out-of-phase sound waves are used for noise reduction
out-of-phase = unsynchronized
Sound travels in waves. Similar sound waves whose peaks (maxima) do not coincide are said to be out-of-phase. If the peaks of one coincide with troughs (minima) of the other, the two waves are said to be 180° out-of- phase. This is the basis of "noice-cancellation" devices.
This site shows how two sound waves that are 180° out-of-phase cancel each other, and this results in noise reduction.
More about the physics of sound waves is at this site
A sound wave has a length. The music note A 440 is somewhere around 20 cm long. If the A 440 is too loud compared to the volume of other mid-range sound, it is resonating, and you should move the speaker one or two cm forward or back.