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<br />RULES OF THUMB <br /> <br />>> WHEN A SOUND INCREASES BY 10 DECIBELS, THE SUBJECTIVE RESPONSE <br />IS A DOUBLING OF LOUDNESS <br />Example: 60 decibels to 70 decibels = twice as loud <br /> <br />PERCEIVED CHANGE IN LOUDNESS <br /> <br />+ 1 dB - UNNOTICEABLE <br />+ 3 dB - BARELY PERCEPTIBLE <br />+ 5 dB - QUITE NOTICEABLE <br />+10 dB - SOUNDS TWICE AS LOUD <br />+20 dB - SOUNDS FOUR TIMES AS LOUD <br /> <br />>> WHEN THE DISTANCE IS DOUBLED FROM A "POINT" SOURCE, THE SOUND <br />LEVEL DECREASES BY SIX DECIBELS <br /> <br />Example: <br />50 feet = 60 decibels <br />100 feet = 54 decibels <br />200 feet = 48 decibels <br /> <br />>> WHEN THE DISTANCE IS DOUBLED FROM A "LINE" SOURCE THE SOUND <br />LEVEL DECREASES BY THREE DECIBELS <br /> <br />Example: <br />50 feet = 70 decibels <br />100 feet = 67 decibels <br />200 feet = 64 decibels <br /> <br />>> A DOUBLING OF ENERGY YIELDS AN INCREASE OF THREE DECIBELS <br />Example: 85 decibels + 85 decibels = 88 decibels <br /> <br />>> THE NOISE SOURCE BEING MEASURED SHOULD BE AT LEAST 10 DECIBELS <br />ABOVE THE BACKGROUND NOISE LEVEL <br /> <br />>> KEEP AT LEAST AS FAR AWAY FROM ANY LARGE REFLECTING OBJECT <br />(EG., A WALL) AS FROM THE SOURCE BEING MEASURED <br /> <br />>> ALL MEASUREMENTS SHOULD BE MADE WITH THE MICROPHONE AT <br />LEAST THREE FEET ABOVE THE GROUND <br /> <br />17 <br />