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.

Discussion…

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)