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Introduction <br />Noise is a pollutant. While its physical and emotional effects are difficult to define quantitatively, the sound <br />level itself can be measured. <br />Sound is an alteration of pressure that propagates through an elastic medium such as air and produces an <br />auditory sensation. <br />Noise is any undesired sound. <br />Waves and sound pressure level <br />Sound travels in a wave motion through the air to our ears. An effective tool to demonstrate wave motion is a <br />weight hanging from a spring. Picture the following diagram as a single weight and spring combination <br />varying as time progresses along the horizontal axis. <br />Weight on a spring - Example of periodic motion <br />• tiff:iiii <br />Weight <br />A <br />Time -I► 4 <br />1 <br />Figure 1 <br />In Figure 1, the first position of the weight on the spring is at rest with no forces exerted upon the system. If <br />the weight is raised above its point of rest and the progression of the weight moving down and up again is <br />observed over a period of time, a sinusoidal wave form is produced. This example demonstrates the <br />relationship between a linear motion, the weight bouncing on the spring, and its corresponding wave form. <br />The amplitude of the moving weight is denoted as A on the diagram and corresponds with the maximum <br />displacement of the weight from its "at rest" position, or the peak of the wave form in the positive or negative <br />direction. We perceive changes in amplitude as changes in loudness. <br />The period of the vibration is the amount of time taken to produce one complete cycle. The number of cycles <br />per second defines the frequency of the periodic motion, denoted by the unit of hertz, or Hz. We perceive <br />different frequencies as higher or lower pitched sounds. <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />2 <br />Period <br />