[Brian Bunge]

These are technically known as dipole speakers. There are a few issues with such speakers. First, is the fact that there is no box to act as an air spring for the midbass/bass drivers so they will not have the same output capabilities as a "box" speaker and it will be much easier to bottom out the drivers due to this. Also, EQ needs to be used in order to boost the low end because the dipole effect causes the front and back wave of the drivers to cancel one another out, thereby causing response in the bass range to be extremely low. This is why you see many dipole speakers in a TM or MTM configuration sitting atop a sealed enclosure with bass drivers to pick up the low end or using multiple dipole subs to give the required bass response.

While I have not built dipole speakers yet, I know that things such as baffle width are important. Also, a dipole design requires 4x the Vd to equal the output capabilities of the same driver in a box. In other words, a dipole subs will need 4 12" drivers to equal the same output of the same single 12" driver in an enclosure.