phase relationships must be considered. At zero phase difference, the resultant value of two identical pure tone sources is 6 dB greater than the value of either single level. Between 0 and 90", the resultant is somewhere between 3 and 6 dB greater than either level. At a phase difference of 90°, the resultant is 3 dB greater than either level, Between 90 and 120 , the resultant is between 0 and 3 dB greater than either level, and with a phase difference of 120 the resultant is equal to the individual levels. At 180 complete can- cellation of sound occurs. The fact that one sound can actually cancel out another is a very important principle in acoustics, as it forms the basis for active noise control. Noise can cellation technology, also referred to as active noise reduction, is increasingly being em- ployed as advances in equipment and instrumentation are developed.