I agree with Onhwy61 about total surface area. In fact, my current speakers have four 8" woofers per side and extend beyond 20 Hz with abundant SPL.
As always, it's more complicated than just size. The drivers have to be designed for bass. If you were to make an array of several midrange drivers they would still be limited, although the SPL within their range would be stunning. The impedance of a typical midrange cone rises at it's lowest limits and an array of them can reach up to 50 ohms @ 50 Hz, depending on how they're wired, etc....
It's possible to have an array of eight 7" drivers that extend no deeper than a single 7", although it'll probably play a lot louder with much less distortion and handle a lot more power.
Darkmoebius: No. It's difficult to explain but well executed line arrays do not have the point source effect due to dispersion patterns, even at nearfield. The concept of pinpoint imaging from a line source is counter-intuitive but entirely possible, actually probable.
As always, it's more complicated than just size. The drivers have to be designed for bass. If you were to make an array of several midrange drivers they would still be limited, although the SPL within their range would be stunning. The impedance of a typical midrange cone rises at it's lowest limits and an array of them can reach up to 50 ohms @ 50 Hz, depending on how they're wired, etc....
It's possible to have an array of eight 7" drivers that extend no deeper than a single 7", although it'll probably play a lot louder with much less distortion and handle a lot more power.
Darkmoebius: No. It's difficult to explain but well executed line arrays do not have the point source effect due to dispersion patterns, even at nearfield. The concept of pinpoint imaging from a line source is counter-intuitive but entirely possible, actually probable.