July 5, 2011

RFZ - what it is and why do you need it

The concept of a reflection free zone (RFZ) is almost exclusively discussed in studio control rooms, but a similar approach is worth considering in domestic critical listening rooms. The approach is simple. Early strong reflections are reduced but diffuse reflections with a longer time delay are retained. The result is greater image focus and clarity without making a room sound dead.

In studios this approach has gained widespread acceptance, but in home audio it is perhaps controversial. According to Toole (Sound Reproduction), side wall reflections are in fact desirable to create a sense of spaciousness. The size of the image is increased and the overall sound is often enhanced. Most in fact would probably prefer to have side wall reflections. So it is best to consider this concept as one worth understanding and trying, even though it might be something that you prefer. Consider it a tool to add to the box of tricks.

Image courtesy of Ethan Winer (Real Traps)
In the above illustration, early reflections are shown in red, longer reflections in blue.

Why early reflections matter

The problem with early reflections is that they are not so easily distinguished from the intial direct sound. The rule of first arrival indicates that the first arriving sound determines our perception of sound origin, however it does appear to be true that early reflections in the first 10 ms, being strong in level, can have a significant impact on imaging. More delayed reflections tend to be lower in level and are perceived as ambient sound that gives each room a sonic signature. As a general rule, the earlier the reflection and the higher the level, the greater the problem.

Let's consider a conventional stereo setup as shown below:
The speakers are arranged in a 30 degree equilateral triangle with a 3m listening distance. The offsets to front and side wall are made different due to SBIR issues. This kind of setup would be considered quite good by most and many would be happy with a room arranged this way with no added treatment at all.
Here the first reflection points are shown in red, with a more delayed reflection off the back wall in blue. The fact that the speakers are brought in and that the listening position is relatively close does help here, but we could certainly improve on this setup.
Here absorbers have been added so that the earlier reflections have been reduced. Image clarity is improved, and the bass traps will also clean up the bass, but the room is starting to sound a little dead. A diffusor on the rear wall takes away the reflection point and avoids making the room too dead. The room sounds bigger.
The next step is to flush mount the speakers in what might be either bass traps, or more solid structures. This design does not need as much absorption, so the room can now retain a little more ambience.
Taking the idea one step further, an added deflector eliminates the need to use absorbers.


  1. Great illustrative post! How is the difference between flush mounted and wall-mounted speakers? They always sound incorrect to me, lacking a sense of space.

    For my purpose (garage room dipoles) I'd use diffusors at reflection points and no absorbents.

  2. Thanks! "Wall mounted" is a bit ambiguous in that it could mean on the wall with a bracket (what many would assume) or it could mean flush with the wall. "Soffit mounted" is often used but technically a soffit is the underside, like an eave or underside of a bulkhead. In studios the soffit is an angled wall. In my case I'm planning something like the last two illustrations. I think of it as "soffit mounted" but strictly speaking that's not really correct.

    In terms of a sense of space, dipoles do it nicely for music but I find them problemmatic for HT since voices don't image with the necessary sharpness. The front wall reflections add a sense of space that I enjoyed for some time, but there is also an element of vagueness in the imaging. I changed my mind about dipoles after a trip to Bathurst and a listen to the system of the infamous Jerry Tones. It had both image sharpness and a huge sound stage - essentially the best of dipoles and box speakers. I have my suspicions that the secret lies in the mix of room treatment and DEQX digital correction where you can not only phase align crossover points but also correct group delay. You end up with one listening chair that is magical.

    Were I to use dipoles again I would probably start with diffusion across the entire front wall. Any first reflection points there will be too soon and probably too high in level. Even in a large room this is true.


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