There is one basic type of low frequency absorber in the marketplace today. This absorber is termed a diaphragmatic absorber. A diaphragmatic absorber is a box or panel that has a face that is designed to vibrate or move with the sound pressure that is exerted upon it. Diaphragmatic absorbers have a membrane or some material type that is stretched across the face of a cabinet which is built to a specific depth.
The cabinet is more rigid and has a much higher mass than the surface material. This design encourages the face material to move more readily and easily. Diaphragmatic absorbers can have their face material made out of gypsum board, wood paneling, MDF, and even lower density materials such as vinyl and sheet metal.
In this video our Chief Product Designer and Acoustics Engineer, Dennis Foley, explains the concept and process of diaphragmatic absorption.
How Does It Work
A diaphragmatic absorber works when sound pressure energy, especially lower frequency energy, strikes the front wall of the absorber. The front wall is set into motion or goes diaphragmatic. This process is similar to a speaker cone moving when electrical current is applied. The face of our diaphragmatic absorber will move but not as fast and as with such force as a speaker cone. This vibrating front wall slows our low frequency energy wave down by changing some of the air born energy to small amounts of mechanical energy.
The wave then passes through the front wall with reduced energy. The low frequency wave then enters the cabinet’s internal space and based on cabinet depth will be absorbed above the cabinet’s determined resonant frequency. The energy inside the cabinet then interacts with the cabinet’s fill material and some of it is converted into heat. The wave then hits the rear of the diaphragmatic absorber and is bounced back through the cabinet fill material and the process just described continues.
Finally, the reduced in energy wave is sent back into the room. To increase some cabinet’s efficiency, some cabinets or panels even have a vacuum inside of them, but this is usually portrayed in molded plastic ceiling panels which are lightweight and do not absorb large amounts of low frequency energy.
New And Improved Diaphragmatic Absorption
What Are The Parts
Our diaphragmatic absorbers have four main parts that must be addressed. We must know the density or mass of our cabinet structure, the density and free floating installation of our front panel, the cabinet depth, and the internal cabinet fill material. Our cabinet for our diaphragmatic absorber must be designed and made to be rigid. It must conform to a certain ratio of rigidity vs. flexibility with the front wall assembly. Diaphragmatic absorbers that absorb low frequency energy are heavy by design and it is this mass that contributes to the cabinet’s performance.
Front Face Panel
The cabinet rigidity encourages movement of the front face section of the cabinet. This diaphragmatic movement of the front face slows our sound pressure waves down before they enter the internal cabinet fill. In order to insure that the front panel moves with maximum flexibility, the panel face must be installed within the cabinet in a manner that allows for this movement. To insure this movement, we install this front face panel using a flexible acoustical adhesive that does not completely dry like a glue would. The sealant stays flexible and thus allows the front face of our diaphragmatic absorber to move in response to air pressure and still provide a good air seal, so our cabinet maintains its air tight seal, but allows for maximum front panel movement.
Cabinet depth is the most important factor that will determine the resonant frequency of our diaphragmatic absorber. Any frequency above the resonant frequency of the cabinet will be absorbed. Frequencies below the cabinet’s resonant frequency will not be absorbed. Cabinet depth must be chosen carefully in order to insure that the design parameters of the low frequency absorber is met. If we are designing a cabinet for 40 Hz. and above, we need to select the proper cabinet depth and match it with cabinet density to insure our resonant frequency goal of 40 Hz. is met. Cabinet depth and cabinet mass or density must be matched to achieve the necessary design parameters of the absorber.
Internal Cabinet Fill
The inside fill material acts as another absorber within our absorber. Internal cabinet fill must be chosen carefully to insure maximum absorption when the slowed downed energy by the front wall fills the cabinet center. Materials used are common building insulation materials. As sound energy enters our diaphragmatic absorber, it meets with the internal cabinet fill. As sound energy interacts with the fibers of the internal insulation materials an energy change occurs. The electromechanical energy of our sound is converted to heat as it interacts with the fibers of the insulation.
Diaphragmatic absorbers are powerful low frequency absorbers that can obtain the necessary rates and levels of low frequency absorption that today’s small rooms demand. With the proper design which includes a “moving” front wall, rigid cabinet, proper cabinet depth, and the correct cabinet fill material, one can achieve a large amount of absorption in a small physical space.
If you would like to build your own we sell a DIY Diaphragmatic Absorber here. Alternatively you can see our full ACDA 12 activated carbon diaphragmatic absorber production units, the most powerful low frequency absorbers commercially available today.
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