Audio buffs have been known to lock horns over all kinds of things - CDs vs. vinyl, Dolby Digital vs. DTS, tubes vs. solid-state, DVD-Audio vs. Super Audio CD, and on and on. But one of the hottest debates of recent years has been over which kind of speakers work best for the rearward surround channels in a multichannel setup: monopole (a.k.a. direct-radiating) or dipole?
In an effort to resolve this debate (or at least lower the temperature a few degrees), I recently replaced my usual surround speakers with a pair of high-end M&K Surround-250 MKII Tripole speakers and settled in with a selection of multichannel music discs and DVD movies for an intense listening session. The M&Ks can be switched between monopole and dipole operation (as well as various combinations of the two), which would help me explore the pros and cons of both speaker designs to see which is better at the often difficult task of reproducing surround sound.
Define Your Terms For those new to the surround sound game, monopole surrounds have a single set of drivers, which are aimed at the listeners; the speakers are usually placed to the sides and slightly behind the listening position. Dipoles, on the other hand, have two sets of drivers firing in opposite directions and wired out of phase. Typically mounted on either side of the listening position, dipole speakers radiate sound toward the front and back of the room, placing the listener in the "null" created between the two sets of drivers. Since most of what you hear from dipole surround speakers is first reflected from room boundaries, the sound is less precisely focused than from monopoles, but that helps give it a sense of ambience and spaciousness.
Of course, all speakers, no matter their design, should deliver a uniform tonal balance (timbre), with a smooth midrange and reasonably extended highs and lows, clean dynamics, and accurate imaging. Imaging - an audio system's ability to reproduce sounds in a spatially realistic way - is perhaps the most difficult to get right. Since people are quite good at pinpointing sounds, recreating a recording's soundstage and accurately placing sonic images between two stereo speakers can be a formidable task. Bad speaker design or placement can draw attention to the speakers' location, destroying the illusion that you're in the space where the action is occuring or the music is being performed. Accurate imaging is even harder to achieve with a multispeaker surround sound system, and there's not even a consensus on what kind of imaging is correct.
Everyone agrees, however, that the front left/right and center speakers in a 5.1-channel system should provide precise front-stage imaging so that dialogue, singers, and musical instruments will occupy stable positions while sound effects move between the speakers. One way to achieve this is to use front speakers that have identical or similar driver elements, so they all have the same (or a similar) timbre and dispersion pattern.
When it comes to the subwoofer, which conveys the ".1" channel, everyone agrees that its sound should be omnidirectional -that is, you shouldn't be able to tell where the low-bass sounds are coming from. And since these frequencies are common to all channels, they should seem to come from all around you. So far, so good.