In our bass absorber DIY panel video, which you can see further down the page, we stated that we’ve tested the acoustic foam in the middle and in a recent Google Hangout I was asked to just explain how that is so given that foam cannot be used as a low frequency absorber just so people don’t get confused between one thing we’ve said and then another.

OK so here’s what we’ve got going on inside of our diaphragmatic absorber. We have two walls whereas most people just use one. Well we found that two are better and there’s all kinds of reasons for that and we get to about a nine to a ten percent improvement adding the second wall.

What is the function of the first two walls or first wall in a standard diaphragmatic absorption? It’s there to slow the pressure wave down. That’s all it’s supposed to do. It’s kind of a barrier although I don’t want to use that term because it’s a sound absorbing device I don’t want to get people confused. But if you think of the absorber as a room, it’s kind of our barrier and it slows that low pressure energy down.

Now that said when you have two walls, those walls have to work in sympathy with each other and if you get the densities right and you get the distance between them correct and you mount them correctly, you will get an additional slowing down of that initial pressure wave. Once the wave is slowed down it enters the inside of the cabinet. What’s inside the cabinet? A perforated absorber filled with carbon.

Perforated absorbers were very popular years back. They’re a way to have a box or a cabinet of 4, 5, 6 inches deep and fill it full with building insulation or some sound absorbing material. Then have holes in the front of it, so perforated, perforation, so thus the name perforated absorber.

Why the perforations? Why the holes?

Because it’s a device that works on air movement. It’s a device that works on molecular velocity. It’s got to have air moving through it, going through the holes and going through the perforated board also. By going through the holes and entering the sound absorbing material inside, you control the depth and it’s just common sense. If you control the depth of the perforated absorber, you control the diameter of the hole, you control how far apart the holes are, you can calculate resonant frequency. You can find out the lowest frequency that unit will absorb at.

When we put carbon inside of it, then we increase the ‘Q’ value or the absorption rate and level of inside the unit. ‘Q’ stands for quality and quality means performance, rate and level. So when you say ‘Q’ value, think rate and level.

When a company says “well so much improved ‘Q’ value” then it means it’s developed a much improved rate and level of whatever they’re doing. Frequency response if it’s a speaker manufacturer, sound absorption coefficients if it’s an absorbing device. So we took an old-school idea of using a perforated absorber and then we added carbon to the inside. We made carbon the sound absorbing material that the air moving inside the cabinet has to go through. When you do that, the ‘Q’ value is off the scale, it’s not measurable.

It’s so great because one gram of carbon has 2,000 square meters of sound absorption surface area. There’s no table we have in existence today that will give you that number. We can approximate, we have that data but the bottom-line here is we take a perforated absorber, we put it in a box that’s very rigid and strong, we ask it to do its job, namely that any air that comes in contact with it, just absorb, just destroy it, get as much of it as you can and keep it inside you. And the front walls of the absorber slow the pressure wave down so it has time to slow down and get a little bit of more molecular movement.

So two different approaches all in the same product. Taking the strengths of both products, combining them together to form a synergy that yields much better performance. With our units, perforated absorbers we assign it a one, we assign the diaphragmatic absorber that we put it in a two, one times two is not two, one times two is ten. You get a synergy when you put two things together, maximize both of their performance rates and levels then you get this synergy that creates so much more absorption in such a small amount of space and that’s the reasoning and design parameters behind the ACDA-10 and 12 series.

With the foam going inside it’s effectively acting as that sponge as well.

If you’re not using carbon, if you’re not buying our production units or you’re not involved in any of our licensing agreements, and by the way those of you out there that are, licensing agreements we’re probably going to stop that the first of the year. So any of you out there that want to get involved, whereby you actually build the unit yourself and license the carbon technology from us we can accommodate you there. Well that’s going to end probably on the first of the new year.

So if you’re using our foam inside of it you will not get the rates and levels of absorption that you get with carbon. You just can’t, it’s not possible. You will have less cost and a lot easier build. You get about 75 to 77 percent of the performance of our carbon unit. But you can get a hundred percent of the performance if you qualify for our licensing agreement.

We don’t give it to everyone and my colleague Ali at will determine whether you qualify or not. So those of you that are thinking about building some units on your own and putting the carbon in it, the first of the year will be the end of that probably.

Dennis Foley

Author Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

More posts by Dennis Foley

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