Doppler effect
Measurement of the Doppler effect using the Fizziq sound library.
Author:
Title 4
Learning objectives :
This activity allows students to understand the Doppler effect and use it to calculate the speed of a moving vehicle. It demonstrates how an acoustic phenomenon can serve as a measurement tool.
Concepts covered
Doppler effect; Spectral analysis; Sound waves; Frequency and speed; Relationship between frequency shift and speed
What students will do :
The student analyzes the sound recording of a moving vehicle using FizziQ spectral tools to detect the change in frequency caused by the Doppler effect. By comparing the sound spectra before and after the vehicle passes, the student calculates the shift in percentage and then uses the Doppler effect formula to determine the approximate speed of the vehicle.
What is required :
Smartphone with the FizziQ application; 'Doppler Effect' recording from the sound library or personal recording of a moving vehicle; FizziQ experience notebook
Scientific background :
The Doppler effect is the apparent variation in frequency of a wave perceived by an observer when the source of the wave and the observer are in relative motion. For a sound source approaching the observer, the perceived frequency is higher than the emitted frequency (higher sound); when it moves away, the perceived frequency is lower (deeper sound). This variation is explained by the compression or stretching of sound waves. Mathematically, for a moving source and a fixed observer, the relationship is: f' = f × (c/(c-v)), where f' is the perceived frequency, f the emitted frequency, c the speed of sound (approximately 343 m/s at 20°C) and v the speed of the source. From this formula, we can isolate v: v = c × (1-f/f'). Analysis with FizziQ uses the frequency spectrum to precisely measure pre- and post-pass frequencies. The frequency shift Δf/f is proportional to the v/c ratio for low speeds. For example, an offset of 3% corresponds to a speed of approximately 10 m/s (36 km/h). The amplitude of the signal can also help determine the precise moment the vehicle passes the observer. This technique is used by Doppler radars to measure vehicle speed, but with electromagnetic waves rather than sound.