I’ll write about more general studio acoustics and my approach to studio design in later posts. For now I’ll dive a little into diaphragmatic absorbers. I’ll preface this by saying that there are a number of things I can’t share as it’s either proprietary technology I’ve developed on my own or trade secrets that I learned from one of my mentors, Ken Goerres of Haikoustics.
There are a couple of general categories of bass trapping which people refer to and use. The most basic is simple porous absorbers which normally use insulation materials such as OC 703 or Roxul/Rockwood. Porous absorbers are very inefficient at providing low end absorption unless very thick panels are used (2’?). Other approaches are needed for effective bass absorption.
The 2 categories which normally come up are membrane absorbers and Helmholtz resonators. Membrane absorbers use a sealed cabinet which essentially act as a drum at certain frequencies and subsequently absorb energy out of the room. Helmholtz resonators use chambers tuned to certain frequencies. I won’t go into more depth on those as you can find plenty of info online and in books.
The problem with these 2 types of absorbers are that they have a narrow bandwidth and are tricky to construct (equations don’t translate to real-world built panels so trial-and-error is often required). The approach which I use in my studio and other studios I’ve designed is diaphragmatic absorbers. Terminology is a little blurred in the industry so you may see others calling membrane absorbers diaphragmatic or use other names for what I refer to as diaphragmatic absorbers.
A diaphragmatic absorber is nothing more than a sheet of some material which is damped by another material. Unlike a membrane absorber, there is no sealed cabinet. This allows the sheet to vibrate in a wider range of frequencies.
In my testing I’ve found the dampening to be critical. A vibrating sheet on its own has the potential to absorb large amounts of energy and cause impressive changes in the measured frequency response of a room. The issue is that as the sheet vibrates with a long decay time, it re-radiates sound into the room acting similarly to a speaker. This causes issues in the decay time the room. By dampening the sheet, you get less absorption but also correct the timing response of the panel.
It’s important to note that diaphragmatic absorbers work based on the velocity component of sound rather than pressure as membrane absorbers. This has certain implications on where they’re most effective in a room. Flexibility in the thickness allows thicker panels to be used where more absorption is required (increasing the porous absorption component of the panel). Multiple diaphragmatic layers can also be used in thicker panels.
What kinds of materials work as diaphragmatic absorbers? Sheets of wood, plexiglass, metal, rubber, paper, and certain kinds of foam. I’ve done extensive research and development testing out a variety of materials to find what works best. The choice of material determines the frequency range of absorption. In the studios I design, I use different materials to target 20Hz - 80Hz, 70Hz - 150Hz, and 100Hz - 200Hz.
The use of diaphragmatic absorbers is in no way my own revolutionary idea. Companies such as Primacoustics and GIK Acoustics offer diaphragmatic absorbers such as GIK’s range limiter options.
One type of panel which I have no firsthand experience with but am keen to experiment with are VPR absorbers. These consist of an incredibly large and heavy metal plate that is glued onto IsoBond. They seem to offer very low frequency absorption at thicknesses of only 4”. The issues are the cost of the metal plates and difficulty in mounting them onto a wall due to the weight. RPG offers this technology in their Modex panels which cost a meager $800 per panel.