on that spectrum the frequency 1644. Then we run the test with the fan on and observe the shift in frequency because the strongest frequency shift to the point 1652 Hz (Figure 2).

Figure 1. Source frequency. Fan OFF.

Figure 2. Spectrum with fan on.

We can verify the quality of data generated by the experiment checking the change in frequency for the full spectrum using the estimated air speed (6.0 KPH) and the data saved as STEP1.TXT and STEP2. TXT.. We import the STEP1.TXT files to a EXCELL sheet and apply the equation of Doppler effect to calculate the change in frequency at low and high frequencies. Those calculated frequencies are compared with the frequencies of STEP2.TXT and we will observe low changes of frequency (mHz) in the low an medium range the frequencies (2-500 Hz). We plot the data from the EXCEL sheet as show the figure 3.

Figure 3. Excell plot of source and observed frequencies.

Teachers notes

How long will the experiment take to setup: The time for set up the experiment can be 30 minutes.

How long will the experiment take to perform: This experiment will take one hour.

TIPS

Select high frequencies for the experiment because a low frequency generates small changes during the test. We recommend you use frequencies in the range 1600-2500 Hz.

Submitted By

You can send your data, comments and suggestions to:

WILSON ARIEL MORENO URBINA

wamu@uole.com

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