On Other Ways to Woof…

By Bryan Geyer

JL Audo E-Sub e-110, 53 lbs.

“Hi-fi” is now in its 72nd year, and two channel stereo continues to be the dominant focus of those who enjoy music at home. Since the start, we’ve learned that good sound is best conveyed by utilizing different types of loudspeakers to reproduce select segments of the audible range—e.g., woofers for bass, tweeters for treble, maybe a mid-range driver too. More recently, we’ve also come to acknowledge that really deep bass (20 to 50Hz) might be that “bridge too far” for a traditional woofer. This prevails because the bottom bass is best achieved by using a relatively rigid, conically shaped driver + long, piston-like pumping strokes, whereas the upper bass needs a more compliant and responsive driver to effectively track rapid shifts in the 100 to 600Hz bass band. Big woofers that are tasked to cover both jobs simultaneously tend to sound muddy and sluggish, not “tight”. Or they might roll off, rather rapidly, below 40Hz.* The ideal solution is to gently separate the low bass from the main input by means of an external active (electronic) crossover network**. Start at a logical crossover point, preferably at some frequency that lies between 80Hz and 100Hz, and channel that bass, in progressively increasing doses, to independent self-powered subwoofers. The subs can then blend with the mains to create a uniform, phase-synchronized† wavefront that’s fully consistent with the source signal and capable of flat response to 30Hz before tapering to infrasonic cut-off. A low pass crossover latency of ~ one wavelength will naturally accrue due to the group delay inherent in this processing. The resulting time-of-arrival difference (10 to 12.5msec) falls well within the accepted fusion zone†† interval of ≤ 25msec when the subs and mains are equidistant. Cumulative delay will remain entirely within that boundary until the subs are spaced > 13 feet beyond the listener-to-main speaker radius. (E.g., if the mains are 10 feet away, keep the subs within 23 feet.) In view of this outcome there’s no need to consider digitally generated corrective timing (e.g., Mini-DSP, DEQX, et al), as any reasonable positioning of the subs will assure that net latency remains inaudible and within the verified fusion limit. Keep this purely analog stage exclusively analog!

Do verify (preferably by measurement) that the subs/mains are accurately phase-correlated, and that they are optimally balanced per the footnote† herein. Also confirm that both subs are placed inside the cited positioning radius. Minimize the allowed spacing when decor constraints permit. Just be aware that you have up to 13 feet available, free of significant performance compromise, if your preferred layout requires such separation.

External xover bypassed

This latter speaker system setup, wherein a pair of self-powered subwoofers handle the low bass while separate stereo main speakers handle the rest, can also yield other benefits. Consider…

Acoustics: The use of paired subwoofers can appreciably improve the overall room acoustics. Two subs, positioned in the front corners and flanked outside the mains, will propel a widely unified bass wavefront that’s approximately opposite in phase to the reflected back wall rebound. This will result in partial cancellation of that resonance, and it will mitigate the disruptive low frequency peaks/nulls that inevitably develop in home listening rooms. Improved clarity will result. Absorbent room treatment materials can also be of significant benefit, but the extra thick*† anechoic pads required to swallow low frequency reflections generally confines such stuff to man-cave environs.

Convenience: Big floor-standing full-range loudspeakers are not the sole solution anymore. Paired (or more) subwoofers, plus modest-sized mains, will present lots of layout freedom. Stand mounting the mains and spacing them several feet from the front wall will generally prove best, but a shelf mount†* is certainly feasible with a sealed enclosure; also with vented models if a port plug is utilized. (The plug will further benefit phase sync tuning.) Smaller main speakers are a lot easier to position (and live with) than big, dominant floor-standers, and subs like those in the JL Audio “E-Sub” series are easy to hide away in the front corners. You’ll like your options.

Bryan Geyer (April 16, 2020)

*There are very few full-range floor-standing loudspeaker systems with woofers that are able to match the performance of a good subwoofer at frequencies from 20Hz to 50Hz. Nearly all of the floor-standers sag from 40Hz down, which is not surprising, given the fact that they’re also expected to reach 600Hz to 1kHz or more. Conversely, good subs are intended purely for 20Hz to 100Hz bass, a demanding but restricted niche that they’re specifically designed to serve—and where they commonly excel.

**Refer “Optimizing Subwoofer Integration, Part 1”, at https://classicalcandor.blogspot.com/2019/02/optimizing-subwoofer-integration-part-1.html.

†Refer “On Optimizing Subwoofer Integration, Part 2”, at https://classicalcandor.blogspot.com/2019/03/optimizing-subwoofer-integration-part-2.html. This procedure details how to precisely (by measured means, with visual veracity) adjust the subwoofer’s input gain and phase angle controls to assure synchronous phase and optimal amplitude of the output at the prime listening position, at a designated crossover frequency. Both such adjustments are present on better subwoofers. (Lesser subs might substitute a simple 0˚-180˚ polarity inversion switch instead of fully variable phase angle adjustment, and that’s not acceptable. You’ll need a continuously variable 0˚ to 280˚ [as referred to 80Hz] control to assure optimal sub-to-mains phase sync.)

††The term “fusion zone” defines the accepted latency interval during which separate sounds (of near-equal amplitude) arrive close enough together to be perceived as the same sound. (Refer fusion zone, section 7.6.4 of Floyd Toole’s Sound Reproduction, 3rd edition [Routledge, 2018, ISBN 978-1-138-92136-8]). Note that this duration (cited therein as 30msec) is actually based on speech, which is regarded as a distinctly more stringent criteria than for music. Similar tests performed using Mozart compositions indicate that the fusion interval for music probably extends to 50msec or more (refer p.211-212 of Toole’s text). In this paper, I apply a maximum limit of 25msec (1/40th of a second) as the cited fusion zone interval; that’s quite conservative.

*†Absorbent padding will have to be ≥ 4 inches thick to have any appreciable effect in the lower bass region. Large drum-type “bass trap” canisters are also appropriate for managing low bass resonances, but their size and appearance is not compatible with most LR decor.

†*Shelf mounting of the main speakers against the front wall becomes somewhat more acceptable when they’re coupled with subwoofers because the most pervasive (most spherically propagated) lower bass frequencies have already been channeled to the subs. Setting a relatively high crossover point, i.e., 90 to 100Hz, can also help, but do verify that your subs are capable of relatively flat output to ~ 130Hz; some subs droop abruptly beyond 100Hz.