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The Tempest
By Scott C. Blaier
31 Mar 1996
The NHT 1259 Woofer and Bass Loading
The Tonegen/NHT 1259 woofer is an extremely well made, high quality, heavy duty woofer. It has gained considerable admiration on the Internet, and has a growing history of successful amateur designs, not to mention its critical acclaim as used in the NHT 3.3 loudspeaker system. It is distributed through Just Speakers, has a thick textured polypropylene cone and dust cap, heavy duty cast basket, large half roll rubber surround, extremely long excursion, and vented voice coil that can absorb 300 watts of power. There is a considerable amount of excellent information pertaining to this driver on the Internet, and I encourage you to research this impressive driver. And if you mention the Internet to Just Speakers when ordering, you can save quite a bit of money off their regular price for the 1259.
The 1259 is optimized for a sealed enclosure. This design uses a 2.8 cubic foot enclosure. As recommended, the enclosure was stuffed with approximately 48 ounces (approx.1 pound per cubic foot) of acoustic grade polyester fill to increase the enclosure's "effective volume"; this lowers the systems Qtc, box resonance frequency, and bass cutoff (F3). Depending on room interaction, this woofer is said to be capable of producing bass into the low 20 Hz. region with significant output, low distortion, and minimum phase disturbance.
Vifa P13MH Midrange
Many designers believe that one driver should cover the critical frequency range of approximately 300 to 3000 Hz. This keeps crossover disturbances and other problems out of the bandwidth where the ear is the most sensitive. However, there are relatively few drivers that can reproduce this wide range well. The Tempest utilizes the Vifa P13MH midrange. The P13MH is the midrange version of the Vifa P13WH woofer. The P13WH and P13MH are both excellent sounding drivers, and considering their relatively low cost, I believe they are among the best polypropylene drivers available in terms of performance to price ratio. Vifa's P13 drivers have not gone unnoticed by several "high-end" loudspeaker manufactures. Notable features include a cast magnesium frame, rubber surround and dust cap, and a robust, vented magnet structure.
The midrange's performance is almost identical to its woofer sibling, albeit with a bit more high frequency extension, at the expense of a slightly less precise high end rolloff symmetry. Its response 30 degrees off-axis is on par with its response on-axis to about 3.0 kHz.
The Tempest uses Vifa's 1.85 liter isolation chamber to subenclose the P13MH and prevent woofer interaction. The chamber (a thick, tapered, plastic cylinder) was filled with acoustic grade polyester to lower the resonance and impedance peak, and attenuate rear cone output so as not to interfere the driver's primary output.
Incidently, the fact that both woofer and midrange cones are both fashioned from polypropylene plastic helps insure they have similar "sonic signature" or "voicing". In addition, the two drivers look good together on the baffle. This is sonically insignificant, but adds to the design's aesthetic appeal.
Vifa D25AG-35 Tweeter
This Vifa aluminum dome is highly regarded, and is one of the best tweeters available today. Its rigid aluminum dome allows piston like behavior to the limit of human hearing. It has a relatively low, well controlled, fundamental resonance provided by an aperiodic subenclosure and ferrofluid damping fluid. It has a very high power handling capability if crossed over properly, and has excellent of-axis response. Its off-axis dispersion stands out among tweeters, as its performance 30 degrees off-axis is almost identical to its on- axis response to 20 kHz. A state of the art tweeter in all regards, and is used in commercial loudspeakers costing thousands of dollars. And at around US $26.00, I believe it represents an unparalleled value among tweeters.
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