I meant that from what I've read, speakers with good off-axis response have no real sweet spot, or more accurately its very wide, which is better for listeners.
Thanks for the explanation! That's what I thought you meant, but didn't want to jump to conclusions.
And yes, I much prefer speakers with a "sweet blob" listening area rather than a narrow "sweet spot".
There is a particular type of off-axis response pattern that is especially good at this. First, let me give some background:
The ear localizes sound by two mechanisms: Arrival time, and intensity (or loudness). If the sound from the two speakers arrives at the same time, the image will be pulled toward whichever speaker is louder. And if the loudness is the same from both speakers, the image will be pulled towards whichever one arrives first. And if they're both the same, you have a strong center image.
So if you have "normal" speakers firing straight ahead, all is well in the middle of the sweet spot, but for someone off to the side, the near speaker "wins" arrival time (obviously) but it also wins intensity, in part because you're a bit closer but also because you're more on-axis of that speaker, where the upper mids and highs are going to be the strongest, and it's the upper mids and highs that convey most of the localization cues. As a result, the center image often moves over to the side farther than the listener has, because both mechanisms support image shift to the near speaker. This is why center-channel speakers are needed in most home theater systems - they're not so much for the person in the sweet spot, as for the people who aren't.
If our speakers have an unusually wide and uniform radiation pattern, there may be little or no discrepancy between the loudness of the near and far speaker for off-centerline listeners, but the near speaker still wins arrival time. So the image still shifts, but not as much.
Now what if there was a way to get one speaker to "win" arrival time, and the other to "win" intensity? The two localization mechanisms would partially cancel one another out, and we'd still get a decent soundstage from well off to the side of the central "sweet spot". Well, it just so happens that there is indeed a technique for doing exactly this. I learned it from Earl Geddes. Here is what we need:
Our wide-sweet-spot speaker's off-axis response should fall off smoothly and fairly rapidly, such that it's -6 dB at 45 degrees off-axis (we's say this pattern is "90 degrees wide") across the top part of the spectrum (ideally from 700 Hz on up but from 1500 Hz on up is still okay). Next, we toe them in aggressively, at maybe a 45 degree angle, such that their axes criss-cross in front of the listening position (and we've voiced them with this configuration in mind). So for a listener off to one side, the near speaker naturally wins arrival time,
but the far speaker wins intensity! This is because the off-centerline listener is on-axis (or very nearly so) of the far speaker, but well off-axis of the near speaker.
The key to this working well is, the near speaker's off-axis sound must fall off rapidly and smoothly. This cannot be accomplished with domes and small cones - we need to use a 90-degree-pattern "constant-directivity" waveguide or horn, and cross over to the woofer where its pattern has narrowed to 90 degrees. A 10" mid woofer has a roughly 90 degree wide (6 dB limits) pattern at about 1.6 kHz. That's close enough for us to work with.
There are technical challenges to making this work of course, but they are not insurmountable.
I have several customers who bought this type of speaker and subsequently sold their center channel speakers. So apparently it can work pretty well.
From my limited knowledge, this is quite a complex subject - e.g. what kind of room treatments are needed, should they eliminate all reflections?
Having experienced what a real professional can do as far as room treatment, I no longer do much "armchair quarterbacking". Room treatment is kinda like crossover design - you want to use the right commponents in the right values in the right places. I suggest Jeff Hedback of Hedback Designed Acoustics (a multi-award-winning studio designer who is still affordable), or the professional of your choice. Getting smooth decay across the spectrum is not easy to do, and a few hundred dollars spent on a professional will work much better than spending that much more on acoustic treatments that aren't the thing you most need.
I remember well my first time in a room Jeff had specified the acoustic treatment for. The room was rich and lively-sounding, and had this unusual characteristic: When you closed your eyes it felt like it was about twice as big as its actual dimensions, and that translated over to music playback in the room.
That being said, in general I prefer a rich, well-energized, lively, and highly diffuse soundfield - in some ways almost the opposite of the overdamped rooms we can get from installing a lot of acoustic foam without really knowing what we're doing.
What kind of room correction takes into account reflections?
I don't like to use room correction anywhere above the bass region. If your speakers have a radiation pattern problem (like an off-axis energy flare at the bottom of the tweeter's range, 2-4 kHz, where the ear is most sensitive), that cannot be fixed with EQ. That's an acoustic problem that can only be fixed at the loudspeaker design stage. If we EQ the room response of this speaker, now we have a dip in the direct sound in that region, and our upper harmonic richness and sense of texture are lost.
Maybe this is overly simplistic, but a grand piano will sound great in almost any room. Yes it will sound best in a good recital hall but probably still really good even in your kitchen as long as you don't bang on it too hard, and it will never outright suck. This is because the reflections aren't a problem that has to be fixed - they are spectrally correct to begin with. Imo that's the way a speaker should behave too.