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Loudspeaker <br />Comparison of periodic motion to sound waves <br />Fl <br />Compress! <br />Com pssion <br />n ompressio <br />Expansion Expansion <br />I <br />Ex ansion <br />Air <br />Figure 2 <br />The graphical representation of sounds in Figure 2 is of pure tones, which are sounds made up of a single <br />frequency. A familiar example of a pure tone is the sound produced when a single key of a piano is pressed. <br />For instance, the middle C key on a piano vibrates the associated wire at a rate of approximately 260 times per <br />second or 260 Hertz. The vibration of the wire transfers its motion to the sound board of the piano, which then <br />vibrates at the same frequency, causing the air adjacent to the sound board to form compression and expansion <br />waves in the air emitting outward from the sound board. When received by the human ear, this is regarded as <br />sound. <br />Most sounds are not pure tones but a mixture of tones of varying amplitude, frequency, and duration. The <br />intensity of sound waves produce a sound pressure level, measured in a unit called the decibel, or dB. The <br />decibel is a logarithmic measurement used to accommodate a numbering scheme that encompasses a large <br />range of values. The logarithm is used because the human ear can detect sounds more than a million times <br />quieter than a jet aircraft during take off. <br />Sound pressure level = 20 Log (Measured Sound Pressure / Reference Pressure) <br />Reference Pressure = 0.00002 Newton's / (meter) <br />Decibel = The ratio between two quantities that are proportional to power. The unit of measurement for sound <br />pressure levels, abbreviated dB. <br />Decibel levels of common noise sources <br />Many different properties affect the noise level of a specific source type. For example, three lawn mowers may <br />have three different noise levels because of differences in each specific piece of equipment. Noise level also <br />depends on the distance from the noise source and the attenuation of the surrounding environment. <br />Figure 3 provides a rough estimate of decibel levels of some common noise sources. <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />3 <br />Corn passion <br />Ex an ion <br />