A phenomenon where a sound is reflected back and forth between two parallel surfaces, such as two side walls in a room. Technically they are created by “room modes” or “eigentones,” which are modes of vibration of air in the room. The sound waves interfere with one another to produce a series of places where the sound pressure level (SPL) at some frequencies is high, and another series of places where they are low. The places are sometimes called peaks and nodes. A standing wave exists in a room where a frequency is such that the distance between any two surfaces is equal to one half of its wavelength. For a given distance there will be many frequencies that will generate standing waves, each a multiple of the fundamental frequency whose wavelength is related to the dimension in question. Standing waves are always detrimental to the acoustics of a room, but can be avoided by careful room design, or minimized by absorbing the frequencies where they build up, which is usually along walls or in corners.