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…”
BG (January 2019)
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