The Decibel (dB). Several acoustic quantities are denoted by the term “decibel” and this is indicated by the use of the word “level” in the text; most commonly used are “sound-pressure level” and “sound-power level”. The decibel is 10 times the logarithm of the quantity measured and, as a result, decibel values cannot be added and subtracted arithmetically; for example adding two levels of 90dB gives a combined level of 93dB. The rules for adding decibels are given in text books and most standard test procedures.