Not all sound enters the body through the outer ear. The inner ear is capable of receiving acoustic energy via bone conduction and tissue conduction. Intense sound waves can penetrate the skull, the torso, and the groin.
To recognize a sound, the ear has to analyze millions of tones, each with a specific pitch and intensity. The American National Standards Institute defines pitch as "that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from low to high. Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and wave form of the stimulus." Loudness is defined as "the intensive attribute of an auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud." Loudness is also influenced by the frequency of a sound and personal judgment of the listener.
Some sounds are pure tones; most are a combination of several tones. Many can best be described as "broad band" sounds; others as "narrow band" sounds. The whine of a jet is a narrow band sound.
The brain attempts to locate the source of a sound. The sightless human is helped in navigating his way by the echoes of sounds received from nearby reflecting surfaces such as walls and building exteriors.
The ear can hear the infinitesimal vibrations caused by a falling leaf and the vibrations caused by the intense sounds of a rocket engine, a range of about 150 decibels. The ear has been likened to an instrument which can measure in yards at one end of its range and yet detect changes of less than one-thousandth of an inch at the other end.
So sensitive is the human organism to sound that even the mere description of an unpleasant sound can evoke a physical response. Scratching a piece of hard chalk or a fingernail on a blackboard can give one goose pimples, and so can the description of such a sound source.
Sound evokes much more than the sensation of hearing. The sound signal is transmitted, via the brain, to almost every nerve center and organ of the body. Therefore, sound influences not only the hearing center of the brain, but the entire physical, physiological, emotional, and psychological makeup of the human being. The received sound wave evokes a combination of responses-auditory, intuitive, emotional, biological, associative. Sound's impact is a profound one.
The sound wave is both a message and a transmitter of messages. The message itself may be verbal or non-verbal. A verbal sound signal may transmit a single fact: today is Monday. The fact might be laden with significance and thus arouse emotion in you: you have just lost your job. Non-verbal signals, such as music, can also convey emotions. Non-verbal signals may also cause irritating physiological and emotional states. In short, sounds can make you ecstatically happy or terribly depressed.
Sound, per se, is a desirable and essential part of our lives and of our environment. There are many sounds that are pleasant in themselves; they can convey warmth and desired information. Sound permits communication, the exchange of thoughts and feelings between people. It enables us to enjoy the beauty of music, the voices of nature: waterfalls, birds, the wind rustling in trees, the ocean surf. It is a means of giving and receiving life-saving warnings.