Focal SUB 10WM Utopia M

Overall summary

In this sample and under these test conditions, the Focal 10WM presents as a compact 10 that is ultimately limited by inductance behavior, not by motor force or suspension travel. The BL shelf is broad for this size, with BL-70 at 20.50 mm one way, and the suspension does not hit the CMS-50 criterion within the same stroke window. Yet the practical clean-use ceiling arrives far earlier at roughly 5.64 mm one way based on the 17 percent Le variation guideline that much of this industry follows. That gap between a generous BL window and an early Le-based limit sets the overall character: usable excursion exists, but clean excursion is constrained by inductance non-linearity.

At 1 volt, the TRF distortion sweep reads as mostly controlled for a semi-compact 10. Total harmonic distortion is about 4.5 percent at 20 Hz is a tad high, but dropping down to about 2 percent at 30 Hz, and about 1 percent at 50 Hz, with H2 distortion and H3 distortion roughly comparable in the lowest octave. At the standardized near-limit of 25 V into the series-wired 8 ohm configuration, which corresponds to roughly 85 watts in free air and about 16.3 mm one way at 20 Hz, the distortion shape shifts into a high, broad plateau rather than a narrow defect. THD sits near 8 percent at 20 Hz, 7 percent at 30 Hz, and 9 percent at 50 Hz. H2 distortion dominates most of the band at this level, while H3 distortion rises below 30 Hz and meets H2 distortion at 20 Hz. The key is that this happens well inside both BL and CMS thresholds, pointing to inductance variation as the most likely governing mechanism for the high distortion.

The large-signal data reinforces that read. BL(x) offers a substantial shelf for a 10, but the curve is not perfectly centered, with a notable symmetry offset that can encourage even-order content as drive increases. CMS(x) trends are progressive without a hard knee within the observed excursion, and the 50 percent criterion does not occur before the BL-70 boundary. In contrast, Le(x) reaches the 17 percent variation limit at only about 5.64 mm one way, and Le(i) modulation is evident, which together explain why the high-level TRF sweep shows a smooth, level-driven rise in H2 distortion and a low-frequency H3 growth even though the cone is not approaching BL or CMS ceilings.

For enclosure planning, the manufacturer’s compact sealed direction is workable but on the tight side. On this sample, their recommendation of 15 liters (0.53 cubic feet) computes to a Qtc of 1.180 using our measured large-signal cold parameters, while a classic 0.707 Qtc requires about 6.3 cubic feet. Because inductance nonlinearity caps clean stroke well inside the motor and suspension limits, medium to large sealed volumes and additional cone area are the reliable paths to cleaner headroom rather than relying on high excursion. Infinite baffle is not a bad candidate assuming appropriate system context considering its QTS, but still, it has too much inductance variance during stroke that is most likely the reason for elevated distortion that it is hard to recommend it.

Bottom line, in this sample and under these conditions, the 10WM’s BL and CMS headroom look somewhat encouraging on paper, but the practical ceiling is set by inductance behavior, which might be why the high-level distortion lands as a broad, elevated plateau with H2 dominance and rising H3 content under about 30 Hz.

Manufacturer's suggested use case

Focal positions the 10WM as a compact sealed subwoofer intended for premium systems where enclosure volume is limited, with a focus on strong dynamics and controlled behavior that integrates cleanly with Utopia M midbass and midrange drivers. The company specifies small sealed enclosures with a stated minimum of 10 liters and commonly points buyers toward 15 liters (0.53 cubic feet) as a practical compact target. The M-profile W-sandwich diaphragm and neodymium motor are presented as engineering choices to reduce harmonic distortion and maintain stable operation at higher drive, while the dual 4-ohm coils provide wiring flexibility to match common amplifier loads. Mechanical elements like the reinforced surround and a progressive spider are highlighted to support stable motion in compact boxes. In short, they seem to say that the intended use is a small sealed installation that prioritizes dynamics, control, and system coherence with the Utopia M front stage.

Our suggested use case

Medium to large sealed is our short summary for suggested use case. The manufacturer recommendation of 0.53 ft³ kind of works but is very under-damped on this sample (Qtc ≈ 1.180). So expect an earlier and steeper low-end roll off below an undesired peak in frequency response, and plan to attempt to smooth that with EQ if you want it flatter. A true 0.707 Qtc needs ≈ 6.3 ft³. Because inductance non-linearity caps clean stroke well before the BL and CMS limits, choose setups that keep current and excursion modest, meaning slightly larger sealed boxes when space allows and adding cone area if you need more output instead of pushing one driver hard. Infinite baffle is technically possible, but not recommended here due to higher distortion at moderate drive; if pursued anyway, plan on multiple drivers and conservative targets to stay in the cleaner range. Overall, considering its price and high distortion with relatively low drive, and large enclosure requirements, it is hard to recommend this subwoofer at all.

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: 25 volts

Approximate electrical power at that limit at 20Hz: ≈ 85 W in free air (8 Ω series wiring of the dual 4 ohm coils) volts

Rated power (published): 400 watts

Power used to hit the standardized limits in free air, relative to their xmax rating free air: 135 watts, which is about 34 percent of 400 watt rating to reach the BL-70% limit of 20.5 mm one way at 20 Hz in free air.

Claimed Xmax vs. measured at BL 70%: 17 mm claim vs 20.50 mm measured, ≈ 121 percent of claim

Xmax @ 50% CMS: Greater than 20.50 mm one way (not reached within protection window)

Xmax @ 17% Le: ≈ 5.64 mm one way, only 33% of the 17 mm claim.

Manufacturer suggested sealed enclosure size (and its resulting QTC): 15 L (0.53 ft³) which nets a QTC of 1.180 on this sample using large-signal cold parameters

Required sealed enclosure for 0.707 QTC: 6.3 ft³

Xmax @ 50% CMS: Greater than 20.50 mm one way (not reached within protection window)

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 1/10th cone diameter plus 2 inches, on-axis. Response measured to 1 kHz and THD to 500 Hz, both with 1/6-octave smoothing. One sweep at 1 V baseline, one sweep at a high level set just under the BL 70 percent point from LSI. Coils were D4 in series to 8 Ω. Near-limit sweep was 25 V, about 85 W in free air, logging about 16.3 mm one way at 20 Hz. All sweeps were repeated 3 times for consistency.

At 1 volt - baseline

THD is about 4.5 percent at 20 Hz, about 2 percent at 30 Hz, and about 1 percent at 50 Hz. H2 distortion and H3 distortion are roughly comparable in the lowest octave. No narrow, level-invariant artifacts are called out on this sample. The curve trends downward with frequency as expected at light drive. The high distortion on the low end is a bit worrying, but the rest is pretty normal.

At high level voltage (25 volts)

This sample shows a broad, high distortion plateau across the intended band. THD is ≈ 8% at 20 Hz, ≈ 7% at 30 Hz, and ≈ 9% at 50 Hz. H2 distortion dominates most of the passband, which points to asymmetry in the system, for example a BL(x) offset and any CMS(x) bias seen in LSI. H3 distortion rises below ~30 Hz and matches H2 at 20 Hz, which is consistent with inductance variation with position and current (Le(x,i)) at higher drive. There are no narrow, level-invariant spikes to suggest a discrete resonance, and the near-limit pass moved ~16.3 mm one way at 20 Hz, well inside the BL-70 and CMS-50 limits, so the high distortion is not from exceeding stroke.

Delta - 1 volt distortion vs. high level distortion

From 1 V to 25 V, total distortion rises across the band and the shape shifts from a mild downward slope to a flat, elevated distortion plateau. Even-order H2 distortion becomes the main contributor through most of the passband at 25 V, indicating asymmetry (e.g., BL(x) offset and any CMS(x) bias) is the dominant driver at level. Odd-order H3 distortion grows below ~30 Hz and meets H2 at 20 Hz, consistent with inductance variation with position and current (Le(x,i)). The 25 V pass is ~16.3 mm one way at 20 Hz, well inside BL-70 and CMS-50, so the increase is not from exceeding stroke limits but most likely from asymmetry-driven even-order content with additional Le-related odd-order growth down low.

What this means in practice

This sample produces high distortion at realistic drive levels. At baseline, the lowest octave is already elevated; at 25 V the entire intended band sits on a flat, high distortion plateau. H2 distortion dominates most of the passband, which points to asymmetry (BL(x) offset and any CMS(x) bias) as a primary contributor at level, while H3 distortion grows below ~30 Hz in a way that is consistent with inductance variation with position and current (Le(x,i)). The near-limit pass remains well inside BL-70 and CMS-50, so the behavior is not the result of exceeding motor or suspension limits. Net result, the distortion character is driven by asymmetry and inductance effects rather than by a lack of stroke, and it remains apparent across the operating band at moderate power.

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

≈ 20.50 mm one way for the 70% BL standard. The curve is a wide, slightly tilted plateau with soft shoulders, not a peak. It holds near-max BL for most of the range and then tapers slowly into the 70 percent limit rather than dropping off suddenly. The BL width isn't the problem here. It is the asymmetry.

Bl(x) symmetry

The BL symmetry point is offset by a lot at 4.56 mm toward coil-out. This bias reduces clean headroom on coil-in and preserves more on coil-out, and most notably, raises even order harmonic distortion. It also shifts the dynamic rest point toward the strong side, and makes compression or limiting start sooner on the inward half-cycle at high drive.

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

The suspension never reaches the CMS-50 percent limit in the measured window, so the point where compliance drops to 50 percent is past 20.50 mm one way. Through most of the stroke it stays around 0.30 to 0.32 mm/N, then it only starts to firm up slowly near the ends. Net, it gives a wide, forgiving window with outward stroke having no sharp knee, but inward stroke having a relatively mild knee showing a shift in compliance.

Cms(x) symmetry

The CMS symmetry point is biased toward coil-out across the working range. The suspension is a bit looser on coil-out and tightens sooner on coil-in, which stacks with the BL bias, raises even-order content, and makes compression show up earlier on the inward half-cycle at high drive.

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

Large-signal Le at rest is about 1.38 mH. The 17 percent Le variance criterion is reached very early on at 5.64 mm one way, far earlier than BL and CMS, and it becomes the practical clean-stroke ceiling, most likely adding noticeable odd-order distortion beyond that point.

Current dependence

Le(i) modulation is present; position and current together modulate the field and elevate odd-order H3 distortion at low frequency under drive.

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

~0.63 cold and ~0.67 warm at rest. The QTS shape is unusual for us, with a dip on coil-out that later turns up, while coil-in trends higher. This lines up with the strong Le(x) gradient and the tilted BL shelf, not CMS, which stays broad and gentle. Coil-out gets more electrical damping because Le drops and BL stays stronger, coil-in loses damping as Le rises and BL relaxes. Le(i) growth with level leans it further. Net, it is stable but biased, so damping is better moving out and compression shows sooner moving in.

LSI takeaway

Based on this sample in this test, the motor has a wide usable BL and a forgiving suspension, but the show-stopper is inductance. Le hits the 17 percent variance at about 5.6 mm one way, well before BL or CMS, so clean stroke is effectively capped there. On top of that, BL and CMS both bias toward coil-out, so inward peaks run out of headroom first, raising even-order distortion and making compression start sooner on the coil-in half. Expect odd-order distortion to rise quickly as excursion and current stack due to inductance variation, with the asymmetry shaping what you hear when levels climb. In practice, keep excursion targets very modest if you want accurate reproduction.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 15 liters (≈ 0.53 ft³) treated as the compact sealed reference for this sample; Qtc ≈ 1.180 using large-signal cold parameters

Sealed volume required for 0.707 QTC on this sample: 6.3 ft³

Applicable for infinite baffle? Not a bad candidate due to high QTS and large enclosure requirements.

T/S parameters