Axiom Home Page Forums General Discussion Technical Questions Editor's Forum: Planar speakers, dipoles & more

A post from BigFoot_Lives in the Advice from Axiom Owners forum got me thinking about planar speakers, dipoles and bipoles.

"Planar" refers to any speaker that uses a flat diaphragm instead of a cone or dome as the radiating element. In conventional, non-planar speakers like Axioms, a rigid but lightweight cone (for midrange and low bass tones) and a dome (for higher frequencies) move rapidly back and forth to generate the sound pressure waves that strike our eardrums. The back waves from a cone woofer are either absorbed inside a sealed box (acoustic suspension) or funneled out a hole or port in the enclosure (bass reflex) to reinforce and smooth the deep bass response of the speaker system. (The back waves from tweeters and smaller cone drivers are usually absorbed in the speaker enclosure and do not contribute to the speaker's overall output. In the case of the Axiom M60ti and M80ti tower speakers, the tweeters are sealed so that no internal cabinet waves can affect them. The small woofers in the M60 and M80 are mounted in their own internal enclosures so as not to be affected by the bass drivers' internal cabinet waves.)

By contrast, a planar speaker uses a large, flat film diaphragm (often a mylar or plastic composite) stretched tight in a frame and usually suspended between two mesh grids that are electrically charged. As the audio signal is applied, electrostatic attraction and repulsion alternately move the flat film back and forth, generating sound waves that radiate to the front and the rear of the speaker.

Planar-magnetic speakers use a flat diaphragm as well, often a foil ribbon suspended between permanent magnets, or some variation of this principle. The audio signal is applied to the ribbon, and the positive or negative magnetic forces move the ribbon back and forth. These planar speakers are almost always dipoles, because the pressure waves radiated from the back of the film diaphragm are out of phase with those coming from the front of the panel. This becomes problematic with bass frequencies below 100 Hz, because the deep bass waves, being non-directional, migrate around the panel and cancel out those from the front. The liability of these dipole planar speakers is that no deep bass is possible unless a subwoofer or auxiliary conventional woofer is used. Moreover, because the range of movement of a flat film diaphragm is more restricted than the excursion of a cone driver, the maximum peak sound levels are usually somewhat limited. If pushed too far, the flat film or ribbon may be damaged.

The virtue of dipoles is that with careful placement and a single known listening position, the midrange and treble sounds from the rear of the speaker will be reflected from the back wall and adjacent room boundaries to supply an enhanced sense of spaciousness to the soundstage.

Conversely, some of these midrange and high-frequency sounds may cancel or interact in a manner that typically make imaging somewhat vague and indistinct.
Nevertheless, a well-designed planar speaker--electrostatic or planar-magnetic--may produce very smooth and neutral midrange and high-frequency response. I say "may" because there are plenty of good and bad examples of each type, just as there are with forward radiating cone-and-dome systems.

Don't confuse dipole speakers with bipoles. A bipolar speaker radiates sound in several directions--typically towards the front and the rear (or in other directions as well), but here the outgoing pressure waves at the front and the rear are in phase, so bass cancellation is avoided. Like dipoles, a bipolar speaker can produce greatly enhanced spaciousness for stereo listening, but again the downside is a lack of distinct and precise imaging. The latter liability makes them unsuitable for use as front speakers in a home theater system. On the other hand, bipolar speakers like the Axiom QS8s (which are really multipolar in nature) used in a surround-sound application can be extremely effective in producing a diffuse and enveloping sense of immersion in the surround sound field. Even dipoles, properly set up as surrounds (and that can be quite a challenge) are also capable of yielding a diffuse and enveloping ambience.