By: Matt Restrepo, MCH Acoustical Consultant
Today is International Noise Awareness Day, so we bring you this brief blog describing the three types of sound absorption. Sound absorption is a useful tool to minimize noise build-up, but is rarely the sole solution to controlling noise.
There are several separate concerns that affect acoustical environments, including sound isolation, HVAC noise and vibration, and finish treatments. Some might think the “age-old trick” of placing egg cartons on wall will do the trick, but that is simply not the case. Here, we will focus on sound absorptive finish treatments.
When sound reaches a surface like a wall, some of its energy is reflected, while some is absorbed. The amount of energy that is reflected or absorbed depends on the properties of the finish material.
There are three different types of absorption, all of which are the result of converting sound energy into heat:
Frictional Absorption
Frictional absorption occurs when sound energy causes air molecules oscillate within interconnected air pockets between individual fibers within fibrous materials, which are soft or “fuzzy”. These are what most people think of for sound absorptive treatments. This type of absorption is most effective in the middle and high frequencies such as for voices, but becomes more absorptive in the lower frequencies as the material thickness increases. Common frictional absorbers are acoustic ceiling tiles, compressed glass fiber wall panels with fabric facing, polyester panels, among others. A common descriptor for the acoustical absorption of these panels is the Noise Reduction Coefficient (NRC), which is an arithmetic average of the absorption coefficients at 250Hz through 2,000Hz. A panel with an NRC of 0.80 would absorb 80% of sound energy.
Flexural Absorption
Flexural absorption occurs when a panel or membrane flexes in response to incident sound. This type of absorption works primarily in low frequencies (i.e., typically 125Hz and below), but is very ineffective at middle and high frequencies. Because of this, the NRC metric discussed for flexural absorption is not appropriate for describing sound absorption of flexural absorbers. Many common construction materials such as glass and drywall act as flexural absorbers. Most rooms with lightweight (drywall) construction and/or windows, already offer some low frequency flexural absorption.
Resonant Absorption
If you have ever created a tone by blowing air over a bottle opening, you have observed resonance. Resonant absorption occurs when a slug of air resonates at a certain frequency within a neck (such as a bottleneck), and can change depending on the volume of air within a cavity (such as the body of a bottle). Devices that function on this principle are known as Helmholtz resonators, named after the German physicist Hermann von Helmholtz. Resonant absorbers are not commonly used in everyday construction; however, they are commonly employed in recording studios where specific problematic frequencies may need to be addressed. Because of its narrow frequency range, the NRC metric discussed for flexural absorption is not appropriate for describing sound absorption of resonant absorbers.
Takeaway
We have discussed three different types of absorption, each affecting different frequency ranges. Frictional absorption is broadband and absorbs middle and high frequency sound energy, whereas flexural absorbers absorb low frequency sound, and resonant absorbers absorb specific frequencies. Addressing sound absorption in a space is an important step to optimizing a space’s acoustics, but it is not a substitute for quiet mechanical systems noise and vibration, and appropriate sound isolation. It is important to use these treatments appropriately to maximize their effectiveness. Consult your project acoustician on which treatments to use, and where to place them.
