Lots of interesting answers here.
The way a speaker radiates and it's interaction with the room will determine it's image size (not the size of the loudspeaker) along with it's ability to cast a holographic image. Relating this to a live performance is difficult. It all depends where you are sitting and what type of venue you are in along with the sound system being used. Image size has nothing to do with volume but, a big image at high volume is very impressive when distortion is low. Unfortunately, the distortion of all speakers increases logarithmically with volume. Getting low distortion at high volumes is not easy.
There seems to be some confusion between bi poles and di poles. Bipolar speakers radiate from both sides in phase. Dipolar loudspeakers radiate from both sides out of phase.
@audio2design, I think what you meant to say was that dipolar loudspeaker's off axis response falls off sharply which it certainly does.
You are right. Most "line source" loudspeakers are not perfect because they act as line sources only at certain frequencies. In order to act as a line source a speaker has to be taller (or wider) than the wavelength of the lowest frequency it is to reproduce. There is one exception to this rule. If the speaker is composed of a stack of multiple drivers the drivers have to be closer to one another than the wavelength of the highest frequency they are to reproduce. This is why you always see tweeters jammed close together and woofers farther apart.
The reason low bass is lost from Magniplanar speakers is because they are too short! At about 6 feet tall, corresponding to a wavelength of around 200 Hz, they act as a line source only down to 200 Hz. Under 200 Hz they do not radiate as efficiently so as you move away from the speaker the bass falls off much more rapidly than higher frequencies.
Well, you might say, if the average room is 8 feet tall that means you can only get a line source down to 150 Hz. That would be wrong. It turns out that if a line source terminates both ends with a solid barrier (floor and ceiling) then it acts as a line source down to 0 Hz. If it is a continuous driver like a ribbon or ESL then it is a line source up to infinity. This is the logic behind floor to ceiling continuous drivers. Magneplaner's big mistake was not making the 20.7 8 feet tall (because their marketers told them it would not sell.) A continuous driver speaker ending at barriers is a perfect line source. A perfect dipolar line source has numerous advantages, aside from the large image size they limit room interaction as they do not radiate up, down or to the sides. The result is a very well defined image.
Speakers that radiate omnidirectionally will produce higher volumes at a given power (depending on efficiency) due to all the added reflections but this plays havoc with image definition and frequency response. Then people wind up spending a lot of money on room treatment. Wide dispersion can make a speaker sound bigger and louder but at the expense of detail and focus.
The best use of subwoofers is to lower distortion in the main speakers particularly at volume when large cone excursions put the suspension in a non linear position in it's range and a lot of doppler distortion is produced. That fact that most subwoofer users do not take advantage of this by using a high pass filter on the main speakers boggles the mind. Do they make a system sound larger? I suppose if you equate low bass with size. I don't as the image size stays the same whether or not my subwoofers are on and I use four of them.
Sounding big and going loud are two separate issues. This single most important characteristic determining image size is the speakers pattern of radiation, point vs line source. Other characteristics are relatively minor.
Any speaker can go loud with enough power. The problem is doing it without distortion. Isolating the main speaker from bass is a great way to limit distortion at volume. Line source dipoles have a beautifully detailed image because they limit room interaction by limiting dispersion. Horns can do the same thing for people who prefer the smaller image of a point source. I also see a lot of tweeters and midrange drivers mounted in dispersion limiting baffles but have not listened to any of them under circumstances where I can tell if this approach works. Don't see why it would not.
