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.
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,
