Doppler Shift Horn

Imagine you are strolling along a street. All of a sudden, you hear the faint siren of an ambulance in the background. With each passing moment, the intensity of the siren increases, which become more and more piercing. Then it goes past you in a whoosh and the siren shifts to a low-pitch. With the rise in pitch, you estimate the speed of the ambulance’s approach and decide in a flicker whether it would be suicidal to cross the street at that instant. What is at work here? Nothing spectacular, obviously. Every school child knows that the phenomenon pictured above is a demonstration of the ‘Doppler Effect’, which was first explained by the Austrian physicist Christian Doppler in 1842 as the apparent change in frequency of a signal caused due to the speed of movement between the source and the listener. Most of us would also have heard the physics teacher remark that police employs this method to trap speeding vehicles on the road (At this point, one of my naïve friends loudly exclaimed wonder at the cleverness of police officers!). If you bother to search this item in Wikipedia, you would find a simple mathematical formula linking the change of frequency with speed. It is really that simple.

Now, think about the reverse mechanism. What if we supply a horn with a stream of signals whose frequency is changing so fast as to invoke the sensation that a vehicle is rushing on to us at a very high speed? The utility of the horn is obvious. People would stampede on their run to be out of the way of this oncoming vehicle. Your right of way would be guaranteed. If the device is used sparingly and judiciously, the driver can find his way in any crowded street.

The new Doppler horn is not difficult to construct. With a square-wave generator whose frequency can be controlled to be in relation to the desired speed, an electronic horn can be built having sufficient amplification to produce sound loud enough to be used in automobiles. ‘Doppler Shift Horn’ might be a good name for the device.

Any takers?