Dear Rives Audio,
First, I'd like to thank you for writing articles about acoustics. I'm glad that you're willing to help audio enthusiasts like me to get the most out of their systems.
In your recent article, you said that you'd like to hear our questions. So here are mine: I have found several articles dealing with acoustics and room treatments, but they all seem to be written for conventional speakers. How would treating a room for dipolar speakers (planars, ESLs, ribbons) differ from conventional box speakers? Since they are mostly line sources, I assume that floor and ceiling interactions are much less of a concern. Does that mean that I can have bare floors, or should I still put a think carpet or rug in the room?
These speakers put out as much sound in the rear as they do in front. Does that mean I should attempt to eliminate as much of the rear waves as possible? Since they are line source speakers, is it better to dampen only the high frequencies radiating from behind the speakers - or will I still need to place treatments throughout the room?
I cannot seem to find the answers I need anywhere. I would really appreciate any help you can provide.
Thank you for your time.
Thank you for a really good question. The basic premise of your question is not unlike a previous one we received regarding speaker interaction with the room. This previous question, answered in RAM:EF1 had to do with designing a speaker that did not interact with the room - or to a lesser degree than conventional designs. Your question is kind of the inverse in that it recognizes that planars and dipoles have a different type of room interaction. Most people will affirm that planars are harder to place and more dependent on the room than most speakers. Let's take a closer look.
First, as you correctly pointed out, the planar has 50% of its energy going behind it. Should we absorb this energy? Absolutely not! This back reflection gives us short reverberation times and with them, the sense of space from the planar. But, should we reflect this energy in its entirety? Probably not. Usually, in such cases, some diffusion works quite well. We want to bring that energy back into the room, but not in an altogether coherent fashion. What I mean by this is that if you took away the direct wave firing at you, what you would have left would be similar sounds/frequencies (the rearward acoustic energy), but diffused in such a manner that they would not be terribly strong or entirely coherent and intelligible. This diffuse energy would mimic what we might naturally hear in a concert hall, emanating from the reverberation off the back of the stage etc - not exactly, but this is the basic principle whereby these types of speakers were designed.
Now let's take it a step further - the line source principle. Is it better to dampen only the high frequencies radiating from behind the speakers; or will you still need to place treatments throughout the room? Well, the answer would be yes, except, we have just diffused and scattered a bunch of energy coming off the back plane. That energy will interact naturally with the floor and ceiling, so you are going to have to treat these surfaces judiciously. In general, the area behind the listener (walls, ceiling and floor) can be even more critical with planars than conventional speakers for this reason. Thus treatment needs to be done in various areas throughout the room. I would definitely try to absorb behind the listener. The extent and nature of side-wall treatments, if required, would depend on the size of the room. The ceiling and floor treatments are also related to and dependent on the size of the room. But, if the floors are bare, an area rug will at least be needed near the listening area, which would serve to reduce any comb-filtering due to the direct reflection from the speakers interacting with the second (early) reflection off the hard floor surface. Also, either some absorption or diffusion, or a mixture of both, would likely be needed on the ceiling unless it is a very high or vaulted ceiling.
Richard Bird and Christopher Huston
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