Sound waves move through the air from a source in much the same way that water waves move outward from a stone tossed into a pool. Pressure of an approaching wave of water can be felt and the lack of push after the wave has passed. The same thing happens to the sound wave: the wave area before the crest is compressed; the area after the crest is rarefield, or less dense.
Now if a second stone is dropped in the water at same spot or at another spot, another series of waves will be started. If these second waves are in phase with the first series, that is, if the compressions of the two sets of waves coincide, abd the two sets of rarefractions also, the power of the wave is increased. If these second waves are of the same power and are in opposite phase, that is, if the rarefractionsand condensations occur together, they neutralize, or cancel out, one another, and the wave will gradually die out.
In sound, when the rarefractions, cancel out or neutralize the compressions, silence occur when the two sets of waves are not exactly in phase or in opposite phase.
Radio men describes another kind of sound interference, a sound of very high frequency, such as 15,000 cycles, can be altered by a sound of lower frequency, such as 1000 cycles. Still another phenomenon in sound interference is known as a zone of silence. In this situation a sound is heard at its source and vanishes in the distance, only to be heard again farther away. This strange occurence baffled investigators until one meteorologist discovered that layers of air of different temperatures can deflect sound waves.As sound waves are deflected diagonally upward from warm to cooler air, sound disappears.