By Jim DeGrandis
Acoustic issues manifest themselves in many ways and from many different causes. Covering all the possible iterations of sound problems, their sources, and solutions would be beyond the scope of this article. Structural vibration mitigation varies by construction type and is far more complex than controlling airborne sound propagating within a space. Therefore, it becomes much more manageable by narrowing the focus down to those “in-room” issues which one can mitigate with diffusion and absorption.
The materials: 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.
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.
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?