I began my own DIY hi-fi hobby time in early 1954, after returning from draft duty in Korea. In that era, virtually all serious speaker systems were big, featuring 12” and 15” Ø woofers that were more efficient than most woofers in common use today. As a result, bass reproduction was always pretty solid; we moved a lot of air. And when more was needed we’d just sink 2 or 3 more Bozaks into a handy side wall. This was in the mono-era, so location and directionality aspects were of little concern. The trend to produce bookshelf-sized speakers came about soon enough, and generally curtailed bass output, but then Edgar Villchur (AR) announced his new “acoustic suspension” design, and those speakers could rival a floor-stander’s bass if you’d just sacrifice some efficiency. The new “AR size” speakers (and the competition that they inspired) materially eased the transition to stereo, and bass output never suffered because bigger power amps soon became pervasive. So sufficient and abundant bass was common until the mid-’80s, when the home theater craze came to market.
Cramming all of that “surround stuff” into a home theater room often caused decor problems, so some speaker systems got downsized, and subwoofers got popular. The “subs” were all highly optimized for bass below 100Hz, with cone suspensions having restricted range but long throw axial excursion (Xmax) capability. This tool was vital to deliver the separate low-frequency effects (LFE) channel of the movie media, otherwise known as the “boom track”. Bass accuracy was never the goal. The express intent was to simulate loud explosions, gunshots, and monster grunts. A single shared (blended) subwoofer was fully adequate.
The application of subwoofers for 2-channel stereo service has evolved more recently, partially in response to concerted efforts to render more realistic and authentic bass, but principally because collective advances in technology have now made that goal attainable. These developments include…
…the creation of Linkwitz-Riley 4th order (phase-coherent, non-inverting) active crossovers.
…the increased availability of multi-sourced low noise monolithic op-amp chips at low cost.
…the significantly improved response of “long throw” low bass subwoofers.
…consistent improvement in the performance of super-efficient Class D power amplifiers.
These assets now make it practical to produce more effective and reliable self-powered subwoofers, as well as the frequency-selectable active crossover controllers needed to define and isolate the deep bass passband that they serve. The possibility of providing accurate deep bass has never been more favorable, but the old mechanical constraints still apply. When the listening room is smaller than a public auditorium, it exhibits a Schroeder frequency* that’s too high to avoid the inevitable peaks and dips that arise due to resonant mode rebounds off the room’s planar surfaces. A traditional means of managing this problem is room treatment. Surface-mounted traps (padding) are added to absorb some of the excessive waveform bounce. The customary 2 inch thick broadband absorbers are ineffective at low bass frequencies, so it’s then necessary to use fatter 4 inch absorbers and/or large canister-style tuned bass traps to tame the low bass (< 100Hz) resonance. That decor is acceptable when the listening room is a dedicated “man cave”, but less tolerable in a shared condo living room.
When that’s the case, multiple subwoofers can be utilized to effectively achieve cancellation of the reflected modal bass over large portions of the listening area**. Two subwoofers will work well; more subs will work better. The subwoofers’ output will naturally be ~ 180˚ out-of-phase with the reflected modal bass, so effective partial cancellation will result when the opposing wavefronts converge.
The potential benefit conveyed by using a pair of widely spaced (along front wall) self-powered subwoofers can be appreciable. Their impact will always be advantageous, regardless of the bass capabilities of the main speakers. The variance implicit in room response will preclude full cancellation of low frequency resonance, but the audible improvement will be obvious. The use of wide-spaced subwoofers can dramatically upgrade the bass in any room—including rooms with big full range main speakers. In the latter instance, let the floor-standers handle all of the mid-to-upper bass, and assign the bottom 20Hz-80Hz passband to the more widely spaced subwoofers. (No matter how costly your top quality main woofers might be, good subs can handle the bottom better; it’s their specialty.) Set the external active crossover controller (e.g. Marchand’s XM66†) to split the passbands at 80Hz. (A lower frequency crossover is seldom beneficial; usually detrimental.) Then optimize the subwoofers’ phase angle and input gain control settings in the manner that’s described in our related white paper††. The critical bass octaves above 80Hz can then be reproduced without compromise, and the bottom bass can be programmed (in fixed ±1dB stepped increments, with ±5dB range on both channels) to provide an output level that best fits the genre of the selected source.
**Refer pp. 234-262 of Floyd Toole’s “Sound Reproduction”, 3rd edition (Routledge, 2018, ISBN 978-1-138-92136-8).
††Refer “On Optimizing Subwoofer Gain & Phase Angle…”