PDs vs Void 18"

Tek, it depends on the type of cabinet.

 

With reflex and at the wavelengths invloved the same applies to mini scoops, the driver unloads below the tuning frequency of the port (Fb). This happens because the output of the port is in phase at its tuning point but below the tuning point just passes air from the rear of the woofer. The rear of the woofers output is 180 degrees out of phase from the front and because the ports output is in phase with the woofers rear output below Fb, the ports output will also be 180 degrees out below Fb. So below FB both front and rear waves can meet. This is like having the woofer in free air, well not quite as the port still adds resistance, but the effect will make the driver seems like its out of a box. Drivers like the V18-1200 have a symetrical suspension and because of its higher Vas needs more input from its suspension to keep things in control. Higher motor strength woofers rely on there motor strength to keep in control, which is great for most of the time but not good when it comes to unloading situations, which can be a lot of the time when playing muisc that has a lot of lows though cabs that are not tuned low enough.

 

A horn will unload the driver when its transmision length can no longer suport a frequency which is one quater its wavelength. So when the frequency being played has a longer wavelength that the horn can suport it disapears and acts like a long port of tube which has no resistance. Longer horns will load to lower frequencies as will larger drivers on the same length horn as smaller drivers. But unlike the reflex box that will allow both front and rear radiation to meet below Fb, a woofer in a horn will still recieve damping from its sealed rear chamber. This in itself can give problems as large woofers in small sealed chambers can undergo very large excusion below Fc. But the problem of unloading is normally worse in reflex designs or horns that act like reflex because of the low frequencies involved.

 

The best thing you can do if you play music with lots of low frequencies is to use reflex cabs tuned to about 25Hz. That way the drivers will never unload and you will keep them for a very long time, well if you don't cook the coils first. You could use horns with a cutoff of 25Hz, but they tend to be house like size and not moveable. Unless its a Rubic horn.......