Similarly, the characteristic chug of a diesel locomotive pulses relentlessly through the body of the engineer. Listening to the monotonous beat for up to eight hours often results in fatigue. Sound waves travel through the air in much the same way as ripples from a drop of rain spread over the surface of a lake. The traditional way of blocking out unwanted noise is by placing a sound-proofing wall between the listener and the source of the sound. However, low-frequency noise components travel far and are difficult to cut off.
Help is now at hand - at least for the train engineer.
A headrest with loudspeakers, microphones and advanced electronics can be mounted on the back of an engineer's chair to create a quiet zone around the head. At lightning speed The microphones detect the low-frequency throbbing in the cockpit and transmit these to the box mounted on the headrest. Here, the electronic circuitry uses digital signal analysis to determine the precise mirror image of the unwanted noise. The speakers subsequently generate the reverse sound in the form of pressure waves in anti-phase with the unwanted noise. Close to the speakers, which are mounted at head level, a quiet zone will result where the noise is significantly reduced.