Equal-amplitude sounds in phase double in amplitude while out of phase they cancel

Evidence

Because a sound wave has periodicity, the pressure from the different sources may have a different sign and amplitude depending on their relative phase. For example, if two equal-amplitude sounds arrive in phase, then their pressures add and the result is a pressure amplitude at that point of twice the single source. However, if they are out of phase, the result will be a pressure amplitude at that point of zero, as the pressures of the two waves cancel. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 230

Categorization

• DEFINE: Clarifies the core mechanism of phase addition and cancellation in wave interference.