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-� .� <br />The reference pressure used in acoustics is twenty (20) micro Pascals dBA — Sound pressure <br />level expressed in decibels, filtered or weighted at the various frequencies is to approximate the <br />response of the human ear. The sound pressure level in decibels is equal to ten (10) times the <br />fogarithm (to the base 10) of the ratio between the pressure squared divided by the reference <br />pressure squared. Where L is the level and A an AB are quantities related to power. <br />L = 10 log (A-B ) dB <br />Therefore, any time that the word "level" is referred to in acoustics, decibel notation is implied. <br />The example chart below is using a starting point for the human ear at OdB. Sound power level <br />(abbreviated as PWL and symbolized LW) would be defined as: <br />LW or PWL = 10 Log (W-Wo) db <br />To bring into perspective different dB levels of common sounds, please refer to the chart <br />below. This chart demonstrates several comparison sources of sound that most persons can <br />relate to. They area as follows: <br />SOURCE dB ratin� <br />170 <br />Turbojet Engine <br />1G0 <br />150 <br />140 <br />75 Piece Orchestra <br />130 <br />120 <br />Large Chipping Hammer <br />I10 <br />Auto on Highway 100 <br />90 <br />80 <br />70 <br />Voice, Conversational <br />60 <br />50 <br />40 <br />30 <br />Voice, Very Soft Whisper <br />20 <br />10 <br />0 <br />The follow�ing chart gives an example of the perception of the human ear to detect change in the <br />dB level: <br />