You can also see where a speaker starts beaming by looking at the frequency charts provided by the better manufacturers with on axis, 30 degrees, and 60 degrees off axis’s. You’ll see the frequency for each one start to diverge from each other at the point of beaming. Take this graph from the Scanspeak Classic 7” mid woofer for example. You can see it starts to diverge as low as 1000 Hz.
Attachment 10816
What many people don’t consider though are all the reflections you get in the tight small space of an automobile. Much of the information you hear is indirect sound as much as it is direct sound and if they don’t match, even when aimed on axis, it will be tough to get everything to sound right. When you LP a driver above it’s beaming point than the indirect sounds will start to become less in amplitude than the direct sounds at the listening position. Having the indirect sound be as close as the direct sound makes tuning easier. This is why using a three way and crossing each driver at or below its beaming point is the best method to use.
Can you be successful with a wide band driver, of course, but you’ll have to be real careful with driver aiming, and you may still not get the same top end sparkle you can get with a tweeter. If you want to use less than ideal factory speaker positions then you may have issues. I’ve seen enough members who went from a traditional two way, replace the tweeter with a wide band and be very excited at first, only to go back to a two way with traditional tweeter, or three way, including myself.