Discussion

When waves with the same frequency, but traveling in opposite directions, pass each other they can interfere to create standing waves, so-called because the resulting wave does not appear to move. Ideally, a standing wave oscillates in a stationary envelope, with nodes, points at which wave displacement is zero, and antinodes, points where wave displacement is maximal.

As the speaker is moved toward and away from the reflecting surface, it passes through several positions at which standing waves can be established. At these positions, sound levels are greatest, because energy from the speakers is most efficiently absorbed by the sound wave, resulting in increased wave amplitudes and corresponding loudness. This demonstration is a three-dimensional version of interference of waves from two point sources.