Three Woofers in a BalancedForce TriLinear Configuration


Three woofers in a BalancedForce configuration.

Chances are you've read or heard at some time that adding a subwoofer to a system built around electrostatic speakers is nearly impossible. Although that's not necessarily true (and we have the subwoofers to prove it), the idea is grounded in experience. For many years subwoofers and electrostats did not blend very well. Turn on the sub and maybe you got the deeper, stronger bass you were looking for but that wonderful electrostatic snap and midrange clarity were, if not gone, now smeared and disconnected.

Descent i subwoofer X-ray view.

Our goal, when we began our subwoofer projects, was to deliver the good stuff – deep, tight, powerful bass – and none of the bad. We wanted to build subwoofers that, when combined with ultra-fast electrostatics or conventional cone speakers, would blend seamlessly to the very bottom of the frequency extreme. Descent i is the full realization of years of engineering by the MartinLogan team.

The most important of the many elements contributing to this achievement are Descent i's TriLinear–, BalancedForce™ driver configuration and its advanced triple-servo monitoring and control system. Three identical drivers – high-performance, high excursion 10-inch aluminum-cone units in this case – are mounted precisely 120 degrees apart from each other in the cabinet. As a result, their reaction force vectors exactly cancel, virtually eliminating cabinet resonance and vibration. Gone with them are the cabinet-radiation coloration and bass blur that afflict even the best conventional subwoofers. The triple-servo system, meanwhile, ensures that the output from the drivers themselves – the output we want – is always an absolutely faithful replica of the audio input signal. It's the power of a charging rhino expressed with the nimble athleticism of a cheetah – performance unsurpassed in any category and unrivaled overall.

No matter how well a subwoofer performs on its own, ultimate satisfaction depends on seamless integration with the rest of the audio system. With that in mind, we've outfitted Descent i with easily accessible top-mounted controls and with LFE and two-channel inputs that allow separate optimization for home-theater and stereo operation in the same system. Descent i is a truly no-compromise subwoofer, regardless of application.

Aluminum cone woofer.

Rigid Aluminum-Cone Diaphragms

Each of Descent i's high-resolution drivers combines an extremely rigid 10-inch aluminum-cone diaphragm with a high-intensity magnet structure to achieve huge excursion and output capability without sacrificing detail. By eliminating cone flexure, even at long voice-coil excursions, aluminum diaphragms help our advanced-technology bass drivers achieve the very low distortion necessary for proper blending with MartinLogan electrostatic speakers.

TriLinear™ Driver Array

Descent i has three 10-inch aluminum-cone drivers, each weighing 12 pounds and possessing a maximum peak-to-peak excursion range of 1.2 inches. That works out to a total displacement of 30 cubic inches – typical for most high-quality subwoofers. Descent i has three of these drivers, however, yielding the equivalent acoustic power of a 36-pound driver with a maximum peak-to-peak excursion of 3.6 inches and a total displacement of 90 cubic inches. And because the thermal load is distributed over three voice coils instead of concentrated in one, the system is less influenced by to temperature-induced compression. The single-driver alternative would be an extraordinarily robust 18-inch unit, which by virtue of its size and very long excursion would inevitably generate considerably higher distortion. Descent i enjoys all the advantages of the smaller, more refined drivers while achieving the output capability of a huge 18-inch driver.

BalancedForce™ Driver Alignment

Conventional subwoofer enclosures vibrate in reaction to driver activity, creating a secondary and highly colored sound source. In extreme cases, the cabinet may even dance on the floor. The typical expedient of adding mass can make the problem less obvious, but it doesn't really make it go away. Resonant bass smear is inevitable unless cabinet vibration is drastically attenuated. MartinLogan's BalancedForce design places multiple identical drivers in exact opposition to one another, so that their forces on the cabinet almost perfectly cancel.

Employing a novel application of the BalancedForce principle pioneered in the original Descent, Descent i locates the three drivers of its TriLinear™ array precisely 120 degrees apart on the cabinet. Their reaction force vectors exactly balance at the center point for near-total cancellation, reducing spurious cabinet radiation by as much as 25 dB (a power factor of more than 300:1).

Triple-Servo Correction System

A highly sophisticated triple-servo control system for each driver compares input to output and applies automatic electronic correction for any deviation from the original signal. These advanced servos combine with the drivers' inherent linearity and their BalancedForce™ reaction-canceling alignment to yield the cleanest, most accurate bass imaginable, exhibiting absolute minimum distortion.

Descent i connection panel.

Comprehensive Inputs and Outputs with Simultaneous-Connection Option

Descent i provides a direct, unfiltered LFE input (RCA and balanced XLR) for connection to surround preamp/processors and receivers with built-in subwoofer crossovers as well as line- and speaker-level stereo inputs that connect to its internal crossover. The LFE and stereo inputs can be connected simultaneously with separate level and crossover settings for each, enabling precise optimization for home-theater and stereo music playback in the same system. For those who desire a multiple subwoofer setup, line-level outputs include a mono jack that carries the sum of all inputs. Further, stereo outputs pass a signal from the stereo inputs either directly or through switch-selectable high-pass filters.

Top-Mounted Controls

It's a lot easier to dial in exactly the right settings when you don't have to get down on your hands and knees to reach the controls. Descent i's top-mounted controls, discreetly hidden beneath an aluminum cover inset into the cabinet, take the drudgery out of performance tweaking.

Descent i control panel.

Precision Low- and High-Pass Crossover Filters

Descent i offers six switch-selectable low-pass crossover frequencies between 30 and 80 Hz, implemented with filters precision-designed for optimum attenuation slope and minimum group delay. This ensures cohesive integration with smooth response through the crossover range with any type of main speaker. The similarly designed high-pass filters on the line-level outputs can be switched between 40- and 70-Hz settings or bypassed. An optional internal module will be available with low-pass filtering custom tailored to the MartinLogan CLX full-range electrostatic loudspeaker.

Custom CLX / Subwoofer Filter

Seamless blending between a subwoofer and full-range loudspeaker is, at the best of times, challenging—and when you're dealing with a no-holds-barred reference level product such as the CLX there is no room for error. The addition of an optional custom 'CLX filter' to the Descent i and Depth i subwoofers takes the headache out of achieving superb low-frequency performance.

The optional Descent i/Depth i CLX filter board contains an optimized low pass filter that precisely matches the 56Hz low-frequency roll off of the CLX. Once this custom board is installed inside of your subwoofer, setup is only a matter of adjusting the phase and level of your sub.

This custom filter is applied only to the left and right inputs of your subwoofer. The sub's LFE input remains unfiltered, which allows surround preamp/processors to send low-frequency information for the center, surround, and LFE channels without compromising the performance of your CLX.

25- and 50-Hz Level Controls

At very low frequencies, all rooms exhibit a phenomenon known as room gain, which produces a mild bass boost. The smaller the room, the higher the frequency at which this starts. Descent i's ±10-dB 25-Hz level control enables you to compensate for excessive room gain by cutting output in the 20- to 30-Hz range, or, if you prefer, to accentuate the feeling of deep-bass energy by augmenting those frequencies. A similar adjustment centered at 50 Hz provides acoustic energy control at a frequency where room resonances often create undesirable peaks or suck-outs.

Full-Rotation Phase Switch

Descent i's phase control can be switched from 0 to 270 degrees in 90-degree increments, allowing precise adjustment for optimum blending with the main speakers.

Subwoofer level control.

Extended-Range Master Level Control

Crave a little heavy-metal thunder from time to time? Ordinary level controls top out at '10'; ours goes to '11'!

Advanced Proprietary Vojtko™ Switching Amplifiers

Descent i's three amplifiers – one for each driver – belong to recently developed class of switch-mode designs that incorporate a variety of innovative techniques to reduce noise, distortion, and heat to the lowest possible levels. In particular, the switching frequency is much higher than in typical subwoofer amplifiers of this type. Each amplifier can produce a true 250 watts continuously, 700 watts peak, for a total of 2,100 watts peak, with vanishingly low distortion at all levels.

Sealed Cabinet with Asymmetrical Driver Chamber

Descent i's sealed cabinet eliminates port and passive-radiator resonance's and minimizes low-frequency group delay. And its internal asymmetry breaks up standing waves within the enclosure to further minimize resonance and coloration.

ETC spikes.

Asymmetrically Configured Energy Transfer Coupler Spikes

Descent i is supplied with sturdy, 3/8-inch ETC™ spikes, which can be used in place of the standard feet to enhance stability on thick carpets or to create tighter coupling between subwoofer and floor. The spikes are arrayed asymmetrically in a configuration designed to minimize the chance of grille damage if the cabinet is tipped.

Aesthetically Extraordinary (Yet Practical) Cabinet Design

Let's face it – most subwoofer cabinets are just nicely finished boxes. That leads people to hide their subwoofers in corners behind plants or to buy models that look like end tables. Descent i's cabinet is the anti-box, with a unique architectural look that is both visually engaging and easy to integrate into a room. It features a selection of wood finishes for the top, which enable the Descent i to blend into any décor. The cabinet body, meanwhile, has a durable, black tough-coat finish, with rubber bumpers at the bottom to minimize damage from stray impacts, such as vacuum cleaners and errant footsteps. Durable, crushproof grilles complete the package.

Descent i, Depth i, and Grotto i.