Its All About "Phase"

Phase is the time difference between to similar waveforms. When two signals are close in frequency and amplitude (level) but out of time with each other, there is a phase difference. One cycle of sound is considered to have 360 degrees. When a second signal starts a half-cycle later, it is called 180 degrees "out-of-phase" with the first signal. When two signals are 180 degrees out-of-phase, the peaks of one signal are in time with the dips of the second signal and the result is a cancellation of the signals’ energy. Its sort of like someone pushing on a door as another person pulls on the same door - it doesn’t move.

When two signals have their valleys start at the same time and their peaks start at the same time, the two signals are said to be "in-phase" and the energy of the two signals will double when the signals are combined. This is sort of like two people both pushing on the same door - it opens faster and/or further.

The Comb Filter

When you have a 50/50 mix of a direct and delayed signal, the delay time puts the delayed signal out of phase with the un-delayed signal at some frequencies. These frequencies will disappear from the mix and other frequencies will be noticeably reduced. At short delay times (less than 20 ms) the noticeable reduction of level occurs in frequencies easily heard. At medium and long delay times, the noticeable reductions happen at frequencies too low to hear (below 20 Hz or below the lowest frequency of most instruments).

The frequency response is called a Comb Filter. The result to the ears is a very thin and unnatural, hollow, sound. The sound can be anywhere from mildly annoying to very objectionable, depending on which frequencies fall in the null. Since the frequencies canceled are harmonically related, it can be a very objectionable alteration of the sound.