i think we are saying the same thing but saying it in different ways.
When you have a rear firing enclosure...the depth of the enclosure also gives a tuning because of the pressure coming back across it
So because an enclosure has a tuning frequency and a sealed portion it's a bandpass? That's incorrect logic. If there is no pressure zone that drives a radiating surface, then there is no ported section. If there isn't a ported section, it isn't going to be a bandpass.
Take these two ghetto illustrations for an example:
1)
2)
Image 1 shows a bandpass enclosure design. There is a sealed chamber that the drivers sit in and they fire into another chamber that has a tunnel smaller than the original that leads to the open environment. The inner chamber will pressurize (hopefully equally) and that pressure will move the mass of air in the tunnel. That pressure will go from positive to negative and will move the mass in and out. That will create a uniform surface that will radiate -- something more commonly referred to as a port. The whole system will have bandpass characteristics.
Image 2 shows a sealed chamber with one end closed with a common chamber directly leading to the outside. This will NOT have bandpass characteristics since there is no radiating surface. It will, however, act like a transmission line for pressure based around the length of the chamber and it's geometry (in this case, it has a uniform line area). This WILL cause gain, just like a bandpass enclosure but with different characteristics. The enclosure will have a high pressure zone in the back wall of the enclosure and as you continue more towards the open end of the line, the pressure will decrease and the velocity of the air will increase. That is how transmission lines work. Once that high velocity end is exposed to another air mass (like the cabin of a vehicle), the air will be excited and it will pressurize. That's how Tommy is able to pull the numbers he can and with the low frequency extension he has.
Transmission lines that look like the one in Image 2 have a low frequency tuning that is 1/4 the wavelength of the length of the line. If the line is about 11 feet long with constant area, it will have an effective tuning of about 25hz. The roll off below that point will be much more shallow than a bandpass or ported enclosure allowing for very, very low frequency extension (sound familiar?) similar to the roll off found in sealed enclosures but with greater output.
Also, everything is relative. Sure all vehicles will show bandpass characteristics OUTSIDE the vehicle but not always INSIDE the vehicle. And more often than not, vehicles have more in common with mass loaded transmission lines than with bandpass enclosures until the ratio of enclosure airspace starts to come close to or be larger than the cabin volume.
