Abby's Switchable Bipolar/Dipole Loudspeakers
This is the front view of each loudspeaker.  The boxes are made from heavily braced birch/maple plywood.  They are vented and damped using a combination of acoustic egg-crate foam, PE's recent Black Hole 5 substitute, and AcoustaStuf.  The driver (one in front and another in back) are from the Onkyo buyouts at PE.  They are 6.5 inch full-range drivers.  The sound is fantastic, especially considering the investment was only $60 for the pair.
This rear view of the boxes shows the second driver, the flared 4-inch port, and the high current switch which changes the loudspeakers from Bipolar to Dipole.  The bipolar configuration is efficient and safe for home amps (8 ohm load, the 4 ohm drivers are series wired).  The bass is deep (F3 below 50 Hz).  Port Clio testing reveals no midrange leakage.  In the dipole configuration the port, of course, is quite dead.  The entire box is dead and the external sound field is much more omnidirectional.  The dipole requires a separte sub for bass below about 150 Hz.
This Clio trace shows the bipolar response.
This is the dipole response curve.