If you look at a typical quasi 6th order, you will notice that it is not much different than a typical bass reflex firing into another area at which, based on even more physical constraints, the resonances within that area also conform to the response. If you look at the word quasi, you will see that in definition it says something about resemblance and looking like something but not all the way. Well, in this particular design, it resembles a BR in a vehicle with a measurement taken just outside the window of the vehicle, 3 ft away. But it also can have an acoustic,al resemblance to a tapped enclosure in which the phase of the rear signal has to be in phase with the output of the second chamber in order to increase efficiency and in turn increase output at a power constant. This is why they can be very efficient, much like a tapped design, but the big difference between the two is that the tapped design utilized back wave propagation for extended low frequency response. The quasi 6th does not. It utilizes resonances and small room acoustics for increased gain even before the vehicle is taken into consideration.
So, basically what happens is, you figure for the response calculation of the rear chamber resonances and response, and figure what it will do in the accompanied space of the second chamber with its response, and figure for a combined response with a specific phase correlation between that and itself, and that within another complex environment in which more resonances and modes are excited creating more gain and loss based on volume and distance, and then conform them together to get a final response and phase at a specific listening position or even an average of the entire room. (take a breath). So, in essence, a quasi 6th order can become very efficient, but uses one chamber to limit its response to a specific tuning factor and then uses that response combined with direct driver radiation into another chamber that again limits that response into a different tuning factor which is combined with vehicle gain at the end.
It is not known that the quasi design acts like a bandpass in a sense that the front or rear chamber is sealed, but in a sense that the direct driver radiation has already contribut3ed to the response before final tuning, where as we know, it only exits one port. And at the end, the terminal velocity and intensity at that port determines the bandwidth, which is like calculation for a single port without combining direct driver radiation as the box volume itself is always considered a filter of some sort. Direct radiation is more broadband, hence the smoother output in be and sealed designs than bps. But I wouldn't call a quasi 6th an typical bp. I consider it quasi and that's it.
