Conduct your own experiment . Take your Logans and press them all the way against the the wall. Move them out about six inches at a time until they are 1/3 the way into the room. Note the results. At some point the will move from thin and diffuse to having slam and coherency. If you use some sort of back wave diffuser this will happen when the speakers are much closer to the back wall. This is why I am so concerned by seeing so many M/Ls so close to the back wall.
Greg, it's possible to misinterpret one's observations from this experiment. Between zero feet and around five feet or so from a back wall, there is enough energy being reflected back against the rear of the diaphragm to actually move it -- turning it into what's called a "passive radiator". If that wave impacts the diphragm out of phase with the wave actively being produced by the diaphragm, then the resulting (compound) wave will be full of all kinds of nasty artifacts -- but primarily phase distortion. So unless there is a large open window right behind the speaker, then regardless of the treatment on that wall, or no treatment at all, it's virtually impossible for the speaker to "get out of the way" of that reflected energy.
If the speakers are sufficiently far away from the back wall, and toed-in just slightly, it's possible to bounce some of that energy off the back wall and around to the left or right of the speakers where it will meet with the front wave, pretty much out of phase with it, and cancel. Many don't realize it, but with all normal height/width proportioned panel speakers, there are two 20-degree "cancellation zones" extending out from each vertical edge (of each speaker), where the direct front radiation and rear wave meet and cancel. With curved panels, those zones face somewhat toward the rear and not straight out from the sides. Interestingly, this unique phenomenon of panel speakers reduces the (soundstage destroying) effect of early sidewall reflections.
If one spends a little time with a nice big sheet of paper drawing ray tracings on a scale plan of the listening room, it's quite amazing what you can learn about dipole speaker placement. Three common mistakes have become very clear to me, over the years, with regard to the way people place panel speakers in small to average sized (13 x 20 ish) residential rooms:
- Too close to the back wall -- well, of course; but don't pull them out so far that they excite standing waves in the room.
- People place speakers too far apart. In a 12 foot wide room, always allow a minimum of 30" from the outside edge of each speaker to the sidewall; and 36" from the outside edge of each speaker to the sidewall for a 13 or 14 foot wide room -- and just let the speaker-to-speaker distance fall where it may! In rooms 15 feet wide or wider, it's first-of-all preferrable to place the speakers on the longer wall -- but either way, in no case should the center-of-panel to center-of-panel distance between speakers be greater than 8 feet max. even if you live in a cathedral! (I prefer 7 feet, especially for the narrower panels like a Summit or Vantage.)
- People use WAY too much toe in. (Goes with WAY too far apart ;--) Always get the speakers close together (even if the room doesn't force you to ;--) and pointing straight forward; and perfectly VERTICAL (not tilting forward, or backwards -- and certainly not leaning to one side or another!! -- what do you think 6 foot levels were made for??) This position is the default starting point for toe-in, which should be carried out in half inch increments, meaning: moving each inside edge of each panel back a half inch from the outside edge of the panel. An easy way to do this is with a string, its ends securely taped around (and stretched between) the outside edges of each speaker (near the floor). As you toe each speaker in, you simply measure the distance between the stretched string and the inside edge of each speaker, a half inch at a time. Then listen. This toe-in offset should NEVER be more than 4 inches. If you go too far (and the soundstage starts to weaken) reduce the toe-in to a previous setting, and begin to increase or decrease the speaker-to-speaker distance in 2" increments. It's a little like dialing in a cartridge, but you WILL nail it . . . . eventually ;--)
Here is a picture of my 12 foot wide by 24 foot long room (8 feet of the length is the kitchen area behind the bookshelf unit.)
http://forum.audiogon.com/cgi-bin/fr.pl?vdone&1109799786&read&keyw&zznsgarch
The distance between the inside edges of the speakers is 31"! The center of panel to center of panel distance is only 4'-10"(58")! and the outside edges of the panels are 29.5" from the side walls. At my listening position, my ears are exactly 5'- 9" from the front of the panels. The soundstage extends well beyond the sides of the room, and the soloists are well focused. The speakers are elevated a foot, and braced, on Sound Anchor special CLS stands, and my listening chair is elevated so my ears are at mid-height of the speaker panels. This is what's called 'nearfield listening' (I call it "control-booth" listening ;--) and is the
mandatory setup for rooms narrower than 14 feet, regardless of the type of speakers one uses.
