Reprinted with permission from Stereophile Magazine, Vol. 19 No. 4.

Dipoles

Audio Artistry Dvorak Loudspeaker System - Sidebar

By Shannon Dickson

The following "visual aids" illustrate how the Dvorak’s radiation pattern differs from that of other major speaker types. Fig.1 shows the polar response patterns of the three major speaker types plus that of the Dvorak/Vivaldi speakers. The diagram is divided into the three main frequency bands to illustrate the consistency, or lack thereof, in the dispersion pattern for each speaker over the full audible band. Note that the shaded areas represent approximate sound pressure level (spl) distribution in the horizontal plane, and don’t reflect changes in dispersion behavior around the transition region between the three frequency bands. Also, the rear wave of the dipole speakers is lightly shaded to denote its negative polarity.

The two most important things to note about the polar response of monopole box speakers (shown on the top row) are the frequency-dependent increase in directivity that changes the forward dispersion from about 250Hz through the midrange and treble, and the spherical radiation pattern in the bass. A common misconception exists that rear-wave radiation is unique to dipole speakers. Fig.1 clearly illustrates that monopoles also have a very prominent rear-wave spanning the lower frequencies, but with the same polarity as the front. It’s this change in directivity from omnidirectional in the bass to unidirectional in the higher bands that causes alteration in the overall sonic illumination of the listening room, emphasizing the bass region.

The second row of images shows the response of a bipolar speaker. As you can see, it has the same spherical bass-radiation pattern and low-frequency standing-wave problems as the monopole box designs, but the midrange and treble dispersions are fairly uniform, illuminating the room more evenly over a wide range of frequencies. However, the bipolar pattern still produces high levels of in-room reverberant energy.

The third group of images highlights planar dipole speakers. With a given on-axis spl, note the significantly lower levels of sound radiated to other parts of the room from all three frequency bands when compared to the other designs. Plus, the rear wave has reversed polarity, resulting in less low-frequency reinforcement. On the down side, the dispersion pattern becomes increasingly ragged at higher frequencies. These off-axis irregularities can generate overt colorations, degrade image quality, and increase speaker placement sensitivity. Fortunately, no radiation occurs 90^o off-axis, reducing side-wall and ceiling reflections and lowering the strength of the overall reverberant field.

The final row illustrates how the Audio Artistry dipoles maintain a consistent dispersion pattern through the bass and midrange frequencies. This uniform dipolar directional radiation, particularly unusual in a subwoofer, minimizes bass reinforcement and standing-wave excitation. In addition, the increasingly directional radiation in the midrange and treble reduces overall reverberant energy.

Perhaps it will help to understand how dipole radiation reduces low-frequency room interactions if we imagine a pair of hypothetical Dvorak main panels in proximity to the nearest rear wall, side wall, and corner on one side of a room (fig.2). (For clarity, I’ll ignore the effects of the other boundaries.)

The left and right dipole panels are labeled "L" and "R," and the listener position is at point A. If we substitute the rear and side walls with an imaginary mirror, a number of "phantom" image soundsources are created. Note that the dipole’s positive- and negative-polarity lobes switch positions in the phantom sources. The negative lobes from the phantom sources representing the rear wall and corner tend to combine with the forward lobe of the actual speaker, resulting in cancellation or attenuation. Reflections from the side wall behave more like a traditional monopole except that the off-axis reflections are significantly reduced in amplitude, even nulling at 90^o. Notice, too, that the nulled axis of the side-wall phantom speaker is aimed toward the listener. It’s this mechanism that helps reduce the reverberant field level and the amount of side-wall (and ceiling) reflections focused toward him or her.

Referring back to fig.1, if you replace the dipole with a monopole, it should be easy to visualize how the omnidirectional, common-polarity reflections from all surfaces combine with the direct sound, reinforcing the bass.

Fig.3 illustrates the effect of reducing the overall reverberant energy relative to the direct on-axis sound-pressure level experienced by the listener with either a dipole or monopole. In the case of a dipole, you could sit 73% farther away from the speaker than with a typical monopole or bipolar speaker before the direct and reverberant fields blended! Consequently, while some acoustic treatment is still a good idea, you’ll generally need less with dipoles than with box speakers.

--Shannon Dickson