Yes, they have a very very narrow vertical dispersion, while maintaining a wide horizontal dispersion.

>>Is a Bessel array always square?

Usually, or close to it, so you can do some neat things with them, like high SPL's or for beam steering applications and frequency shading and some other things. JBL has a tech paper on beam steering and freq. shading for bessel arrays and the principles behind it, see link below.

Phillips corporation holds the patent and licensing for Bessel arrays so if you're going to use them commercialy(sp) you may need a license.

HTH
Bill W.

• JBL tech paper on bessel array layouts