Optimizing Subwoofer Integration, Part 2

On Optimizing Subwoofer Gain & Phase Angle Using an External Active Crossover Control, e.g., Marchand XM66

By Bryan Geyer

Objective: To optimize each subwoofer’s input gain and phase angle adjustment, at a stated crossover frequency, in a manner that optimally complements mini-monitor main speakers. (I use Spendor’s Classic S3/5R2 speakers, as derived from the BBC guideline).

Conditions: Set subwoofers for internal crossover off (bypass mode), polarity = 0, and phase angle = 0˚. Set the external crossover controller for your chosen frequency, with the polarity switch (XM66 labels this control “phase”) at +, Low Cut and Sum off, Hi-Pass = 0, Lo-Pass = 0. The “Damping” control setting is optional; see below.

Position a microphone + step-up transformer, on a stand, at listening position. Monitor mic output via a AC millivoltmeter. I use a common low impedance dynamic microphone (Shure’s SM57-LC, plus their related A85F step-up transformer), and a Instek GVT-417B millivoltmeter with (old-style) analog moving cursor display; it makes the null obvious.

Input a sine wave signal (I use a Instek GAG-810 generator) of 1 Vrms at the crossover frequency. Use the system’s main volume attenuator to set the final output SPL, and…

(1) Set the external crossover’s “Damping” control as desired. I prefer damping = +2. This introduces a mild, localized +2 dB response bump at the selected crossover notch.

(2) Set both subwoofers for power off. Drive both main speakers to a high SPL (approx. 82-86 dB, C-wtd.) at the chosen crossover frequency, using the system’s main volume attenuator. Take note of the exact position of that control knob; set same in next step.

(3) Set left channel subwoofer for inverted polarity. Then turn on that subwoofer to output the crossover signal from the left channel low-pass output, using same volume setting as above. Alternately adjust left subwoofer input gain and phase angle controls to attain minimal SPL. The consequent null will display on the millivoltmeter as a very distinct notch. When done, restore normal polarity setting to left subwoofer.

(4) Repeat same process for the right channel subwoofer; left subwoofer power off.

(5) When finished, assure that both subwoofers are properly reset for normal “auto-on” operation; also for normal (not inverted) output polarity.

Summary: This procedure assures that output from each subwoofer will be optimally phase-coincident with the combined output from both main speakers at the assigned crossover frequency and specified listening position. Also that the individual subwoofer-to-main speaker output ratio will be ~ 2:1 (subwoofer up by +3 dB). Extended listening experience indicates that this 2:1 weighting is aurally optimum with most programming. To accommodate special exceptions (or satisfy alternate bias), use the XM66 lo-pass and hi-pass stereo level controls. They’re all stepped (±1 dB/step, with ±5 dB range), so it’s easy to alter or restore (and visually verify) this baseline 2:1 output ratio when desired.


This method for setting subwoofer gain and phase angle is based on the approach promoted by ace subwoofer veteran Barry Ober, a.k.a. The Soundoctor.”  We differ on some details and means (Barry doesn’t utilize any instruments), but this classic inverted polarity/nulling concept works well, and it’s more accurate than other alternatives. The key advantage stems from the fact that this procedure simultaneously combines the sonic output from both sources (mains + sub) at the same time that both adjustments are applied. This simulates actual use, and encompasses the impact of room resonance. An abrupt and distinct SPL cancellation will occur when the converging bass waveforms are of equal amplitude and opposing polarity (phase shifted 180˚) at the monitored location and frequency. Adjust subwoofer input gain and phase angle to attain minimal output (maximum metered null). Later, after normal polarity of the subwoofer has been restored, this same adjustment will then assure that the crossover wavefronts exhibit coincident phase and maximum SPL.

The normal guideline for accomplishing this process suggests setting each channel independently, by matching one subwoofer against one main speaker at a time. This is instinctive, and it will assure the desired phase matching. But it will also yield a precise 1:1 subwoofer-to-main speaker output ratio, and that’s not aurally appropriate with most programming. The low bass will clearly need ~ +3 dB more emphasis to sound optimally balanced. While it’s always possible to increase the subwoofer weighting later, simply by advancing each subwoofer’s input gain, it’s more elegant (and likely more accurate) to optimize this gain adjustment at the same time that the phase angle matching is done. And it’s quite easy to do this—just phase-null each subwoofer’s output against the sound emerging from both main speakers, rather than from just one main speaker. That will yield a 2:1 individual subwoofer-to-individual main speaker output ratio, and a 2X aural power boost is = +3 dB.

This procedure will accomplish close phase matching of the subwoofer-to-main speaker outputs at the crossover frequency, at the listener position*. However, an inherent time-of-arrival disparity will remain because the related low-pass filtering entails group delay on the order of some 12 - 16 msec. in the crossover region, so subwoofer output will lag the main speaker by about one wavelength (equiv. 13.5 - 18 ft.). Regardless, after the two signals are accurately phase-matched at the listener site little evidence of the offset will be apparent. Its effect is obscured because the delay is still well short of the known fusion interval (30 msec.) that acousticians identify as the minimum interlude needed to discern separate sounds as separate events**. The impact will be perceived primarily as a small boost in the low bass response. In sum, this time-of-arrival disparity is too brief to be of practical concern.

The best way to re-synchronize time-of-arrival within the context of a two channel stereo system would be to digitally delay the main speaker outputs. The circuitry required to implement that delay isn’t commonly provided in a subwoofer crossover control unit. While there’s little apparent need to institute this “sync fix”, it could prove useful in special situations, e.g. when the subwoofers must be spaced at longer distances from closer main speakers, thereby increasing the subwoofer offset delay.

*Boundary limitations implicit in most home listening rooms make it unrealistic to expect wide area phase matching over an extended low frequency range without adding extensive acoustic treatment. Regardless, this reality should not be construed as a significant shortcoming—refer 4.8.1 and 4.8.2 of Floyd Toole’s opus Sound Reproduction, 3rd edition (Routledge, 2018, ISBN 978-1-138-92136-8).

**Per Toole, refer fusion zone,” section 7.6.4 of “Sound Reproduction,”  3rd edition.

BG (December 2018)

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Meet the Staff

Meet the Staff
John J. Puccio, Editor, Publisher, Reviewer

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on the Big Jon and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.
Karl W. Nehring, Contributing Reviewer

For more than 20 years I was the editor of The $ensible Sound magazine and a regular contributor to its classical review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simpleminded, but I know I appreciate reading reviews by others that do the same for me -- point out recordings that I think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises an Arcam CDS50 CSD/SACD CD player, Goldpoint SA4 Passive Preamp, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. I also do a lot of listening while driving in my 2016 Acura RDX with its nice-sounding ELS Studio sound system through which I play CDs (the ones I especially like I rip to the Acura's hard drive so that I can listen to them whenever I want) or stream music through the system using my cell phone. For more casual listening at home when I am not in my listening room, I often stream music through the phone into a Vizio soundbar system that has remarkably nice sound for such a diminutive physical presence. And finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom Bluetooth speaker for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can't imagine life without music and I am humbly grateful for the technology that enables us to enjoy it in so many wonderful ways.
Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

William (Bill) Heck, Contributing Reviewer

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to that classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Recently I’ve rebuilt--I prefer to say reinvigorated--my audio system, with a Sangean FM HD tuner and (for the moment) an ancient Toshiba multi-format disk player serving as a transport, both feeding a NAD C 658 streaming preamp/DAC, which in turn connects to a Legacy Powerbloc2 amplifier driving my trusty Waveform Mach Solo speakers, supplemented by a Hsu Research ULS 15 Mk II subwoofer.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

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"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa