[hokator]

Interesting application for geeks, for measuring speed of cars or RC airplanes. Seems like it uses sound generated by object flying or driving around and doppler effect.
If you know how to use it its kind of precise.
Sound Speed Gun
SoundSpeedGun | Windows Phone Apps+Games Store (United States)

[Jazmac]

Is this YOUR app?

[hokator]

Yes

[ttsoldier]

Does it work like if you're IN the plane.. Or, IN a race car?

[hokator]

As it is explained here Doppler effect - Wikipedia, the free encyclopedia
You have to be outside of that object and has to pass around you.
Example: you are standing on the field and airplane flies over your head.
Or, you stand next to road and car pass around you.
Important is that the object is emitting constant sound.

[Siggi90]

How is the accuracy, and how loud has the moving part to be, so it can be measured?

[hokator]

Accuracy depends only on background noise. If there is none it is +- 5km/h.
Important is that application hears the main frequency of the object. (constant sound of engine running 5000rpm for example)
Airplane sound of engines spinning constant speed.
It can have a problems on the street where is sound pollution from 10 different cars same time