Most rooms can be worked with, some are going to give better results then others, but unfortunately, there are rooms where you will end up with awful sound no matter what you do. My 12x11 room is one of them. I have literally tried placing the subwoofer all over the room and it sound very boomy and measures terrible no matter what I do. When speaking about good vs bad room dimensions, I’m really only looking at low frequencies, since this is the hardest to control, and you are pretty much stuck with it. Below 100hz, room treatments are impractical, especially in domestic settings. Some rooms sound good without fussy placement, some rooms can enhance the low frequency extension, and some rooms measure so poorly that multiple subs and heavy usage of eq still leaves you with sloppy, bloated bass response.
In most cases, larger rooms with dimensions longer than the width provide a flatter response, although you do lose some boundary gain. Since longer dimensions generally push the room modes down below 30hz, where a majority of subs start struggling, a peak at say, 20hz is only going to help rather than hurt. In my experience, room dimensions that give modes spaced an octave apart have a positive effect on low frequency response and sub efficiency. Two room modes with a center frequency of 50hz and 40 hz will give a boomy one note response, on the other hand, if we have modes at 20hz and 50hz, you will get a fairly even gain across the low frequency range, response below 30hz will be extended, and the gain will more or less raise the entire low frequency spectrum evenly, since you will also have gains at 40hz, 80hz, and 100hz. This is the type of room where a sub is likely to sound good in a corner.
I do agree that a room is sometimes a bigger factor in sound quality than speakers themselves, especially when it come to low frequencies, but at the same time, at higher frequencies, it’s often the speaker to blame for poor sound in room, assuming the room isn’t overly reverberant, which would cause temporal smearing.
When it comes to the speakers influence on the room at midrange and high frequencies, I’m a big fan of controlled dispersion. Since the timbre we hear is a combination of direct and reflected sound, it’s important that the reflected sound is spectrally similar to the direct sound. The speakers I’m currently using have a controlled dispersion of +-45 degrees up to 14khz. within this 90 degree area the response measures nearly the same off and on axis. Further out than 45 degrees the high frequency response falls off evenly, with no major anomalies. The speakers are two feet from the side walls, 3.9 feet from the floor, and a little over 4 feet from the ceiling. Since these distances are well within the coverage pattern, the sound is more or less the same no matter the location in the room. The reflected sound is similar to the direct sound, and outside of normal aberrations at lower frequencies, I can measure a +- 5dB response from anywhere in the room.
All speakers will narrow in dispersion as frequency increases, and widen as it decreases, it is nearly impossible to build a speaker with perfect directivity because single drivers must cover frequencies that vary in wavelength in proportion to the diameter of the driver, however, reasonably good control over the highest frequencies can be achieved with horns and waveguides, while matched directivity across woofer and lower frequencies can be achieved through the use of multiway systems, and smart crossover points, a majority of systems are crossed over at 2.5-3khz, at frequencies this high, many 5.25” and 6” driver begin beaming, while the tweeter has a very wide dispersion because it is small in relation to the wavelength. Using a crossover lower crossover point can help maintain similar directivity between the tweeter and woofer, which reduces lobing off axis. Lenard from Lenard audio is a big supporter of 4 way systems for this reason. See
http://education.lenardaudio.com/en/05_speakers.html
Another great article on directivity
http://www.acousticfrontiers.com/category/blog/acoustics/speaker-directivity/
A speaker with increasingly directional dispersion at high frequencies , or worse, uneven dispersion and lobing, is going to vary significantly from room to room, and from seat to seat, a speaker with controlled dispersion, void of lobing off axis, will provide spectrally similar sound to every seat within the defined coverage pattern, and the effect on the timbre from room reflections will be minimal. A majority of cinema speakers are designed with a coverage pattern of 90x60 horizontal/vertical. The two major providers of cinema speaker systems are JBL and Klipsch, I believe QSC also designs cinema systems, but I don’t know enough about them to comment. Both companies have put a great deal of effort into achieving controlled directivity with an even off axis response across the coverage pattern. This is why the timbre sounds more or less the same regardless of whether you sit all the way in the left or right seats(horizontally off axis), or in the front or back of the cinema (vertically off axis)
Floyd toole advocates speakers with wide dispersion, geddes advocates speakers with narrower dispersion, but both of them agree on one thing, that controlled directivity and an even response off axis is paramount to getting good sound in room. I think for the purposes of home theater, where there are multiple seats, many of them at least somewhat off axis, a wider dispersion is desirable up to a certain point. A speaker can have a flat response on axis within 2dB, but if the response rolls off steeply at high frequencies off axis, or becomes uneven, you’ll never hear that flat response in a real room, because that ragged off axis sound is what will be reflected off the walls.
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