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pressure is approximately one million times greater than the weakest <br /> sound. Such a pressure would produce a sound pressure level of about 120 <br /> dB relative to the reference pressure . This level would be approximately <br /> what you would experience standing next to the amplifiers oi f a hard rock <br /> band . A soft whisper would be rated at about 40 dB. <br /> In everyday use , sound pressure levels are often referred to as <br /> "noise levels." And the reference to 20 micropascals can bile , and often <br /> is , omitted when it is clearly understood that the dB quantity is sound <br /> pressure level . You might describe the sound of an engine repair shop <br /> this way : "The noise level in the shop is about 90 dB.1" <br /> Addition of decibels <br /> Question : The noise from an airplane engine , as you tear it on the <br /> ground outside the plane , is measured at 100 dB. The nolise from the <br /> engine in a nearby plane is measured at 95 dB. How much noise , in <br /> decibels , are the two planes generating? <br /> Answer : Did you say 195 dB? Wrong . Decibels cannot be added <br /> algebraically. They are logarithmic values and they mutt be added <br /> logarithmically. <br /> A simple table for adding decibels is shown below <br /> TABLE 2. METHOD 1: ADDING DECIBELS. <br /> When two decibel Add the following amount <br /> values differ by: to the higher value: <br /> 0 or 1 dB 3 dB <br /> 2 or 3 dB 2 dB <br /> 4 to 8 dB 1 dB <br /> 9 dB or more 0 dB <br /> 7 <br />