1. Doppler Effect David William D. Ecoben III-Archimedes

2. The Doppler Effect on Waves  The apparent change in pitch produced by moving objects is called the Doppler Effect.  Sound waves undergo the Doppler Effect. When the source of the waves moves at a constant speed relative to you, the observer, the frequency of the wave as observed by you is shifted.

3. For sound, the shifted frequency f observed, in hertz, is given by:  f observed = f source x (v source - v observer ) (v source - v sound ) Where: F source = frequency in hertz at the source of the sound V source = the speed in m/s of the source of the sound V observer = the speed in m/s of the observer, v source – v observer = the relative speed in m/s of the source with respect to the observer, and V sound = the speed of sound in m/s.

4. Example 12.5 Emergency A fire truck siren emits a sinusoidal wave with a source frequency f source of 400 Hz. The speed of sound is 340 m/s. Find the wavelength of the siren while at rest. If the truck is moving with velocity v source = 40 m/s, find the wavelength of the siren going nearer to and then away from a stationary observer.

5. Given: Source frequency f source 400 Hz Velocity of source v source 40 m/s Velocity of the observer v observer 0 Find a. Wavelength if the source is at rest b. Wavelength if the source is nearing the observer c. Wavelength if the source is going away from the observer

6. Solution:  A. When source is at rest, using the equation v = f λ Solving for λ, λ = v source f source λ = 340 m/s / 400 Hz λ = O.85 m

7.  B. In front of the siren, since the object is getting farther, then the speed of sound relative to the observer is v – v s, so  λ = (v – v source ) / f source  = (340 m/s – 40 m/s) / 400 Hz  λ = 0.75 m

8. c. Behind the siren, since the object is going nearer, then the speed of sound relative to the observer is v + v s, so λ = (v + v source ) / f source = (340 m/s + 40 m/s) / 400 Hz λ = 0.95 m