phasing

"signal is delayed and then mixed with the original signal. In the middle of the audio spectrum delays of less than about 100 μs will produce no noticeable effect, whilst delays greater than about 30 ms will produce a distinct echo. A delay between these limits will give the required `phasing` effect. Of course, a fixed delay time will not have the same effect on signals of all frequencies. If, for example, a 1 kHz signal is delayed by exactly 1 ms and mixed at equal amplitude with the original, then the result will be a signal with twice the amplitude of the original, since the delayed signal has in fact been phas(rshifted by 360°."