Stereo Integrity SQL 10" D2 (Series 2)

Overall summary

In this sample and under these test conditions, the Stereo Integrity SQL 10 Series 2 shows unusually high distortion for a modern 10 inch subwoofer, and it does not scale benignly with drive. The 1 V baseline already sits around 30 percent THD at 20 Hz, with both H2 distortion and H3 distortion wandering rather than collapsing to a low floor. At our standardized near-limit pass, distortion at 20 Hz remains roughly 25 percent and is dominated by H3 distortion below about 35 Hz, which aligns with the very messy large-signal behavior seen in BL symmetry and inductance variation.

Suspension and inductance limits are similar, near 19 to 20 mm one way on this sample. During the LSI run, the analyzer aborted the run due to the voice coil thermal protection limits being hit after about five minutes due to a 60 °C rise in voice coil temperature over ambient. At that moment the cone was at 20.62 mm one way and input power was only about 480 watts, roughly half of the 1000 W RMS claim. None of the 30 or so other subs we tested tripped this protection under the same procedure, which is unusual for a product marketed around high xmax and power and carrying the SQL moniker for ‘Sound Quality Loud’. Extrapolating the BL curve suggests a potential BL 70 percent in the mid-20s, but measured symmetry is extremely poor, probably the worst we have seen in this test batch, actually, so we cannot verify the advertised 28.4 mm based on a completed run.

In practical terms, this is not a clean low-frequency driver in our setup. If you keep asks modest and avoid strong low-frequency boost, it will make bass, maybe even a good amount of it, but the measured distortion profile and the early thermal cutback leave a lot to be desired for accurate reproduction and high output. Infinite baffle is not advised due to the low Qts as well as the elevated distortion in low frequencies, where IB users care most."

Manufacturer's suggested use case

Stereo Integrity positions the SQL line as a “Sound Quality Loud” subwoofer intended to offer strong bandwidth and output from smaller enclosures. For the Series 2 SQL 10, marketing materials cite an overhung motor they call SGLC with a 2.75 inch voice coil. They state the presence of an aluminum Faraday ring to reduce inductance, and a stiff paper cone with a tall roll rubber surround. The published numbers call out 1000 W RMS, a one-way linear xmax of 28.4 mm, and small recommended enclosures, including a sealed 0.5 ft³ option and a ported 0.6 ft³ tuned to 30 Hz. Marketing copy also suggests very low F3 capability and highlights transient attack and symmetry benefits from the motor and suspension package.

Our suggested use case

In small sealed builds where enclosure volume is the hard constraint, the SQL 10 Series 2’s cold large-signal parameters do show that it can be used in unusually compact sealed enclosures. On this sample, the published 0.5 ft³ lands near Qtc ≈ 0.496, and about 0.175 ft³ yields ~0.707, so it can physically work in very tight spaces with simple sealed implementation. I would still probably recommend sticking to 0.5 cubic feet as I cannot picture needing less than that considering its depth, and it offers closer to critically damped QTC (0.500) over flat maximum amplitude response (0.707), which is nice.

That said, I do cannot in good faith recommend it for pretty much any use. Baseline distortion is already extreme at 1 V, about 30 percent THD at 20 Hz, and at our standardized near-limit pass it remains roughly 25 percent at 20 Hz with H3 distortion dominating below ~35 Hz and H2 distortion around ~5 percent near 40 Hz and up. Large-signal behavior explains the result, with poor BL symmetry, a CMS 50 percent point and an Le 17 percent point both clustering near 19 to 20 mm one way, and inductance variation likely driving the H3 distortion growth. The BL run terminated on Klippel thermal protection about five minutes into the test, during a gradual level ramp, at 20.62 mm one way and roughly 480 W, a condition none of the ~30 other subs in our batch triggered under the same procedure.

Infinite baffle is not advised given the low Qts and the elevated distortion right where IB listeners care most. If someone insists on using this model and basing it on the measurements extracted with this sample, expectations of clean output need to be modest at best and the cleaner path to headroom would be additional cone area, not pushing a single unit harder.

Testing and linearity limits vs. what is advertised

What it took to reach our high-level sweep limit, and how that compares to the published specs.

High-level sweep rule: Set just under the BL 70 percent point from LSI

High-level sweep limit for this sample: 40 volts

Approximate electrical power at that limit at 20Hz: ~400 W into its 4 Ω nominal load (dual 2 ohm wired in series), real power varies with frequency and impedance volts

Rated power (published): 1000 W RMS

Power used to hit the standardized limits in free air, relative to their xmax rating free air: ~40 percent

Claimed Xmax vs. measured at BL 70%: 28.4 mm claim vs not resolved before thermal abort. Extrapolation suggests the mid to high-20s, but the BL curve is highly asymmetric, so this is uncertain and cannot be counted as verified. At the thermal abort, excursion was 20.62 mm one way, which is only ~72.6 percent of the 28.4 mm claim and already past the CMS 50 percent (≈19.7 mm) and Le 17 percent (≈19.1 mm) thresholds on this sample.

Xmax @ 50% Cms: 19.7 mm, only ~69 percent of the claimed 28.4 mm

Xmax @ 17% Le: 19.1 mm from 17% Le(x)/Le(i) variation on this sample, only ~67 percent of the claimed 28.4 mm xmax

Manufacturer suggested sealed enclosure size (and its resulting QTC): 0.5 ft³ results in a QTC of 0.496

Required sealed enclosure for 0.707 QTC: 0.175 ft³

Xmax @ 50% Cms: 19.7 mm, only ~69 percent of the claimed 28.4 mm

Overall performance snapshot

This is our subjective interpretation of the objective data. How we derive these scores can be found on the home page of the testing section.

Distortion & frequency response - TRF measurements

Method recap: Method recap: Nearfield mic positioned at 1/10 the cone diameter plus 2 inches, on-axis. Response measured to 1 kHz and THD to 500 Hz, both with 1/6-octave smoothing. One baseline sweep at 1 V, one high-level sweep at 40 V set by the standardized rule of just under 70% BL, which achieved ~18.7mm of excursion at 20Hz. Distortion is shown both as percent and by harmonic. All observations tie back to LSI behavior on this sample

At 1 volt - baseline

In this sample, the 20 Hz point sits around 30 percent THD, which is the highest we have measured at baseline for any subwoofer in this testing batch. Both H2 distortion and H3 distortion features are visible up through the passband rather than collapsing to a low floor above about 40 Hz. The response shape is not especially unstable at 1 V, but the distortion content never clears in a way that would justify a use for an SQ based installation, especially in the lower frequencies. This baseline is the first sign that the ceiling will be set early by non-linearities that are not primarily suspension driven.

At high level voltage (40 volts)

Below about 35 to 40 Hz, distortion growth is led by H3 distortion. At 20 Hz we measure roughly 25 percent THD, with the lower frequencies clearly dominated by H3 distortion. Moving upward toward 40 Hz, H2 distortion becomes the larger component at roughly 5 percent, which indicates an even-order contribution that is consistent with the significant BL asymmetry seen in LSI. There is no convincing “collapse” to a clean region in the 40 to 120 Hz band at this drive level. The distortion signature matches a system in which the motor’s position and current dependencies, together with asymmetric BL, are setting the ceiling rather than a simple suspension knee.

Delta - 1 volt distortion vs. high level distortion

From 1 V to 40 V, distortion does not scale benignly. Magnitude stays high and the shape shifts toward H3 distortion below roughly 35 Hz. The trend aligns with the LSI picture, where BL symmetry is poor and inductance variation is non-trivial. There is no evidence that this sample becomes cleaner at moderate stroke, so planning around low distortion is, based on this sample in these tests, seemingly impossible.

What this means in practice

At real listening levels, this sample under these test conditions is not a good candidate for clean ~20Hz up to ~40 Hz. The combination of a very high baseline distortion, H3 distortion growth at high level, and an early thermal abort means you have a very uphill battle to fight if accurate reproduction or high output are either of your goals.

Motor & suspension linearity - LSI measurements

Method recap: Klippel LSI large-signal identification for this unit, cold and used for enclosure computations. Standard thresholds in this project are BL 70 percent, CMS 50 percent, and a 17 percent inductance variance criterion. Commentary below ties the large-signal behavior to the acoustic results.

Bl(x)

Bl(x) shows how much motor force a speaker produces as the voice coil moves, B is magnetic field strength and L is the wire length in that field. A high, wide, symmetrical BL curve means linear control and low distortion, a steep or uneven drop means earlier output limits and rising distortion, which is why BL(x) is often the most telling single Klippel LSI indicator of real performance.

Bl(x) window and shape

BL 70 percent did not resolve on this sample because the run was aborted by the analyzer’s thermal limit after a quick 60 °C rise over ambient at only roughly 40 to 46 percent of the published power.
During BL(x) identification, the analyzer followed the standard gradual level ramp and tripped thermal protection about five minutes into the run at 20.62 mm one way. Back-calculated input at the moment of abort was roughly 460 W, which is only about half of the 1000 W RMS claim. None of the roughly 30 other subs tested under the same procedure triggered this protection.

Extrapolation of the partially collected curve suggests a BL 70 percent in the mid-20s, close to the 28.4 mm claim, but symmetry is very poor. The center region shows notable tilt, which maps to even-order content at level.

Bl(x) symmetry

The curve is biased toward the inward stroke. That asymmetry explains the visible H2 distortion component around 40 Hz and up at high level, and it contributes to the lack of a clean collapse above about 40 Hz. For context, the SQL 15 Series 2, which also claims 28.4 mm xmax and uses a similar 2.75 inch coil diameter, measured 23.62 mm at BL 70 percent in our lab. Taken together, the picture is inconsistent and messy for a line marketed explicitly for symmetry and linearity.

Cms(x)

Cms(x) is suspension compliance versus displacement, the inverse of stiffness. When the curve is broad and symmetrical, motion is linear and distortion stays low. Early roll off or offset indicates progressive stiffening or mis-centering, which adds mechanical distortion and caps clean excursion.

Cms(x) window and shape

Using CMS for suspension commentary, the 50 percent criterion occurs at roughly 19.7 mm one way of excursion. The curve is reasonably smooth in the center, then tightens progressively as stroke increases in both directions. There is no hard knee inside the standardized window.

Cms(x) symmetry

Centering is not great but is not the dominant issue in the measured window. The bias we do see is minor compared to the BL asymmetry and the inductance behavior that shows up as H3 distortion growth at high level.

Inductance - Le(x) and Le(i)

Le(x) and Le(i) measure how a subwoofer’s voice coil inductance changes with position and current. These curves show how stable the motor’s magnetic field is under real movement and drive conditions. When inductance varies heavily, it causes distortion, uneven response, and a loss of upper-band clarity, which is why Le(x) and Le(i) are critical for evaluating how clean and consistent a motor’s behavior really is.

Level and shape

Le(x=0) is ~2.20 mH. Position dependence is not flat. The 17 percent inductance criterion lands near 19.1 mm one way, which clusters with the CMS 50 percent point and sits well below the manufacturer claimed 28.4mm xmax. This maps to the practical clean stroke, since H3 distortion becomes the dominant component below about 35 Hz at high level.

Current dependence

Le(i) rises with drive and, together with the position dependence, explains the strong H3 distortion signature. The inductance ceiling is effectively in play by the time you approach the 19 to 20 mm region, even though BL might theoretically carry farther if it were thermally and symmetrically well behaved.

Qts(x)

Qts(x) is the driver’s total damping versus excursion, combining electrical and mechanical losses. Stable, symmetrical Qts(x) means consistent control, while large variation or asymmetry signals uneven damping that can shift response, raise distortion, and cause compression.

Qts stability

Qts at rest is low on this sample, roughly 0.29 cold and about 0.34 warm. With excursion and temperature it does not track evenly, and the Qts(x) curve is asymmetric, tilting toward the inward stroke. That asymmetry indicates unbalanced damping across stroke and aligns with the even-order distortion we see at level, contributing to why the response never settles into a clean, stable region. The low center Qts is compatible with small sealed alignments, but the asymmetric Qts(x) behavior is part of the broader non-linearity picture that limits how cleanly the driver behaves as stroke rises.

LSI takeaway

In this sample and under these test conditions, the practical clean one-way stroke is set by inductance and suspension around ~19 to 20 mm, with BL symmetry adding even-order distortion at level. BL 70 percent could not be verified before thermal abort, and the poor symmetry undercuts the projected window. The acoustic data reflects this directly, with high baseline distortion and H3 distortion growth with drive.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 0.5 ft³ nets a QTC of 0.496 on this sample

Sealed volume required for 0.707 QTC on this sample: 0.175 ft³

Applicable for infinite baffle? No. Low Qts and elevated distortion in the target region are not compatible with a clean IB experience.

T/S parameters