Comb filter

When a sound is mixed with a delayed version of itself, each sinusoidal component of the delayed sound has a unique phase offset compared to the original, so each frequency is accentuated or attenuated differently. For example, if a 1000 Hz sinusoid is delayed by 1/1000 of a second (1 millisecond), the original and the delayed version will still be perfectly in phase, so that frequency will be increased in amplitude when the two versions are added together.