This is somewhat similar to the switching that occurs in the telephone exchange to allow one person to speak to another. This pattern-matching event may be very weak, resulting in a weak perception of sound. A strong pattern match results in a loud and intrusive sound perception. The original purpose of this ability to amplify small signals and to suppress others was to facilitate the detection of potential threats in the environment. The Meaning of Loudness Under normal circumstances we hear more intense sounds as seeming louder than quiet sounds, but our perception of loudness is not dictated simply by the strength or intensity of the sound arriving at the ear.
Some sounds become loud, intrusive and unpleasant because of their meaning or association.
This is almost universally true for the sound created by scratching chalk on a slate, or the sound of a burglar alarm down the street. Very often the over-sensitivity for sounds is begun by an irrational fear which nevertheless becomes a very strongly held belief. These weak sounds are turned them into very loud intrusive and unpleasant perceptions which become constantly audible whether we like it or not. The standard 'pure tone audiogram' measures the quietest sound you can hear. This is called the threshold of hearing.