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Discussion Starter · #1 ·
If I understand things correctly, the first reflection (from side walls... ignoring the floor and ceiling for now) causes comb filtering distortion at frequency intervals dependant on the path length differences between direct and reflected sound.


Removing the comb filtering by using absorptive material increases imaging by reducing this distortion, giving you "pinpoint" placement, while diffusing the sound "smears" the early reflection, and results in a wider and more "spacious" soundstage but lacking a bit in the pinpoint imaging.


My own crude experimenting has revealed this to be roughly true, and with my planar speakers I seem to prefer the diffuse sound (though I've been living with reflective sidewalls for so long it could simply be that I'm more "used" to that type of sound).


What I'd like to know is what you guys' opinions are on this subject. Is there any concensus on what the "best" approach is? What variables are most important to which "sounds" better? Room size, speaker type, speaker placement, source material, listener preference? Can surround sound processing (like Logic7) restore the width and realism of the soundstage in a "dead" room while retaining the pinpoint imaging?
 

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I agree with you 100% about treating the room.


I've never done any experiments with multichannel audio in a "dead" room but my minimally educated opinion is that it would be no better than two channel in a dead room.


Nothing sounds worse to me than a dead room. I'd rather listen in a room completely made of glass and wood! More speakers would simply mean more direct sources of sound with no reflections. Moreover, if you have a dead room with 7 speakers even with excellent signal processing the most points in the sound field you could obtain would be... 7. FAR fewer than the hundreds of late reflections and reverberations in a lively room. I've also never been too impressed that those "hall" and "studio" modes sounded good and to my ears. No amount of surround processing has ever come close to good stereo imaging. Now multichannel SACD is (sometimes) another story!


I think the consenus is

1. Absorption at the first reflection points with broad enough spectrum so as not to skew the tonal balance of the room. This will sharpen the imaging.

2. Diffusion in the middle of the front wall and/or middle of the rear wall. This will increase spaciousness and depth and the sense of "being there".

3. Bass traps in the corners.


My opinion on importance of various variables... This is hard.


1. Speaker placement and type. These need to be combined because the demands of planars is different from the demands of cones which is different than the demands of horns. Also no amount of absorption and diffusion will equalize an improperly setup room. My favorite cheap tweak is a tape measure.


2. Room size is important, but unless you have a cube for a room usually if you have enough time and money you can treat just about any room for reasonable use. It is quite difficult to treat a bad bass suckout, however.


3. Source material- Even the best room can't reconcile a lousy disk. I had to throw away all of my favorite 80s pop music because with a high-resolution room and system they sounded absurdly bad. Regardless- source material is easy to change. And some of those lousy 80s disks have now been remade into SACD and have bloomed to life (example- The Police, Pink Floyd, The Stones, and even Billy Joel).


Nothing, and I mean NOTHING, has EVER made my music improve more than proper room setup and sound treatment. Nothing has EVER even come close. No amp, no CDP, no speakers... NOTHING! Is that clear? Treat the room first. Now I can really tell if my speakers are 1/2" out of position or not properly toed-in.
 

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Planar speakers generally does not require much side room treatment. This is due to their dipole radiation nature, where very little sound emits off toward the side. Since it emits sound from the front and back of the speaker, it would be preferred if some type of diffuser is used in the back wall, with some directly behind the speaker using ficus tree, etc. This should somewhat tighten up the image.


The comb filter effect could be due to toe-in issues, since the drivers are wide and side to side, there could be frequencies where cancellation occurs.


As surround formats go, I think Meridian's Trifield works great for planars. Besides creating a center fill by the center speaker, which alleviates the tendency for the image to pull toward the speakers if they are too far apart, it also has special directional psychoacoustic algorithms which actually makes the imaging much more precise, a perfect combination with magnepans.
 

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Discussion Starter · #4 ·
Any other thoughts? I limited my earlier comments to sidewall early reflections. What about ceiling and floor? Diffusive or absorptive?


Comb filtering has been shown in tests to contribute to the "spaciousness" of the soundstage. Removing it increases imaging accuracy. It's a form of distortion... but how much do we want?
 

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If you have big planar speakers, then ceiling and floor reflection should not be an issue. The way sound dispersion works, the wider the driver, the narrower the dispersion, relative to the freq of the sound. So in effect, everything longer than 1/2 wavelength of the driver's length would have its off-axis dispersion start to fall off. That is as long as you are sitting roughly near the center (vertically) of the driver.
 

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Ceiling and floor surfaces are boundaries that absorb, reflect and diffuse as are the walls. What complicates our ear/brain response is that as humans we are not "conditioned" to exposure to diffuse (at least reasonably broad band) reflections off of floors and ceilings, but rather mildy absorptive and/or specular reflective. Since we walk on the ground, there was and is always a boundary there. The ceiling is a little more complicated in that we spend a good part of our time in spaces with and without an overhead boundary. The same pschoacoustics applies however. Experiments with listener response suggest direct reflections and diffusive reflections that bounce off these boundaries and arrive at our ears too close in time to the direct sound will tend to diminish the solidity and precision of the "imaging" many people like. An averge floor standing speaker's midrange/tweeter heights and the listening distance will yield the physics of the situation. The ceiling is just an inverse of the floor (assuming its flat). How much diffusion vs. absorption etc. cannot be answered specifically since it depends on the listener's tastes. Some people enjoy ambiance and "immersive" "enveloping" soundfields, others like precision "imaging" and apparent localization of sources.


Bill Lummus is dead on with his comment about source material. Nearly all music produced in major studios with the expectation of mass radio play or playback on high volume consumer gear (boom boxes, inaccurate stereo systems, mpg players, computers, etc.) is poorly recorded and can minimize the satisfaction of the most accurate/best sounding system and listening room. Why this is so, I don't know since the marginal cost to record it well is small relative to the production budget I'm sure. There really should be no such thing as "audiophile" recordings. They should all be high quality given the state of technology and production knowledge.
 

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I thought the main difference between wall effects and floor/ceiling effects was to do with the fact that our ears produce a stereo image in the horizontal, or I hope they do as they are on each side of my head not the top and bottom!!

Doesn't this work a bit like eyes and the two together produce the image by triangulation? Is that spelled right?

Maybe someone could explain if this isn't so.
 

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I'm not sure I quite understand your comment. The "stereo image" (difficult to define) does not have a plane. It is sort of a "spatial" resultant of the ear-brain response to the interrelationship of source locations, boundary interactions, acoustic properties of the boundaries and space, etc.. The list is long. This is why everybody banters about how some speakers did or didn't "image" well at a show in a certain room or set-up. Everybody's listening set-up is different and much of what is criticized about equipment quality/faults might be better attributed to the environment its used in.


My point regarding floor and ceilings, (mostly regarding floors) is that since we (humans) have always listened to sound with a ground boundary, if we were to "effectively" remove this boundary or change its acoustic characteristics radically (pure broadband absorber/reflector or diffuser) then we could expect a psychoacoustic response greater than the effect of changes to other boundaries, which occur naturally as part of our daily lives. As far back as the evolution of humans, we walked around experiencing sound bouncing off the ground with various other boundaries being present or not. Thousands of years of doing this has got to influence how our ear-brain works. I've never participated in an experiment where there was no floor or the floor was made of 6 feet deep cone absorbers and the walls and ceiling were typical materials and construction, but I bet the experience would be interesting.
 
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