What are the advantages and disadvantages of bandpass subs? Do they add gain or do they offer greater bass extension or both? I am looking at some options right now. Ranier
Two advantages. One, you don't have to do as much electronic filtering to remove unwanted high-frequencies from the passband. Two, the added mass loading of the driver by the second chamber lowers the potential passband frequency. It is possible to tune a bandpass box to achieve some gain, but so doing also narrows the passband, so that is not the usual reason for using a bandpass box.
..is the reduced diaphragm excursion, maintained over almost the entire bandpass of the sub. This also mean you can EQ a bandpass sub in the bottom end. I use a BP-sub (DIY) with a Peerless 10" driver in a box of ~80 liters (active volume) - it goes from 28Hz to 75Hz (no EQ'ing), -3dB.