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.
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.
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.
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.
You can send your data, comments and suggestions to:
WILSON ARIEL MORENO URBINA
L.A.N.Z.O Sweden AB