1. Doppler effect Eeeeeee – yowwwwwwwwww
A change in frequency or pitch of a sound detected by an observer.
Unless specified – assume that the speed of sound waves in the air is 343 m/s
This velocity is a fair estimate under most daily conditions

2. Stationary source production of sound waves
Velocity of source (vs) = 0
Both women hear the same sound (pitch)

3. Moving source production of sound waves
Velocity vector in direction (vs) has positive value
Hears lower pitch – longer λ hears higher pitch – shorter λ

4. HOW DOES IT WORK??? Let’s start with a review of equations:
λ = V(wave) / f units: m (per wave)
f = V(wave) / λ units: waves / s
T = λ / V(wave) = 1 / f units: s / wave

5. How about a visual here Stationary source: 1 pulse/s (siren) 
Moving pulse: 1 pulse/s (siren v = .5m/s)

6. Δ λ = V source (λ) / V sound
λ = 1m
λ’ = .5m
λ’ (in m/wave) = the wavelength that the observer hears as the sound source moves toward them
λ’ = λ - V sound source x period
λ’ = λ - (V source)(T)
And another:
Δ λ = V source (λ) / V sound

7. Fun and exciting question for you:
If a siren wave frequency is 1600 Hz
And the V of the siren source is 25 m/s
Then what is the frequency of the siren as heard by an observer (as the siren moves toward the observer)?

8. You need the λ and the period to solve
λ = V sound / f = 343 / 1600 = .214 m
T = 1/f = 1/1600 = 6.25 x 10-4 s/wave
Now calculate λ’
λ’= λ – V siren (T)
=.214 m/wave – (25 m/s x x 10-4 s/wave)
= .198 m
f’ = V sound / λ’ = 343 / .198 = 1732 Hz

9. One more fun one for you!!!
A motorcycle with a wavelength exhaust note of .83m (414 Hz) is moving away from you at 54 m/s. What wavelength do you hear as it moves away?
0.965 m (a frequency of 355 Hz)