Mike Sorensen (@mikesorensen06) is a master cabinet maker, structural engineer and the author of the AcousticFields.com sound diffusion audio blog. I think you’ll find his research on activated carbon for acoustic absorption very interesting.
If you’re going to make a great recording you have to find a solution to the room acoustics. Like the black sheep of any family, room acoustics are somewhat left un-talked about given their unsexy nature. Yet they are key to helping you produce the best sound from your recordings whether in the live or listening room.
Some people think that throwing up a bit of foam here, dampening the sound there and generally shutting the door and turning off the extractor fan will do it but alas no. There is a big science that goes into it and I want to share some of my years of experience with you today so you can start to consider some of the treatments and how they work in conjunction with your studio space.
Diaphragmatic absorbers are powerful, low frequency, absorbing technologies. One must build a solid, sealed box that has a front wall that can “move” in reaction to sound pressure waves. This front wall movement slows the wave down, so that it can enter the inside of our sealed cabinet. Yes, the cabinet is sealed without any air holes. Low frequency waves that are 40 and 50 feet long do not care about some 1/4″ air holes in any type of absorber. With low frequencies we are dealing with waves of energy not rays.
The cabinet that supports the front wall must be as inert as possible and not move. It must be rigid, so that only the front wall is moving in response to the sound pressure exerted upon it and not the sides or cabinet rear wall. Rear wall construction must be thicker than the side walls in order to obtain the proper rigidity ratios between the sides, the rear wall, and front wall. Since the side walls are shorter in length, they possess more rigidity and thus we need to add mass to the rear wall to keep our rigidity ratios between the surfaces correct and balanced. Viseo-elastic damping compounds between the cabinet’s layers of materials are a good way to minimize cabinet sides and rear wall vibrations and maximize cabinet rigidity.
To achieve the proper resonate frequency of our diaphragmatic absorber, we must calculate cabinet material density and the depth of the cabinet itself. We are designing a cabinet that has a certain density in the materials we are using and has a certain depth inside the cabinet. If we do our calculations correctly, we build a cabinet that has a certain resonant frequency. This resonant frequency number is our sound absorption baseline. Any frequency above our diaphragmatic absorbers’ resonant frequency will be absorbed and any frequency or wavelength striking our cabinet that is below our resonant frequency will not be absorbed.
Inside The Cabinet
Inside the sealed cabinet is an air space. Conventional thought usually declares that one should fill this cavity with a fiberglass, insulating material, or some other foam based product. I have even seen spray foam used inside the cabinet. None of these materials address the issues necessary to achieve a low frequency absorber that absorbs greater than the physical dimensions of the cabinet tell us. They are cabinet fillers which are supposed to minimize the cabinet’s internal resonances. Minimizing the internal cabinet resonances is short sighted and does not take into account the power that a diaphragmatic absorber is capable of. We need to lower the internal cabinet’s Q value to a level that will radically increase the unit’s absorption capabilities.
A cabinet’s Q value is a ratio of how well the given space we are discussing performs in relation to the size of the space. A cabinets Q value is the bandwidth of our resonating system or diaphragmatic absorber internal space. With traditional cabinet fill materials such as foam and fiberglass, all we are doing is minimizing the cabinet’s internal resonances. Granted this minimizing does increase the cabinets bandwidth and thus performance but not to the degree necessary for frequencies below 80 Hz. If we are going to achieve the high rates and low levels of absorption necessary in our diaphragmatic absorber in the smallest cabinet size possible, we need to increase the cabinets Q value through a process termed acoustical compliance enhancement or ACE.
Acoustical Compliance Enhancement
Acoustical Compliance Enhancement is a process where we make something perform better than its physical size and contents tell us it should perform at. If you take a car with an engine that uses fuel and air through a carburettor into the engine, that engine will produce a certain amount of horsepower for the size of that engine. If we take that number and use that number as a benchmark, we can increase the horsepower of that engine by adding fuel injection and replacing the carburettor. We have not changed the size of the engine, but simply the method in which the engine receives its fuel and air for combustion. We have made our engine more efficient without increasing its size.
Acoustical Compliance Enhancement or ACE is the same process one can use to increase the cabinet’s internal Q value or bandwidth of our diaphragmatic absorber. Instead of replacing a carburettor with fuel injectors, we use a substance called activated carbon inside the diaphragmatic absorber. Activated carbon or charcoal has a high degree of porosity and is a powerful absorbing material. It is used to filter water and air. Activated carbon granules look like miniature meteors. Each granule of charcoal has numerous holes or pores in it. Each pore is a perfect place for sound to enter into and be absorbed. Each activated carbon granule has many pores and these pores translate into a large amount of surface area. If we could unfold one gram of activated charcoal it would equal anywhere from 500 to 1500 m2. One teaspoon of activated charcoal powder (about 3.3 gm.) has about the same surface area as a football field. This surface area translates into a tremendous potential to “absorb” large amounts of sound energy.
Diaphragmatic absorption is a sound absorbing technology that has been around for years and is used extensively in professional studio and home theater construction. A diaphragmatic absorber is a sealed box that has a surface that vibrates in sympathy to sound pressure waves. Inside that sealed box is placed building insulation type materials and even construction foam which assists in minimizing internal cabinet resonance but does nothing to actually make the absorber more powerful. One can increase the performance of a diaphragmatic absorber by using a different fill material called activated carbon.