Cyclic sound paths create standing waves when path lengths match half wavelengths

Evidence

When a room is excited by an impulse, the sound energy is reflected from its surfaces. At each reflection, some of the sound is absorbed, and therefore the sound energy decays exponentially. Ideally the sound should be reflected from each surface with equal probability, forming a diffuse field. This results in a single exponential decay with a time constant proportional to the average absorption in the room. However, in practice, not all the energy is reflected in a random fashion. Instead, some energy is reflected in cyclic paths, as shown in Figure 6.30. If the length of the path is a precise number of half wavelengths, they will form standing waves in the room. These standing waves (resonant modes) have pressure and velocity distributions that are spatially static and so behave differently from the rest of the sound in the room in the following ways: ■ They do not visit each surface with equal probability. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1948

Categorization

• DEFINE: Explains the physical mechanism by which resonant room modes arise from cyclic reflections — a core concept for the reader’s perceptual framework.