Doppler effect in a
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.