addition Sound from two loudspeakers can combine coherently or incoherently. This is generally different at different spots in the room, and the following refers to one specific spot. Coherent addition means all of the pressure peaks and lows of the sound wave from one speaker arrive at exactly the same time as the peaks and lows of the other. The pressures then add. Twice the pressure means 4 times the acoustic power, so the sound level increases 6 dB. Incoherent addition means the peaks and lows from one arrive at random times compared to the other. In this case the power adds and the sound level increases 3 dB. For frequencies below 100 Hz, where the sound tends to be monaural, and the distance between pressure peaks and lows is more than 5 feet, addition tends to be coherent. At higher frequencies addition tends to be incoherent.