Sound generated within a room will eventually strike a boundary such as a wall, ceiling or other surface in the path of the sound wave, and be reﬂected back into the room. Diffusion has the goal of controlling these reﬂections and redirecting sound.
While both absorption and diffusion have the effect of reducing sound intensity, diffusion preserves the sound and spreads it over a larger area, rather than having it disappear. The listener’s perception of an expanded space is created by the combination of its spatial and temporal components.
A perfectly reﬂective, smooth surface will absorb no wave energy, but return all of that energy back to its originating space. One example of a very reﬂective acoustic surface is a plaster wall.
A totally absorptive material will not reﬂect any energy back into the room. Materials that absorb sound are generally porous, ﬂuffy and lightweight such as ﬁberglass insulation.
Diffusion simultaneously increases the distribution of sound, and modiﬁes its direction (red line) without removing energy from within the space (spatial response). Also, the diffused sound energy is distributed in time (temporal response). Bookcases with random-sized books, and quarried stone (or shingled) walls have some sound diffusing properties.
Over the years many shapes of sound diffusors and scattering devices have evolved. In the late twentieth century it was discovered that a panel with series of wells, would cause sound hitting its surface to be spread. This phenomenon is analogous to light’s behavior.
A butterﬂy’s wings may appear to be many brilliant colors when ﬂoating in the sunlight, only to be revealed as dull brown when observed up close. The color perceived is a result of the light being refracted as it bounces off of the small surface variations of the wing and breaking down into its component parts as if passing through a prism.
Smoke and Mirrors
The science of acoustical diffusion involves a lot of math, but it isn’t necessary to be a mathematician to understand and beneﬁt from the use of diffusors. Taking a cue from the thought that some science seems to be smoke and mirrors, we used a fog machine and light reﬂection to illustrate diffusion.
The figure above shows a pair of parallel laser beams reﬂected from a ﬂat mirror surface. Nothing changes other than their direction, opposite but equal.
The figure above shows an acoustical wall panel. Like sound entering the absorber, it vanishes.
In the figure above, a pair of laser beams is reﬂected off of D-series sound diffusor, with a custom mirrored ﬁnish. Four distinct beams leave where only two entered, each at lower intensity than the original beam and surrounded by additional blooms of light scattered and diffused by the mirror-surfaced elements.
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