Day 167: Examining Doppler Shift in Logger Pro

doppler2College-Prep Physics: One student is looking at the relationship between the speed of her car and the apparent shift in the frequency of the horn. She used her iPhone to video her dad driving the car and honking the horn (yay for parental involvement!). Then in school, she played back the video on her phone and held a Vernier microphone to the iPhone speaker. Looking at the FFT graphs created by LoggerPro, she was able to see the shift in pitch at various approaching and receding speeds.

TIP: We used the LoggerPro template file called “Mathematics of Music” found in the “Physics with Vernier” folder. The FFT graph will display the whole spectrum and indicate the peak (loudest) frequency. However, we found that sometimes, depending on the exact moment when we sampled the horn, the different frequencies in the horn’s spectrum were the peak frequencies, even though all the same frequencies were present on repeated samplings.

We discovered this because the student was originally looking solely at peak frequency and was finding an inconsistent pattern between car speed and pitch. So I helped her out and looked at her data. By looking at the whole spectrum (rather than just the peak frequency),  we could see see the “fingerprint” of the horn. Then we could map the shift in the “fingerprint” for different speeds.


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About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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