Fighting back
With the exception of some neighbour noise and some warning signals, no one would be likely to complain if noise were to be eliminated. Can this be done?
In those (unfortunately rare) cases in which noise is highly regular, consists of one or few frequencies, and is bothersome only in a small and well-deflned location, active noise cancellation (ANC) can be highly efiective. ANC relies on the fact that sound waves consist of compressions followed by rarefactions. On a pressure plot such as Figure 1, this is clear from the fact that the line goes flrst above and then below the midpoint. If a second sound wave were superimposed on the flrst such that at every point at which there is a compression in the original wave there was an equally intense rarefaction in the new one, perfect silence would result, the acoustic energy being converted entirely to heat. However, in practice this can only be achieved over very limited areas, such as plane cockpits, the driver’s head area of some cars and—perhaps most usefully—the space inside ear defenders (Figure 23).