Mirror, Mirror On the Wall (RFZ Analysis)

I wrote this brief article describing a simple "mirror analysis" method to help you develop a Reflection Free Zone (RFZ) control room design with flush mounted monitors. To find the RFZ of your design you simply draw imaginary lines that extend the walls adjacent to the walls the monitors are mounted in. Then you mirror the images of the monitors across these lines. Fig. 1 below illustrates the simple case of a rectangular room having front monitor walls angled at 30 degrees. Imaginary lines are drawn extending the center front and side walls, plus images of the monitor mirrored symmetrically about those lines. Each adjacent wall reflects sound from the monitor as if those mirrored images were actually there. The blue lines represent some of the "lines of sight" from the images into the room. The red lines are the real sources of the reflections from the monitor itself. The image source lines can only pass through the specific wall they are mirrored about. And, since no image source lines can pass into the light blue shaded area, we call this the RFZ. Notice that in Fig. 1, however, the monitor axis line does not cross the green centerline of the room within the RFZ. The listening position, of course, needs to be on the centerline of the room equidistant from both monitors. There are two solutions to this problem. We can move the monitor position to the left, closer to the center of the room, so that its axis crosses the center axis within the RFZ (this solution has a disadvantage that we will discuss later). Or we could place a sound absorber in the orange area of the side wall. In this case we would need a good broadband absorber that extends from the highest frequencies down into the upper bass - not a trivial matter. But such an absorber would extend the RFZ out to the pink shaded area. And it doesn't need to be so large that it will make the control room too dead sounding.

Note: Notice that in all this I use the term "monitor wall" rather than "soffit". Soffit implies the downward angled mounting structure close to the ceiling you typically see in studios. This traditional arrangement is a compromise made in order to leave space for the control room window. If you don't need such a window, then its much better to mount the monitors at ear level using the entire wall as its baffle.

Figure 2 illustrates and augmentation of the basic design that eliminates the need to move the monitors closer to the centerline or use an absorber on the side wall. Here we have designed a secondary angled wall between the monitor wall and the side wall. Once again we draw an imaginary line extending this wall and mirror the image of the monitor about it. The situation, however, is more complex because we now have two reflective walls to the left of the monitor. We can't forget to mirror the monitor image about the side wall as. The blue source lines from each of the images only pass through the corresponding wall that they were mirrored about. Drawing the source lines we se that we have significantly extended the range of the RFZ. This dual angle arrangement is very common in RFZ designed control rooms. Now comes the question of the best distance to place monitors from the wall intersections. Ideally you want the distances from the monitor to the intersections to be as large as possible. However, it's also a good idea to mount the monitor off center. Why? If you look at the diagrams you can see that walls and their angled intersections actually create horns that load the monitor's output. The closer the monitors are to the intersections and the more acute the angles, the more they will horn load the monitor, and the higher in frequency that horn loading will occur. So, when we came up with the solution in Fig.1 of moving the monitor towards the center, we can now see that what we are doing is also moving the monitor towards the center of a horn created by the front wall and the monitor wall. Additionally, if the monitor is mounted in the center of the monitor wall, and the angles of intersection on either side are similar, then the horn loading effect of each side will reinforce the other in the same frequency range. Placing the monitor slightly off center help break up this mutual reinforcement. The ideal situation is to make the monitor wall as large as possible, and the intersection angles as wide as possible, such that the horn loading effect just blends into the overall low frequency room loading effect - which you then try to fix with bass trapping. Hope this all makes sense, and you find it useful!:) Thomas

:!: Thanks Thomas, Eventhough it's not yet clear to me 100%, it is indeed very useful. The good thing to know is that by creating that one more angled bit of a wall between the monitor and the window extends the RFZ, as you described....it all looks logical when you draw it out.. Thanks :!:

Hello Thomas, well don't laugh, if this file even opens. I was blown away by this thread you've started. I'm posting this drawing I did over a year ago as a test to post drawings here. It was a study, just to see if I could figure out exactly what your illustrating here. It was by no means my studio. Just doodling. In fact, seems that is all I do :D Now that your showing the real stuff, maybe I can straighten this out. I do have a couple of questions. If you are designing a room from scratch, is there an ultimate or acoustically "correct" RFZ configuration that designers strive for, or is this a matter of ergonmics. Or some other goal? It would seem all designers would use this template to design similar studios, or to standardize some sort of tested response whereby you could garantee that template design would test the same from studio to studio. Or is that the goal :D I do understand RFZ is a registered trademark, but is there a patented or registered actual configuration that exists today? I'm sure the surrounding environment in a room and differances in monitors, console, architecture all have a bearing on a room designers ultimate choices and restraints. But shouldn't the goal of all acoustical room designers, especially for control rooms, all be the same? And if so, what are those goals? And secondly, how to you test a RFZ in reality. Is there a technique to prove the boundarys in 3 dimensional space at the monitoring position? How do you tell the difference between reflected sound, and direct as if you were in a geometric bubble(zone) of direct sound only where time differences in real time are so short? WHILE there are reflections completely encasing the zone without entering? Thirdly, if a reflection free zone is the goal to eliminate reflections for direct monitoring, then why would someone need a Diffuser ANYWHERE? That is a reflection, no? Alton Everest stated in one of his books that to use a room full of absorption, shortchanges the "precious energy" that a diffuse soundfield requires. That is NOT a quote btw, so correct my interpretation if I'm incorrect in my thinking. But that only heightens my curiosity about what seems like conflicting set of opinions. Is not a diffuse sound field reflected? Or a mixture of direct sound and reflected sound? This is why differences of opinion, artistic licence and ears of the beholder, all tend to confuse me when it comes to figuring out exactly what you are trying to do when designing a control room besides satisfy ergon0mic considerations. I'm trying to say is, I don't think an ultimate acoustical set of goals for a control room exist do they? Otherwise why wouldn't all designers use the template? Which brings me to the last question. If that is the case, which of course it is, then of what value is a RFZ, if someone with a different opinion about its value in a control room makes a Top Ten CD REGARDLESS of the fact he didn't monitor in a RFZ? Does it matter? Thomas, I have many questions regarding all this stuff, but another day, another question. Anyway, lets see if I can attach this file. DON"T LAUGH!! It's only a test :evil: :D fitZ