Sound Absorption Basics You Need to Know
Sound absorption is the use of acoustic sound absorption panels to absorb sound waves that hit the panels and prevent them from reflecting back to the sound source. Thus, distortion such as echoes or reverberation are greatly reduced or eliminated. Even on a bare concrete wall, sound absorption can happen but at a very much lower absorption level. The bulk of the sound energy is reflected away, a fraction of it is absorbed and the rest transmitted through the wall. The ability of a material to absorb sound is determined by its sound absorption coefficient, α (alpha) which ranges from 0 (no sound being absorbed) to 1.00 (sound being totally absorbed). All materials, even the human body, have its sound absorption coefficient.
Here’s a table of absorption coefficient for some common materials.
Note: The absorption coefficient varies with the frequency of sound.
Common Sound Absorption Material
Soft and fibrous materials absorb sound better than hard (high density) materials. When sound waves strike on an acoustical material its intensity diminishes with distance. Which is why the thicker the acoustic material is, the better is the absorption.
The two most common sound absorption materials for acoustic treatment are high-quality acoustic foam, which is the standard material provided by most acoustic panel makers, and acoustic fiberglass. One common misconception is the use of conventional foam for sound absorption. The differences in the foams are beyond visual, though many would argue that the conventional foam is good enough to serve its purpose in acoustic treatment. It would however, depend on the level of acoustic appreciation. For example, if you try to arrest those low bass frequency, firmness of the foam plays an important part in absorbing the low frequency sound waves. Acoustical foams are specifically developed, or calibrated to achieve optimum absorption according to frequency ranges. A higher ILD (Indentation Load Force Deflection) foam, which is firmer is best use for low frequency absorption, such as bass traps. Whereas the lower ILD foam (softer) is good for absorbing high frequency sound waves, as seen in most ready made acoustic panels. This difference in ILD is what differentiate a bass trap from an acoustic panel, which both are for the purpose of sound absorption.
What makes the firmness of the acoustic foam is in the foam’s cellular structure and pores density. Usually express in PPI (Pores Per Inch), the acoustic foam will have a PPI of about 80 versus the conventional foam used for mattresses and sofas at around 65 PPI or even lower, and about 25 PPI in the case of pillows or cushions.
Another important point to note is the fire retardancy properties in acoustic foam. Most makers of acoustic foam or studio foam will have their products treated for fire safety, as most sound absorbers, especially for soundproofing, are incorporated as part of the wall, hence they are subjected to the Building and Construction regulation on fire safety. However, that may exclude those stand alone acosutic panels or wall-hung panels that are mobile.
Build Your Own Sound Absorption Panels
Before you begin to make your first sound absorbing panel, make sure you have a proper plan for your listening room or home theater to minimize trial and error, and cost. Do a little research online, gather as much information as you can on the type of acoustic treatment you need, this will save you valuable time and avoid common pitfalls.
Without the luxury of all the resources like a professional studio or a concert hall, treating your room with DIY acoustic sound absorption panels on the cheap is the next best thing you can do, and they cost only a fraction of those ready made panels. The only trade-off is perhaps on the aesthetics. The ready made panels are more aesthetically pleasing, with different designs, shapes and colors to suit the interior or architecture appearance. But with a little creativity, you can match up with some designs on the wrap.
Most folks who DIY their own sound absorption panel would go for fiberglass instead of foam. Such as the Roxul Rockboard 60 which is 2 feet by 4 feet and 2 inches thick and cost less than $10 a piece. You can easily buy them in Home Depot or online at Amazon.
Video – How to Build DIY Acoustic Absorption Panels
Building your own sound absorbing panels and customizing the acoustic treatment to suit your room can be a lot of fun. With each changes you will experience a new acoustic perspective. Keep experimenting with different materials, size, thickness and placement of the acoustic sound absorption panels. And don’t forget to mix in some acoustic diffusers, which can add texture to the acoustics in a subtle way. That’s Acoustic Sound Absorption hope you have got the basic now.