The numerical difference in decibels of the average sound pressure in two areas or rooms. A complete measurement of the reduction in noise consists of the simple level difference between the two rooms due to the common partition and also takes into account the background level in the receiving room and the amount of sound absorption in the receiving room. The noise reduction is a function of frequency typically increasing with increasing frequency and is measured in third octave bands from at least 100 Hz to 3150 Hz.