As part of my blog series on speaker off axis performance (
http://www.acousticfrontiers.com/whats-new/, latest post:
http://www.acousticfrontiers.com/whats-new/2014/3/5/speaker-off-axis-controlled-directivity-speakers) I've reviewed a lot of speaker off axis measurements.
The ones that I've been impressed with have been:
YG
KEF
Revel
JBL
Adam
Vivid
All of these companies obviously pay attention to what the sound is doing off axis.
If we accept the premise that most of the sound that reaches our ears in a normal listening room is reflected or reverberant sound, and/or the premise that the reverberant sound can have a significant influence on perceived timbre (singing in the shower), then the off-axis sound matters. How much it matters and where the goal posts are for off-axis performance may be subject to debate, but "it matters" is a good starting point.
At the risk of either over-simplifying or over-complicating, in my opinion there are four basic catagories of radiation patterns for speakers that "get the off-axis sound right". And just to be clear, imo "getting the off-axis sound right" starts with the spectral balance of the off-axis energy being similar to the spectral balance of the on-axis (or first-arrival) energy.
First up, we have "wide pattern smooth". The speakers that I'm familiar with from Nyal's list all fall into this category. This is where the -6 dB limits of the radiation pattern form an arc more than 90 degrees wide in the horizontal plane across much or most of the spectum. This is my arbitrary definition and anyone is free to disagree. I would add Gradient to the list, as their Revolution and Helsenki models have exceptionally well-behaved radiation patterns, due in part to the unorthodox midbass and bass loading techniques designer Jorma Salmi uses. Most (but not all) speakers in this category are three-way (or more) direct-radiatior systems, some two-way concentrics being exceptions.
Next, "narrow pattern smooth", wherein the radiation pattern is 90 degrees or less in the horizontal plane, typically with radiation pattern matching in the crossover region. To the best of my knowledge Altec pioneered this approach with their Model 19 and its siblings, which were improved on in the landmark JBL model 4430 studio monitor, and whose modern optimized incarnation would be epitomized in the GedLee line. Most in this category use constant-directivity horns or waveguides.
Then we have "omnidirectional" or nearly omnidirectional speakers, in the horizontal plane anyway. MBL, Ohm, Mirage, Duevel, and last-but-not least the amazingly affordable Morrison Audio come to mind. This approach gives the richest timbre of them all, as spectrally-correct reflections are especially beneficial to timbre (which is why good recital halls tend towards being diffusively reverberant instead of anechoic).
The final category I'll call "polydirectional", a term coined by Richard Shahinian, and this category of course includes his speakers as well as many dipoles, bipoles, and others. Even using a rear-firing tweeter to fill in missing reverberant field energy in the upper octaves qualifies as polydirectional in my book. The SoundLabs were mentioned earlier in this thread, and imo when set up correctly they may well be the ideal example of a good polydirectional: Very smooth first-arrival sound across a 90 degree arc, and then a generous helping of spectrally
identical reverberant energy whose onset begins after a significant path-length-induced time delay (assuming they're positioned well out in front of the wall).
All of these approaches have their advantages, all involve tradeoffs, and in case you didn't notice I have opinions as to which of ones yield the best net results. But as long as intelligent thought is applied to rendering the reverberant energy supportive of, rather than detrimental to, the first-arrival sound, imo we're barking up the right tree.
Some posts in this thread have talked about DSP. The one thing DSP
cannot fix is the radiation pattern. So as DSP improves in power and sound quality and becomes more practical, more affordable, and more widely used, I think we'll see radiation pattern smoothness become increasingly valued as a basic requirement for a high performance loudspeaker.
Imo, ime, ymmv, etc.