Way Down Deep II ServoDrive Contrabass

ServoDrive Contrabass

Back in 1987, ServoDrive's belt-driven ContraBass subwoofer seemed to rumble onto the market out of nowhere. But the unusual device, with its pair of belts looped around a rotary-shaft DC motor to drive opposing 15-inch cones, had a literally field-tested progenitor. The Pachyderm 6 was founder Tom Danley's answer to a request from Cornell University for a sub-subwoofer small enough to fit in the back of a Pathfinder, for use in elephant research in Africa. (Elephants communicate with each other by vocalizing in a range that is subsonic to humans.) The ContraBass's ability to play deep and loud has helped it become a favorite of owners of theme parks, planners of military war games, promoters of rock concerts, and homeowners with a taste for explosive effects in the very bottom end.

ServoDrive recently updated the ContraBass to lower belt noise for home theater applications, and it was this "theater calibrated" version that I reviewed for this survey.

Frequency response at 80dB (Fig.9): 17–80Hz, +/-6.5dBl –10dB at 16.5Hz. Peak at 76Hz, with downward-tilted response below that at approximately 8dB/octave to 26Hz, flattening out to 17Hz.

Fig.9: Frequency Response vs. Output

Frequency response at higher levels (Fig.9): Response becomes noticeably more linear with increased drive level: At 90dB, 20–80Hz response is +/-3.5dB; responses at 100dB and 105dB playback levels require tolerance windows of +/-3dB or less. Response in the deep bass (20–40Hz) remains strong even to 120dB (not shown). Overall usable response limit (–10dB referenced to nominal level at 47Hz): 16Hz.

Dynamic range: Some minor power compression evident at the highest level shown (Fig.10), but virtually none up to 110dB. Very high peak output before audible distortion on Linkwitz shaped tone-burst tests (Fig.11), ranking second among the 12 subwoofers in the survey overall, and equaling the first-place Genelec HTS6 at 25Hz. Peak levels below 50Hz registered 4–6dB higher than the next best performer.

Fig.10: Power Compression

Fig.11: Shaped Tone-burst Peak Output

Total harmonic distortion: Unusual pattern: THD actually decreases as playback level increases, up to approximately 105dB, beyond which distortion resumes the more familiar pattern of increasing with increased playback level (Fig.12). Minor cabinet rattle noted on the 16 and 20Hz THD signals; none noted at higher test frequencies.

Fig.12: Total Harmonic Distortion vs. Frequency & Level

Sensitivity: 90.5dB/2.83V/m at 40Hz.
Impedance: Nominally 3ohms throughout the 15–100Hz range tested, dipping to a minimum of 3.1ohms at 15Hz. The ServoDrive will be a manageable load for most any modern power amplifier rated to deliver >300W into 4ohms.

While most speaker engineers I spoke with readily acknowledged the ingenuity of ServoDrive's system for converting rotary to linear motion, physics exacts a price: motor stiction and drive-train noise. Stiction shows up as roughness in the shaft's motion at very low signal levels—a "twitchiness" or "chunkiness" that originates at the commutator-and-brush level in all such motors. This results in the unusual rise in distortion at the lowest playback levels—the opposite of the usual trend. But this is one of the few subs in the world that actually gets better as you turn it up.

As for drive-train noise, I'd noticed a low-level hhh sound riding atop otherwise pristine sinewave sweeps during the pure-tone measurements. During the listening tests, I thought I detected a gauzy layer of belt noise diminishing the vividness of my test tracks, though the effect always seemed to reside near or below the threshold of audibility.

The ContraBass was one of only two subs in the survey able to summon enough subsonic slam to make the T. rex's final clomp at the end of "Jurassic Lunch" (Figs.A1 and A4) sound and feel like the real, shuddering deal. The air simply throbbed. With the main speakers disconnected, there was no sign of power compression—that is, every dB of increased drive level produced a dB more peak level—up to 114dB, beyond which the unit began to emit cries of distress. The waterfall plot (Fig.A4) confirms my listening session notes on this passage: The ServoDrive belted out more energy in the pants-flapping sub-20Hz range than did any of its competitors.

The depth charges in U-571 (Figs.B1 and B4) came across with a concussive, hull-twisting fury to match the gathering devastation depicted onscreen. With infrasonic-rich, sub-only peak levels hitting 110dB, they made for a gut-gripping movie experience. When I turned these up to 113dB (1dB short of their maximum level) and reconnected the main speakers, I had to resist the occasional urge to dive under my lawn chair or run for cover.

I then backed down the volume by 10–15dB and repeated the clip, and three factors conspired to drain realism from the experience: the ServoDrive's idiosyncratically weakening response; the ear's fading sensitivity to deeper bass at lower playback levels; and something hearing scientists call acoustic reflex—the natural contraction of small muscles in the middle ear during and after exposure to loud sounds, with the result that subsequent inputs sound less loud. The first of these effects can clearly be seen in the downward tilt in the ServoDrive's 80 and 90dB frequency-response traces.

I was eager to hear how the belt-drive approach would handle the sustained artillery barrage in my Pearl Harbor selection (Figs.D1 and D4). I chose this clip in part because the big gun discharges follow each other so closely in time that any energy storage (ringing) in the system tends to fill in the spaces between blasts, creating a turgid, sludgy effect. Such "articulation loss" is inescapable in a home theater, as all rooms reverberate in the bass (the vast majority excessively so). Conducting the tests outdoors allowed me to pose and answer the question, "Is any of the sludge caused by the subwoofer itself?"

The ServoDrive unit—belt drive and all—did not produce more audible blur on this torture track than the small, so-called "fast" drivers, such as the 12-inchers used in the Wilson WatchDog and Genelec HTS6 reviewed in Part 1. If you sense a thick, out-of-focus character to the bass coming from a ContraBass playing near or above cinema playback levels, it's almost certainly room-related.

The ServoDrive ContraBass Theater Calibrated also came up huge on Black Hawk Down (Figs.C1 and C4). On this track, about 35 minutes into the movie, it was rivaled only by the coffin-sized Genelec. The 21-inch Bag End could manage 18Hz, but by now the careful reader should need no reminding that deep reach can be a shallow accomplishment if it can't muster enough level—at least 100dB at the listening position—to be perceived by the audience.

The final cannon discharge in the 1812 Overture was solid, impactful, and clean, and the big bass-drum smack in Holst's First Suite was crisp and compression-free up to 117dB. I expected to hear some drive-train grunge overlaying the exposed, slip-slidey fretless bass on Béla Fleck's "Flight of the Cosmic Hippo," but I never did.


ServoDrive ContraBass magnetically shielded, passive subwoofer in cabinet of 14-ply, 3/4"-thick Baltic birch plywood
Drivers: 2 opposed 15" cones of epoxied pulp driven by rotary DC motor via belts, 2 opposed 18" flat passive radiators
Frequency response: 14–125Hz, no tolerance window specified
Inputs: Neutrik Speakon
Power handling: 200W continuous, 500W for peaks (<15 seconds)
Finish: black epoxy paint
Dimensions: 37" x 22.5" x 18" (WxHxD)
Weight: 120 lbs
Price: $5000

ServoDrive, Inc.
(847) 724-5500