Acoustical Diffusion
How diffusers play a critical role in achieving optimum sound.
by Nick Colleran

The two basic elements for shaping and controlling sound in the church sanctuary are absorption and diffusion. Absorption has been well defined over the last century. Diffusion has become more understood and measurable only in the last two decades.

Absorption is all about limiting reflection and controlling reverberation. Materials that absorb sound are generally porous, fluffy, and lightweight. Examples include fabric-covered, high-density fiberglass wall panels and vinyl-encapsulated, lower-density hanging baffles.

Diffusion has the goal of controlling reflection 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. Diffusion provides clarity for spoken words while preserving and redirecting reflected sounds that enhance traditional music. At the same time, diffusion creates the acoustical environment of an apparently larger space by making the room boundaries less obvious.

Diffusers
Professor Dr. Manfred Robert Schroeder, a German teacher, physicist, and mathematician discovered that a panel with a series of wells spaced in a sequence based upon prime numbers, and having a depth equal to one quarter of the sound wavelength to be affected, would cause sound hitting its surface to be spread rather than reflected back in a straight line.

The first major commercial adaptation of Schroeder's diffuser, and possibly the best known, is the quadratic residue diffuser. It is a single-dimension device comprised of a repeating pattern of wells based upon a prime number sequence.

Other diffusers followed, based upon this formula as well as other mathematical sequences. Some of them have the well sequence running in two directions providing diffusion in both horizontal and vertical planes.

Still others took the approach of projecting forward. These may be seen as quadratics without the well dividers, or as what appear to be stacks of blocks projecting outward into the room space rather than as wells recessed into the wall cavities.

Polycylinders and Bass
Over the years many shapes of sound diffusers and scattering devices have evolved. Earlier designs included pyramids, splays, and poly-cylinders, commonly called "barrel diffusers." The polycylindrical barrel diffuser ("poly") serves double-duty by being both a diffuser and a low-frequency bass trap. Unlike many corner-mounted acoustical devices that are incorrectly called bass traps, the poly has significant absorption only in the lower frequency ranges. Above 500 Hz the device ceases to have any meaningful absorption but provides useful diffusion characteristics.

Trapping bass is a counter-intuitive solution to improving low frequency sound in the sanctuary. The name "bass trap" implies that bass is taken away, but the opposite is true. By trapping the bass, the low frequencies are not allowed to reflect back and cancel themselves by combining "out-of-phase." If bass is heard "building up" in the corners of the sanctuary, it may be more correctly said that bass is disappearing in the middle of the room. Either way this is expressed, the effect is the same: where you sit will determine what you hear and how you understand.

Figure 1: Polycylindrical absorption rises at lower frequencies (right to left) as panel absorption drops.

Because the poly absorbs bass tones, its use is often specified in conjunction with standard sound-absorbing wall panels to extend their effective range. A typical array (or bay) will consist of a four-foot-wide column of polys sandwiched between two four-foot-wide columns of two-inch panels. The height may be twenty feet, accomplished by stacking four-by-eight-foot units above and below a row of four-by-four-foot units, providing both effective performance and a pleasing visual presentation.

The graph in Figure 1 shows how the ability of a one-inch fiberglass panel to absorb sound drops rapidly as the frequency gets lower, while a poly has the ability to absorb these low frequencies. Increasing the thickness of the fiberglass panel extends its absorption range lower, while doubling the size of the poly will shift its bell-curve absorption "hump" a full octave lower.

Another popular diffuser shape is the pyramid, in particular the offset apex version, a design that dates back to the ancient Greeks. It scatters sound in four different directions and, when used in multiples, may be rotated to give four times the dispersion of a single unit. It should be noted that some molded versions have their offset in one direction only and provide only three different angles of reflection and, consequently less useful scattering.

Many other shapes and sizes are available from a variety of manufacturers. All irregular surfaces can provide useful scattering, and their cavities, if any, will serve to absorb sound as well. While all acoustic diffuser manufacturers are concerned about brand differentiation, underlying principles remain the same across all brands.

Smoke and Mirrors
Taking a cue from the thought that some science seems to be smoke and mirrors, we use a fog machine and light reflection to illustrate diffusion. If a picture is worth a thousand words, this one should complete the story.

Figure 2 shows a pair of parallel laser beams reflected from a flat mirror surface. As expected, the beams bounce off of the mirror at the same angle they enter and continue in parallel. Nothing changes other than their direction.

Figure 2: Laser beams on a flat mirror surface.

In Figure 3, a pair of parallel laser beams is reflected off a commercially manufactured diffuser, a "binary array" in a custom, mirrored finish. Although the laser light beams approach the unit in parallel, they emerge quite different. Four distinct beams leave where only two entered, each at a lower intensity than the original beam and surrounded by additional blooms of light scattered and diffused by the mirror-surfaced elements. Acoustical diffusers work in the same way, taking incident sound waves and sending them in multiple directions.

Figure 3: Laser beams on diffuser.

The science of acoustical diffusion involves a lot of math, but it isn't necessary to be a mathematician to benefit from the use of diffusers. Much like learning enough law to know when you need a lawyer, the best and usually least expensive course to solving acoustical problems is to learn enough to understand what your consultant and sound contractor are saying to you.

Nick Colleran is a member of the Acoustical Society of America, past President of the Society of Professional Audio Recording Studios (SPARS), former president of the Virginia Production Services Association, VPSA and is currently active in acoustical design for houses of worship, new acoustical products, and performance venues.