RealTraps RFZ panels are a perfect way to treat the first reflection points on the side walls and ceiling. They have a beveled front surface for an attractive appearance, and can be mounted securely flat on a wall either vertically or horizontally.
Their 32 by 42 inch size also creates a larger Reflection-Free Zone than typical 2 by 4 foot panels. RFZ panels are 2 inches thick to absorb to a lower frequency than MicroTraps, and each weighs 15 pounds. [Metric: 81 x 107 x 11 cm, 7 kg]
A useful goal for any room where music plays through loudspeakers is to create a Reflection Free Zone (RFZ) at the listening position. The concept is very simple – to prevent “early reflections” (also called first reflections) from obscuring the stereo image. This occurs when sound from the loudspeakers arrives at your ears through two different paths – one direct and the other delayed after reflecting off a nearby wall.
Just as damaging is when sound from the left loudspeaker bounces off the right wall and arrives at the right ear, and vice versa. Similarly, early reflections off the ceiling and floor can also harm clarity and imaging. In all cases the reflections obscures fine detail and make it difficult to localize the source of the sound or musical instrument.
The drawing at left, viewed from above, shows the three main paths by which sound from a loudspeaker arrives at your ears. The direct sound is shown as black lines. The early reflections – a single bounce off a nearby surface – are the red lines, and late echoes and ambience arrive as shown in blue. In truth, the blue lines are much more complex and dense than the single path shown here, but this is sufficient to explain the concept.
The general goal of a Reflection Free Zone is to eliminate the red early reflection paths by placing absorbing panels, such as HF type MiniTraps, on the side walls in key locations. Not shown, but just as important to avoid, are early reflections off the ceiling, floor, and mixing desk if present.
When a direct sound is accompanied by an echo that arrives within 20 milliseconds or less, the ear is unable to distinguish the echo as a separate sound source. So instead of sounding like an echo or general room ambience, the sounds coming from different directions combine, which obscures clarity and confuses the stereo image. You can still tell when an instrument is panned all the way to the left or right, but the in-between positions are not as well defined. Put another way, listening to music in a Reflection Free Zone is similar to listening with headphones – musical instruments sound clearer, and their placement in the stereo field is much better defined.
Another important reason to control early reflections with absorption is to reduce comb filtering. This is a very specific type of frequency response error that’s caused when a source and its reflections combine in the air. Depending on the difference in arrival times, some frequencies are boosted and others are reduced. The graph at left shows the comb filter frequency response measured with and without MicroTraps at the first reflection points. You can learn more about this effect in the Comb Filtering video on our Videos page.
When the budget allows for dedicated construction, early reflections can be avoided by angling the side walls and sloping the ceiling upward. The control room at left was designed by noted studio designer Wes Lachot, and offers a beautiful example of such construction. Given a large enough angle – at least 35 degrees – the reflections are directed behind the listening position without having to apply absorbing materials to the walls or ceiling. This lets you better control the overall ambience in the room because you don’t need additional absorption just to get rid of the reflections. But most people do not have the luxury of building new walls, so the only option is to apply absorption at key locations.
The easiest way to tell where to place absorption to avoid early reflections is with a mirror. This is the thin green object to the right of the listener against the wall in the drawing. While you sit in the listening position, have a friend place a mirror flat against the side walls and move it around. Any location in which you can see either loudspeaker in the mirror should be covered with absorption. It’s a good idea to treat a larger area of the wall than you identify with the mirror, so you’ll be free to move around a little without leaving the RF Zone. Once the side wall locations are identified do the same on the ceiling. Although it’s more difficult to slide a mirror around on the ceiling, one way is to attach a hand mirror to a broom or garden rake with rubber bands.
Finally, it’s important to understand that the range of frequencies absorbed affects the quality of the RF Zone. Some people use thin panels made of fiberglass or foam, or blankets, and believe that’s sufficient. But those materials absorb reflections at higher frequencies only. A MiniTrap is therefore a better choice because its absorption extends to a much lower frequency, thereby making the RF Zone effective down into the bass range.
The three photos at left show the left, center, and right sides of a room that’s been treated to avoid all early reflections. There’s a High Frequency MicroTrap on a microphone stand on each side, halfway forward between the loudspeakers and listening position, plus two HF MiniTraps on the ceiling. Notice that the small drink table in front of the couch has a towel draped over it to prevent reflections off its glass top surface.
FIRST REFLECTIONS: CEILING AND FLOOR
I recommend treating all of the first reflection points, not just those at the side walls. Finding the ceiling and floor reflection points is easy. Assuming your ears are at the same height as the tweeters, which they should be, the reflection points are exactly halfway between you and the loudspeakers. So if the speakers are six feet in front of you, the center of the ceiling absorber will be three feet in front of your head, as shown in Figure 1. If your floor is reflective I recommend placing a small throw rug directly under the absorbing panel on the ceiling. For larger rooms, or when the speakers are farther away or farther apart, you’ll want two overhead panels (and two throw rugs).
Note that the main and center speakers should form an arc. This puts the center speaker slightly farther forward in the room than the mains, though they remain at the same distance from you. (The surrounds should be that same distance too.) If you’d like you can average the distances between the mains and center speaker. Or else you could calculate each distance separately and verify that the panels are large enough to cover both reflection points.
This brings up a related issue, because the notion of a single point is valid for only a single listening position. If you have more than one seat you should determine all of the reflection points, and be sure your panels are placed to handle them all. In smaller rooms a single 2×4 foot panel is probably adequate, but larger rooms will require two panels, or even more if you have a second row of seats.
Another way to look at this is to pretend you have a mirror image of the room on the other side of the wall. This is the extra loudspeaker shown in Figure 4. If you draw a line from the phantom speaker to the listening position, that line will pass through the wall at the reflection point.