Why is good, deep bass so difficult? - Myths and their Busters

The Rythmik Direct Servo Feedback Subwoofer system is a very sophisticated design (patented, for what that’s worth)---many subs are nothing more than a woofer and an amplifier in a box---that accomplishes a number of goals for it’s designer, PHD Brian Ding. For instance, all loudspeaker driver voice coils heat up with use, slightly altering the driver’s electrical characteristics. The Rythmik DSF compensates for changing woofer coil temperature, keeping the driver’s electrical characteristics consistent. All woofers, in fact all drivers, would benefit from that.

All drivers have a rise time (when hit with a signal), and a return to "rest" capability (when the signal stops), ideally not over-shooting the "at rest" position of the voice coil within the magnetic field of the driver’s motor when attempting to do so. Servo-feedback systems, found in the woofer columns of the Infinity IRS and RS-1b (which I use to own), have long been known for affording superior inter-transient silence, another term for non-overshoot driver performance. There are a few ways to achieve high performance in that regard, servo-feedback being a cost-efficient means of doing so. The Rythmik subs excel at that performance characteristic. Just as the Eminent Technology LFT driver has been described as "quiet" (very low "noise"), the Rythmik subs have a very high degree of inter-transient silence. They are unusually good at blending with planar loudspeakers, sounding "leaner" (no bloat) than most other subwoofers.

The Rythmik sub designed in collaboration with GR Research’s Danny Richie, the only OB/Dipole sub in the world featuring servo-feedback woofers, is State-Of-The-Art. A pair of those subs (usable up to 300Hz), combined with the Eminent Technology TRW-17 Rotary Subwoofer (designed to be used for reproducing 20Hz and below!), greatly exceeds the capabilities of any other subwoofer in existence. Not cheap (the TRW-17 especially), but cheaper than the woofer section of the $100,000 and above loudspeakers available to the well-heeled.

Rythmik owner/designer Brian Ding has a very detained explanation of his designs on the company website, for anyone interested enough to read it all. Warning---it’s quite technical!

Not a recording studio, but Sterling Sound Mastering in NYC have three pair of Rythmik F15 Direct Servo Feedback Subwoofers in their monitor systems, each with a 15" woofer. Killer bass. Rythmik owner/designer Brian Ding has been asked about the question of woofer diameter versus "speed", and his answer is that his 8" and 12" woofers are no "faster" than his 15", but that the 15" has higher maximum SPL output. Of course, the woofers in his Rythmik subs are servo-feedback controlled.

I have both 12" and 15" Rythmik Subs, but the woofer size is the least important difference between the two. The 12" are used in pairs mounted in Open Baffle H-frames, the 15" in a 4cu.ft. sealed enclosure. OB subs are very different sounding than both sealed and ported, for a number of reasons I won't go into here, but the point is the size of their woofers is not responsible for the difference in sound between the two.

To state "a 10" woofer will not keep up with a 6" midrange. Too much mass." is to oversimplify the situation. For one thing, different 10" woofers and 6" midrange drivers have differing amounts of mass; there are some 10" woofers with less moving mass than some 6" midrange drivers. But more importantly, the moving mass of any given driver is only one factor determining it’s "speed"---the size of the driver’s motor (magnet) is a huge factor. A driver with higher moving mass and a bigger motor can outperform a driver with lower moving mass but a smaller motor. By the way, the perceived "speed" of a driver is more a matter of how fast it stops moving when the signal does, and returns to "rest", than it is of how fast it starts moving. The cone of a dynamic driver not stopping when it should is called overshoot. As Newton’s Laws of Motion state, "A body in motion tends to stay in motion"---my own over-simplification!

Likewise, the statement "They must use very stiff suspensions to bring them back to neutral position after every excursion" is not universally true. The stiffness of a suspension is only one factor in the design of any driver, one needing to be balanced against other factors. Some very high performance woofers have stiff suspensions, some don’t. Acoustic Suspension designs (sealed enclosures) require suspensions with far less stiff suspensions than do Bass Reflex, for instance.

Woofers ARE often placed in MDF enclosures, but they don’t have to be, and sometimes aren’t. Baltic Birch plywood is a popular material used by some makers of high-performance subs. That plywood is far stiffer than MDF, it’s resulting resonance at a higher frequency than that of MDF, optimally far above the x/o frequency of the subwoofer.

"Massive objects resonate at frequencies that are hard, very hard to deal with". Sorry, also not necessarily the case. Internal bracing very easily and effectively deals with subwoofer enclosure-wall resonances, the braces pushing those resonances above any frequency the sub will be called upon to reproduce, reducing their audibility to below perceptible levels.