# Formula Acoustic Doppler Effect Observer frequency    Emitter frequency    Emitter speed    Observer speed

## Observer frequency

Unit
Frequency perceived by an observer (who may hear a loud ambulance, for example).

## Emitter frequency

Unit
Frequency emitted for example by the siren of an ambulance.

## Speed of sound

Unit
Speed of sound at which sound waves propagate. In air, the speed of sound is: $$c ~\approx~ 340 \, \frac{\text m}{\text s}$$ at 20°C.

## Emitter speed

Unit
Speed at which the emitter (for example an ambulance) moves relative to the observer.

"$$c ~-~ v_{\text s}$$" is used when the emitter moves towards the observer. "$$c ~+~ v_{\text s}$$" when the emitter is moving away from the observer. If the emitter is stationary, then $$v_{\text s} = 0$$.

## Observer speed

Unit
Speed at which the observer moves relative to the emitter.

Use "$$c ~+~ v$$" when the observer moves towards the emitter. "$$c ~-~ v$$" when the observer moves away from the emitter. If the observer is standing still, use $$v = 0$$.