My name is Alexander FufaeV and here I will explain the following topic:

# Acoustic Doppler Effect: How Frequency Shift of Sound Occurs

## Formula

What do the formula symbols mean?

## 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 $$v = 0$$.

Explanation

## Video

The basic idea of the acoustic Doppler effect is similar to the Doppler effect for electromagnetic waves such as light, but here it is applied to sound waves. When a sound source or an observer moves relative to the medium in which the sound is transmitted (usually air), there is a change in the perceived frequency of the sound.

Formula anchor

Consider illustration 1: An ambulance drives to the right. It has a siren that emits a certain transmitter frequency $$f_{\text S}$$ from its point of view. Depending on how an observer moves, they perceive the transmitter frequency differently. The frequency that they perceive is referred to as $$f$$.

• When the ambulance and the observer move away from each other, the observer hears a deeper siren sound.

• When the ambulance and the observer approach each other, the observer hears a higher-pitched siren sound.