Doppler found this effect by experiment use of sound such as on train sound. This sound actually have different hear by different people in many location or in different distance from this train. Doppler than want to know what happen on this phenomenon.
The Doppler effect was named after Johann Christian Doppler (1803-1853). This Austrian physicist observed and explained the changes in pitch and frequency of sound and light waves, as well as all other types of waves, caused by the motion of moving bodies. The general rule of the Doppler effect is that the wave frequencies of moving bodies rise as they travel toward an observer and fall as they recede from the point of observation.
While Doppler, in 1842, demonstrated the phenomenon named after him in the area of sound waves, in the same year he also predicted that light waves could be shown to exhibit the same response to the movement of bodies similar to those of sound waves.
This effect of sound actually applies in the bridge building technology that is used of wire to holding this bridge. The bridge building like this also many build to cross the wide river and cross the sea. They key on this building should intense to the frequency of wave that merge on the bridge.
The principle rule of frequency that is merged in the wire can strengthen or weaken each other. On wrong design the frequency can strengthen each other and can make the bridge break. If the frequency of each wire then weaken each other than make the wave on the bridge will vanish and the bridge safe. This theory also apply to the guitar and piano, this principle should be applied on designing of flute, drum, guitar, lute, lyre, lyra, violin, clarinet, saxophone and harp.