Absorption and diffusion: Creating optimal acoustic spaces

Photos courtesy Acoustics First

By Jim DeGrandis

Acoustic issues as to the richness and functionality of sound in a space manifest themselves in many ways, and from many different causes. Covering all the possible iterations of sound problems, their sources, and their solutions would be beyond the scope of this article. However, it becomes much more manageable by narrowing the focus down to those issues which one can mitigate with diffusion and absorption.

Materials to have in the toolkit

When approaching any task, it is necessary to understand the available tools. In this case, the tools at hand are materials for absorbing and diffusing sound. Knowing the strengths and limitations of these materials is paramount to effectively mitigating acoustic issues in a space and tuning that space for the best sound possible.

The broad overview of sound absorption and diffusion is this—absorbers reduce acoustic energy, and diffusers reflect energy in a way that contributes to a diffuse sound field. However, the devil is in the details, and not all these materials or constructions are created equal—and for good reason. Sound comprises a wide range of frequencies, and some of those frequencies have different properties and characteristics to their propagation. High frequencies are relatively easy to absorb and scatter due to their short wavelengths, while low frequencies are not so easy to absorb. Human hearing also plays a role in this, as it is more sensitive at some frequencies than at others.

Two types of absorption

Given, absorbers reduce acoustic energy. This is a vast oversimplification of how these materials work. Only by examining how they reduce energy does one understand why certain absorbers are suitable for certain tasks. There are two general types of absorbers by function: broadband absorbers and tuned absorbers. The practical difference in these two is “broadband absorbers” affect a wide range of frequencies, while ‘tuned absorbers’ focus on one particular range more than others.

Broadband absorbers

The general classification of ‘broadband’ is a bit of a misnomer. Broadband absorbers are usually constructed from materials classified as ‘frictional absorbers’—which is related to how they reduce acoustic energy within a space. This is where high school physics classes become useful. The First Law of Thermodynamics states energy cannot be created or destroyed—merely converted from one form to another. What does this mean in acoustics?

Electrical current is converted into electromagnetism which moves a loudspeaker, which in turn, creates varying pressures through the air as it moves. This energy is propagated from air particle to air particle in all directions—lessening in intensity as it travels away from the source, because it is spreading that energy out in accordance with the inverse square law, which states with every doubling of distance away from the sound source, the sound will be four times less intense. That is, until it hits something.  Now, in the case of a frictional absorber, that “something” is generally a lightweight, porous material, which converts that acoustic energy into heat energy. As acoustic energy hits a porous surface, it tries to move the material, and the resistance to moving is what causes acoustic energy to be converted into heat.

The materials one generally finds in frictional absorbers are fiberglass tile or boards of different densities, felts and other fibrous materials, or foam. These materials all have different properties in density by weight, fiber length, thickness, binder materials, rigidity, and pore size. These variables affect the acoustic resistance of absorbers, and hence their effectiveness at different frequencies.

There is no frictional, broadband absorber which is “ruler-flat” in its frequency response throughout the entirety of the human hearing range. However, many of them are nominally effective through a wide range of frequencies—and mostly the higher frequencies. High frequency sounds are more effectively reduced by frictional absorbers than low frequency energy—this goes back to the laws of energy conservation.  High frequencies oscillate over a shorter wavelength than low frequencies, and when travelling at the same speed as low frequencies, attempt to oscillate the fibers in frictional absorbers faster than low frequencies. Low frequencies retain more of that energy over the long travel of the wavelength than high frequencies—and penetrate deeper into those frictional absorbers due to this fact.

This then leads the conversation to material thickness. With a frictional broadband absorber, the general rule is: the thicker it is, the better the low frequency absorption. As stated above, high frequencies absorb quickly and easily, but the low frequencies retain energy and penetrate deeper into the frictional material; therefore, the solution to low frequencies is “just make it thicker.” When looking up absorption test results for a frictional absorber material, one will find many of them are tested repeatedly at different thicknesses. This is where one truly sees why they are called ‘broadband’ absorbers—go thick enough and they can absorb almost everything.

Why not just exclusively use broadband absorbers? If they are “thick enough,” can they not absorb almost everything? Firstly, “thick enough” is a broad term, pardon the pun. When a situation calls for thicknesses up to 914 mm (36 in.) deep or more to absorb the lower frequencies, broadband absorbers start to become impractical. Second, it is very rare for a space design to entail absorption of “everything.” Anechoic chambers which block outside sound and reflections do not make superb listening rooms—they are muffled and claustrophobic sounding. High frequencies will attenuate naturally, just by travelling through the air… and because they are also easily absorbed. An apparent low-frequency buildup remains due to the imbalance in the absorption rate of the various frequencies, resulting in a space with bass heavy environment.

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