Definitive Technology BP8 Floor standing speakers
Definitive Technology says that its bipolar speakers are designed to create a spacious, natural soundstage, closer to that of a concert-hall performance than that produced by most conventional (forward-radiating) models. The BP8, the newest addition to that series, has two identical sets of speaker drivers in a single compact, unobtrusive cabinet. One pair faces forward conventionally, while the other faces the rear, so that its direct output is reflected from the wall behind the speaker enclosure. The two sets of drivers, which share a common enclosure volume and port, operate in phase (with their cones moving inward or outward simultaneously), so that their low-bass outputs tend to reinforce each other instead of canceling as happens with dipole (out-of-phase) radiators.
Each driver group consists of a single 5-1/2-inch polypropylene-cone woofer and a 3/4-inch soft-dome tweeter with magnetic-fluid cooling and damping. The crossover, at an unspecified frequency, is of the Linkwitz-Riley type to optimize the phase and frequency response through the crossover region. The enclosure was designed with a newly developed computer program and is said to rival the bass-loading characteristics of a transmission-line enclosure with a simpler and less expensive cabinet.
Another goal of the BP8's designers was to minimize diffraction, commonly caused by grille frames and external cabinet discontinuities, which can alter a speaker's imaging characteristics. The BP8 is a columnar design with rounded edges, just wide enough to accommodate the bass drivers. Instead of having a conventional grille, the entire cabinet (except for its top and bottom) is enclosed in a black (or, optionally, white) fabric "sock" that fits snugly around its full length and can be rolled down for access to the drivers. The top plate and the visible edges of the base are finished in either glossy black or genuine oak. The input connectors-multiway binding posts spaced to accept dual banana plugs- are in the bottom plate of the cabinet, which is slotted at the rear to accommodate the connecting cables.
We placed the BP8 speakers about 2 to 3 feet from the side walls and 18 inches from the front wall of the room, angled slightly inward, as suggested by the manufacturer. Their "raw" room response, averaged for both speakers, was unusually uniform across the full audio range, even in the bass region where room-boundary effects are usually quite prominent.
The close-miked response of the front woofer and its port (the rear response was identical) was flat within ±3 dB from 30 Hz to about 3 kHz. The composite frequency response, formed by splicing the woofer measurement to the averaged room curve corrected for room absorption at high frequencies (they overlapped for about three octaves, from 300 Hz to nearly 3 kHz), was flat within +3 dB from 30 Hz to 20 kHz. Quasi-anechoic MLS response measurements showed an overall rise of 5 dB from about 1.2 to 15 kHz, with a couple of ±2-dB variations along the way.
The high-frequency horizontal dispersion of the system, over a 45-degree angle to its forward axis, was typical of systems using a 3A-inch tweeter. There was less than a +2-dB change up to 10 kHz, above which the off-axis output fell by about 6 dB at 15 kHz and 12 dB at 20 kHz.
The system's sensitivity was 90 dB, as rated. At a standard 2.83-volt input (equivalent to 1 watt into 8 ohms), the woofer distortion was between 1.5 and 2 percent in the upper part of its range (200 Hz to 2 kHz), reaching 5 percent at 53 Hz and 10 percent at 30 Hz.
Definitive Technology refers to the impedance of the BP8 and its companion bipolar systems as "compatible with 8-ohm outputs." That characterization is probably justified by the system's relatively high sensitivity, which enables normal listening levels to be generated with less input power than many comparably priced speakers need. Nonetheless, we measured a minimum impedance of 3.2 ohms at 240 Hz and dips to approximately 4 ohms at several other frequencies. The crossover to the tweeter appeared to be at about 3 kHz, judging from impedance and group-delay measurements.
Single-cycle tone-burst measurements indicated that the BP8 has an exceptional ability to absorb high-power transients without damage or audibly offensive distortion. We were able to "bottom" the woofer cones at 100 Hz with an input of 785 watts, although they absorbed 1,100 watts at 1 kHz (the amplifier's maximum into the speaker's 5.5-ohm impedance at that frequency) without difficulty. The tweeter, like most, was not damaged by high burst levels at 10 kHz, and the amplifier clipped at 1,470 watts into its 6.7-ohm impedance.
Listening to music via the BP8 speakers produced, in addition to the expected performance, at least one surprise. Although solidly built, with internal bracing and 1-inch-thick high-density fiberboard front and back panels, the BP8 looks very compact, and, regardless of how it performs in the upper octaves, one does not normally expect a speaker of its modest proportions to fill the room with deep bass. Even when close-miked woofer measurements suggest extended bass response from such a speaker, it will seldom be capable of a really satisfying output level at the very bottom. After all, that is why we have subwoofers!
To get right to the point, the BP8 puts out enough clean sound in the 30-to 35-Hz region to create a room-filling, skin-tingling quality that one does not expect (and very rarely experiences) from an inconspicuous column and a pair of small cone drivers. To be sure, adding a good subwoofer can still make a real difference-but only with program material that contains significant deep bass in the first place. For most of what we think of as bass, down to 40 Hz or so, these speakers need no assistance.
Their imaging was excellent and often included the sense of space and "air" that is one of the desired benefits of a bipolar design. These speakers "grow on you" as you live with them (in contrast to some that outgrow their welcome rapidly). Considering its performance, modest size and cost, and ability to blend with almost any decor, we consider the Definitive Technology BP8 an outstanding value among floor-standing loudspeakers.
