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APPENDIX D NOISE EVALUATION MEASURES AND STANDARDS y Sound is mechanical energy transmitted by pressure waves in a compressible medium such as air. Sound is characterized by parameters that describe the rate of oscillation of sound waves, the distance between successive troughs or crests, the speed of propagation, and the pressure level of energy content. The sound pressure level , or decibel (dB) level , has become the most common descriptor used to characterize loudness of sounds in an environment. Decibel levels are measured using a logarithmic scale since noise intensity levels can vary over one million times within the range of human hearing. A human response factor is added to noise levels by the process of "A-weighting", expressed as dba, since the human ear is not equally sensitive to all sound frequencies. Figure D.1 shows A-weighted decibel levels associated with a variety of common activities. Unwanted sound is referred to as noise. Certain sound frequencies are more disturbing to people. A description of a particular noise includes amplitude and/or amplitude variation of the acoustic wave, frequency (pitch) content of the wave motion, and duration of the noise. Certain noises are only acceptable if they occur for limited durations. In addition, short duration noise levels which contrast sharply with the ambient environment can be disruptive. Time variations in noise exposure are usually expressed in terms of a steady-state energy level equal to the energy content of the time varying period (called Leq) , or as a statistical description of what sound level is exceeded over some fraction of a given observation period. (L10, L50, or L90) . Leq corresponds to the equivalent constant level of a fluctuating sound. LIO is the A-weighted sound level which is exceeded D-1 104-61.R4 4/10/89