Understanding FAST and BAD, just a TAD better
I have to admit that I love acoustics. I know it sounds a little obsessive, but I do love the science, the physics and the creation of architectural devices that actually have both form and function. It seems, at times, that we are epitome of a “niche” industry and that my love for acoustics isn’t necessarily shared by the average person, but I love it just the same.
Real love requires real honesty and a shared devotion to common principals (at least in my opinion). I have been concerned for some time about certain information that gets disseminated to the public that may be slightly misleading. Most of this comes from manufacturers who might just be trying to convince themselves of their convictions and claims, but don’t have the empirical evidence to back it up.
Unfortunately, much of this information falls into the hands of users who do not have the background to really understand the physics of sound. They take the manufacturers word that the devices do what they say they do at face value. In all honesty, they should be able to do that. It only makes sense.
We have recently introduced a new product called the FAST Panel. FAST stands for Frequency Absorbing, Specified Tuning. You may notice that it resembles a couple of other products on the market and that it exhibits similar results in testing. We feel that the principals of the actual performance of the product are sound (pun intended) and that this product is something that is useful to the acoustician or sound designer.
You will notice one difference in our product description, however, we don’t call it a diffuser. The acronym we have chosen (FAST), and the name behind it, describe it perfectly.
I have watched in amusement for some time, RPG and Kinetics go back and forth about which of their products (BAD and TAD) has the most validity as a diffuser. Kinetics even went to the trouble and expense of buying a bunch of BAD panels and having them tested, alongside their product, by an independent laboratory. That testing is very revealing and tells more of a story than most might understand.
Let’s talk about reality here. For an acoustic diffuser to have any real meaning it needs to have efficiencies well in excess of the absorption curve it exhibits. When I design and test a diffuser (as I have many) I consider average results in the operating range of the diffusion, where the absorption exceeds the diffusion or scattering, a failure. The problem is energy, or lack thereof.
As we have shown in the last couple of years with the development of the Geometric Uniform Diffuser (GuD Panel) efficiencies are important. With a diffuser, the more absorption we can eliminate, and replace with pure diffusion, the better the device is going to function in the real world. With the development of the GuD panel, and its subsequent ACAD (Asymmetrical Cellular Acoustic Diffuser) patent, we have developed the most efficient acoustic diffuser in the world.
In a very real sense, absorption and diffusion have conflicting purposes. For instance, when Schroeder came up with the now famous equation for designing quadratic residue diffusers he wasn’t really looking at absorption in the equation. It was, and is, an artifact that comes along with the physical design of these devices. In a perfect world, you would have no absorption in a diffuser. As we have shown with the GuD panel, absorption affects tremendously the overall efficiency of a diffuser.
It’s not the diffusion that everyone likes about the FAST Panel (or similar products), it’s the absorption. In reality, it’s the lack of absorption in certain frequency ranges that we really like. In essence, the FAST panel acts to limit high frequency absorption, thereby keeping this energy, so easily absorbed by conventional absorbers, in the room.
It's not to say that there isn't diffusion taking place in the FAST panel, there is. With the new proposed ASTM diffusion testing standard that I helped develop with Ron Sauro of NWAA Labs, we can now see diffusive characteristics in the 15-16KHz range. This adds a pleasant HF envelope that many appreciate.
One other artifact that has been reported, by a well-known acoustician, is the low frequency absorption (well under 100 Hz) of the FAST Panel design. This is primarily due to diaphragmatic absorption of the perforated face in reaction to the LF sound waves. However, this is relatively minimal overall and should not, in my opinion, be considered in the overall design equation.
There is no question that the pattern, design and implementation of the perforations in the FAST panel make for better performance. The placement and amount of reflective vs. absorptive surface plays a large part in the test results. We’re not going to confuse the issue by calling it a diffuser. It would be disingenuous of us to do so.
As a sound engineer for many years, I like to think of the FAST Panel as an acoustic equalizer. Have you ever noticed when you look at sound systems with 1 or 1/3 octave graphic EQ’s in them, that most people tend to dip the midrange out and boost the lows and highs. Why do they do that? It’s quite simple, because it is pleasing to the ear.
There’s nothing really too astounding about it, to be honest. The human ear is generally much more sensitive to midrange frequencies and so we want to reduce those and enhance that which brings more aural balance. That is exactly what the FAST Panel is doing, only acoustically, not electronically. It is, essentially, a focused absorber.
The advantages to correcting these issues acoustically is that you are not altering the original signal (pre-recorded music or otherwise). You are also reducing the reverberation time of the room naturally which is a benefit whether using the room for music/video playback or simply just speaking in the room.
We hope you will take a look at the FAST panel for your projects. It’s a great solution for a lot of different situations. I hope that we have shed some light on the subject that is beneficial for you and look forward to your comments.
President, RealAcoustix LLC