# Doppler effect in sound waves

While studying doppler effect we study that as the locomotive banging up the horn is moving the produced waves come closer, making wavelength decrease and frequency increase ,and so is the wave heard as shrill and as it is moving ahead the waves in opposite direction move farther,therby increasing wavelength and decreasing frequency so my question is this that is it necessary for vehicle to move faster than speed of sound for Doppler effect to be observed

In response to your subsidiary question (in your comment to C. A. Powers) about supersonic objects: If the object making the sound is moving above the speed of sound (supersonic), then instead of the Doppler effect, a sonic boom is what occurs (example, watch for a few minutes from this point).

Nothing is now heard in front of the object, only behind it, so there can't be a Doppler effect as such1. The Doppler effect is all about the difference between the frequency (or pitch) of what is heard in front of and behind the moving object. That's what C. A. Powers was referring to.

Here is another good explanation:

Sonic booms occur when the source travels faster than the speed of sound. If the source is traveling at the speed of sound, the waves pile up and move along with the source; when the source travels faster than sound, a shock wave (also known as a sonic boom) occurs as waves pile up. The angle at which the shock wave moves away from the path of the source depends on the speed of the source relative to the speed of sound.

1Unless we argue that the difference is now infinite because no pitch exists in front, while waves pile up behind, etc.