# Doppler effect frequency

• arpon
In summary, when the observer is moving away from the source, the apparent frequency is decreased. However, if the observer is stationary and the source is moving faster than sound, the sound will be played in regular order.

#### arpon

As we know, when the observer is moving away from the source, then the apparent frequency is,
##f_{observer} = f_{source} (\frac{v_{sound} - v_{observer}}{v_{sound}})##
But, if ##v_{observer} > v_{sound}## , ##f_{observer}## becomes negative.

With sound waves being vibrations in air, if you move faster than sound away from the source, those pressure waves hit your eardrums in the reverse order. Windiness aside, the sound would be heard as being played backwards.

arpon
jfizzix said:
With sound waves being vibrations in air, if you move faster than sound away from the source, those pressure waves hit your eardrums in the reverse order. Windiness aside, the sound would be heard as being played backwards.
And what it would be, if the observer is stationary and the source is moving faster than sound, i.e.
##f_{observer} = f_{source} (\frac {v_{sound}}{v_{sound} - v_{source}})## , and ##v_{source} > v_{sound}## ;

you would hear the sound in regular order:
The first pressure wave the source emitted would be the first thing that hits your eardrum, and the second would be the second, and so on

However, the sound would be slowed down:
The pressure waves would hit your eardrum less often because each time the wave has to travel a longer distance from the source at the same speed.

jfizzix said:
you would hear the sound in regular order:
The first pressure wave the source emitted would be the first thing that hits your eardrum, and the second would be the second, and so on

However, the sound would be slowed down:
The pressure waves would hit your eardrum less often because each time the wave has to travel a longer distance from the source at the same speed.
When the source is moving faster than sound and moving towards the observer, the source has to go ahead of the observer to reach sound to the observer. So, in this case, I think, the formula for the source coming to the observer is not applicable. Only, when the source is moving away the observer, the sound can be heard.

arpon said:
When the source is moving faster than sound and moving towards the observer, the source has to go ahead of the observer to reach sound to the observer.
Right, and after the super sonic source passed the stationary observer he would hear the previously played sounds in reverse order.

arpon said:
So, in this case, I think, the formula for the source coming to the observer is not applicable.
Why not?

A.T. said:
Right, and after the super sonic source passed the stationary observer he would hear the previously played sounds in reverse order.

Why not?
I think, the observer will hear two sounds, one coming from in front of him and the other coming from behind him.

Sound coming from in front of the observer (frequency ##f_1##) will be fast but played backward. And sound coming from behind the observer (frequency ##f_2##) will be slow but played in regular order.
##f_1 = f_{source} (\frac {v_{sound}}{v_{source} - v_{sound}})##
##f_2 = f_{source} (\frac {v_{sound}}{v_{source} + v_{sound}})##
And, look, for sufficient velocity of the source,##f_1## may be less than ##f_{source}## !

## 1. What is the Doppler effect frequency?

The Doppler effect frequency is the perceived change in frequency of a sound or light wave due to the relative motion between the source of the wave and the observer.

## 2. How does the Doppler effect frequency work?

The Doppler effect frequency is based on the concept of wavelength. As a source of sound or light moves closer to an observer, the wavelength of the waves it emits becomes shorter, resulting in a higher perceived frequency. Conversely, as the source moves away, the wavelength becomes longer and the frequency is perceived as lower.

## 3. What causes the Doppler effect frequency?

The Doppler effect frequency is caused by relative motion between the source of the wave and the observer. This can occur with both sound waves (such as a car driving by) and light waves (such as stars moving in space).

## 4. How is the Doppler effect frequency used in science?

The Doppler effect frequency is used in various fields of science, including astronomy, meteorology, and medicine. It is used to measure the speed and direction of objects in space, to study weather patterns, and to assess blood flow in the body.

## 5. Does the Doppler effect frequency only apply to sound and light waves?

No, the Doppler effect frequency can also apply to other types of waves, such as water waves or radio waves. Any wave that travels through a medium and is affected by relative motion can experience the Doppler effect frequency.