Sound waves can both add together and subtract from one another. When soundwaves add together they are described as being ‘in phase’ with one another. When soundwaves subtract from one another they are described as being ‘out of phase’. This cancelling out of the soundwave can result in some very strange sound effects and is sometimes deliberately applied to sounds. Phase cancellation (the subtraction of one sound from another) can happen when the same sound is played from multiple loudspeakers and the soundwaves meet, or when sounds reflect off of a surface and the reflection meets the original wavefront.
