The roar of an aircraft engine, the noise of the city outside the window, the morning roar of the neighbor’s drill in our lives regularly invade the senseless and merciless sound, elastic waves of mechanical vibrations. Any sound the hearing begins with vibrations of the eardrum, which is passed along the chain of little bones in the skull and then becomes a signal running through the auditory nerve to the brain. All this work is often not only unnecessary, but even harmful: the word “noise” we are opposed to “signal”, complaining about the noise, from the noise trying to escape or to defend himself.
The loudness of any sound depends on the amplitude of waves in some medium (be it air, water, or even solid) reaches our ears, that is, from the magnitude of fluctuations of particle environment — the more “humps” of the displacement, the louder the sound. To make any sound quieter, it is necessary to reduce its amplitude until it reached our ear drums. Usually the fluctuation is reduced, erecting all sorts of obstacles in the path of wave propagation. The street noise is weakened, if you close the window in my room. The blare of the neighbor punch the wall can be saved with the help of earplugs. However, this approach is not always convenient and effective: in the cabin, heard the roar of the engines, because the walls of the housing to protect against this noise is not enough, in a room with a closed window simply becomes stuffy, and the fixtures for the ears it is difficult to combine good sound with a small footprint.