Stereo Integrity BM-11

Overall summary

Interesting name for a company all things considered.... In this sample and under these test conditions, low-frequency distortion is incredibly high (the highest levels of distortion out of any that we have tested) and clean headroom is even limited by the suspension and inductance well before the standard 70% BL xmax limit, which also fell very short of the manufacturers stated 14mm one way xmax. At 1 volt, distortion is pretty high across the board, and below about 40 Hz already rises sharply, with large H2 and noticeable H3, and the spikes below 30 Hz are pronounced. At the near 70% BL limit sweep of 17 V, roughly 70 W into 4 ohms Rdc, the driver reaches about 9mm one way excursion at 20 Hz and total harmonic distortion at 25 Hz is at about 45 percent. Distortion above 40 Hz in the 20 to 120 Hz band is also elevated above most other drivers on this sample, and a mild amount of level-dependent distortion shape drift is visible.

The large-signal data explains the trend. The BL shelf exists within a small amount of excursion, but falls below the 70 percent standard at about 9.3mm one way, and is non-symmetrical. The suspension reaches the 50 percent CMS standard at only 6.8mm and is very strongly biased outward, so the inward side runs out sooner and favors even-order distortion growth on inward peaks. Inductance is high at rest, but also varies strongly with position and current, which puts a practical clean one-way limit near 4.7mm going by the commonly used 17% Le standard, and aligns with the rising odd-order distortion content, especially at higher drive.

For real use, sub-bass duty is off the table if clean, low-distortion performance is your desire. The suggested sealed volumes yield extremely high QTC alignments on this sample, about 1.275 at the manufacturers recommented 0.40 ft³ (remember, 0.707 is the accepted standard goal for general use cases), and a classic 0.707 alignment is not achievable from the measured parameters. The only plausible niche is maybe as a front subwoofer to help fill the gap between an more capable rear subwoofer and the midbass drivers, use with a steep high-pass at ~50Hz, minimal power, minimal to no low-end boost, and possibly even limiters (which are not really available in car audio) set to keep excursion near or below about 5 mm. Disclaimer: These observations apply to this exact unit, this test setup, and these procedures only.

Manufacturer's suggested use case

According to Stereo Integrity, the BM-11 is a shallow-mount 11 inch subwoofer built for tight spaces and small sealed enclosures, using a Dual Gap (XBL2) neo column motor with a brass shorting ring to lower and linearize inductance, a flat carbon fiber sandwich cone, a custom low profile S-surround, and a 3 inch 8-layer copper voice coil with dual 2 ohm coils. Mounting depth is 3.09 inches and weighs about 16 pounds. They also state 14 mm one-way linear excursion and 500 watts RMS power handling, with sealed box recommendations of 0.35 to 0.70 ft³, 0.40 ft³ being typical, and they also list it for infinite baffle use. The product page positions the BM-11 as a broad-bandwidth shallow subwoofer, citing a very wide operating span, and a design focus on quick, low-distortion bass where mounting depth is limited.

Our suggested use case

In this sample and under these test conditions, clean headroom is set well before the nominal BL window by inductance behavior and suspension asymmetry. Practical one-way clean motion is near 4.7 mm via the 17% Le standard, 6.8mm via the 50% CMS standard, and 9.3mm via the 70% BL standard. All of which seem to be contributing to the high levels of distortion that are present in both the 1v standard, and our 17v test (which achieves 9mm of excursion at 20Hz). As a result, sub-bass duty is not advised. Let me get this out of the way right now. On this specific sample and in these test conditions, distortion, overall performance, and it’s design are bad enough that I have not a single suggested use case for it and I do not suggest using it for any reason.

For sealed use, the manufacturer’s suggested 0.35 to 0.70 ft³ volumes compute to extremely high QTC values on this sample, and do not approach classic alignments. Ported alignments are also not recommended given the high Qts and the strong low-frequency nonlinearity observed. Infinite baffle is feasible I guess, but the low practical xmax caps meaningful output, so IB is not recommended.
I guess the only possible application that I could say might be halfway acceptable is as a front subwoofer used more like a mid-bass to sub-bass helper. High-pass at 50 to 60 Hz with a steep slope, keep power conservative, avoid any low-frequency boost, and in general try to stay below about 5 mm of excursion. Even then, its not a good fit.

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: 17 volts

Approximate electrical power at that limit at 20Hz: ~70 W if treated as a 4 Ω resistive load, real power varies with frequency and impedance volts

Rated power (published): 500 W RMS

Power used to hit the standardized limits in free air, relative to their xmax rating free air: ~14 percent. Hits 9mm xmax in free air with ~70 watts of power.

Claimed Xmax vs. measured at BL 70%: 14.0mm claim vs 9.3mm measured, only ~66% of the claimed 14mm.

Xmax @ 50% CMS: 6.8 mm. Only ~50% of the claimed 14mm.

Xmax @ 17% Le: 4.7 mm from 17% Le(x)/Le(i) variation on this sample, approximately only 35% of the claimed 14mm xmax.

Manufacturer suggested sealed enclosure size (and its resulting QTC): Qtc at manufacturer recommended 0.40 ft³ is a whopping 1.275. Yikes, that is very high.

Required sealed enclosure for 0.707 QTC: Not possible with this driver. A 0.800 "equal ripple response" QTC requires a 7 ft³ sealed enclosure.

Xmax @ 50% CMS: 6.8 mm. Only ~50% of the claimed 14mm.

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 microphone positioned at 1/10th the cone diameter plus two inches from center, on axis. Sweeps at 1 volt and at a high level set just under the BL 70 percent point from LSI. Plots shown in percent with 1/6 octave smoothing. In this sample, the near-limit sweep was 17 V, about 70 W into 4 ohms Rdc, producing roughly 9 mm one-way excursion at 20Hz in free air, just under the BL 70 percent criterion.

At 1 volt - baseline

In this sample and under these settings, low-frequency distortion is already very high relative to most drivers we have tested. Below about 40Hz the curve begins rising sharply, with large H2 and noticeable H3 components, and below 30Hz the spikes are pronounced. This baseline picture indicates that low-frequency nonlinearity is present even at small excursion on this unit.

At high level voltage (17 volts)

With excursion near 9 mm one way at 20Hz when applying 17v of input, distortion increases substantially. At 25Hz, total harmonic distortion reaches about 45 percent on this sample. The rise begins around 40Hz and climbs as frequency drops, consistent with what we observed at 1 volt but magnified at higher stroke. Distortion above 40Hz is also abnormally high. Level-dependent shape drift is apparent and aligns with the LSI findings for suspension asymmetry and inductance variation.

Delta - 1 volt distortion vs. high level distortion

Both drive levels show unusually high low-frequency distortion for this sample, with the high-level sweep preserving the same rising pattern that starts near 40 Hz and peaks by 25 Hz, only at much larger amplitudes. Practically speaking, this means low-frequency cleanliness is the limiting factor well before the nominal BL window, in this unit and under these test conditions. I am trying to stay neutral on these, but its hard to in this case, so big yikes on distortion performance overall.

What this means in practice

In this sample and under these test conditions, achieving clean output in its intended frequency range is not realistic. Distortion is already very high at minimal excursion, and by the near-limit sweep of 17 V the driver is around 45 percent THD at 25 Hz with only about 9 mm one way at 20 Hz. Frankly, I couldn't imagine using this driver in any scenario as distortion is just way too high, so I won't waste anyone's time trying to come up with a use case suggestion for it.

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

In this sample, the 70 percent BL standard lands around 9.3mm one way, indicating a relatively narrow usable window for a driver with a 14mm xmax claim. The desired flat shelf is present near center but drops off sooner than expected, falling below 70 percent well before the claim would suggest. In practice, that early roll-off helps explain why low-frequency distortion rises quickly as stroke increases.

Bl(x) symmetry

BL symmetry shows a clear, non-linear outward offset, with the inward side falling away faster. Practically, inward motion runs into the weaker side of the motor sooner, which tends to raise even-order content, especially H2, and trims clean headroom on inward peaks in this sample.

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 50 percent CMS standard occurs at only 6.8mm on this sample, a few millimeters inside the BL limit, so the suspension becomes a primary constraint and a contributor to distortion relatively early, especially on inward motion.

Cms(x) symmetry

Suspension asymmetry is very strong and biased outward by about 6mm. That is an extreme amount for a sub of this size and it typically adds even-order components like H2 and promotes earlier compression on the inward half-cycle, which matches the unusually high low-frequency distortion pattern seen in the TRF sweeps for this sample.

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

Inductance at rest is high for a shallow design and varies strongly with position. Using a common 17 percent variance guideline, the practical inductance-based displacement limit occurs around 4.7mm one way on this sample. The large swing may be related to how the coil, shorting ring, and dual gaps interact in this compact layout, but we can only speak to what was measured here.

Current dependence

As drive current increases, inductance changes with it. In plain terms, turning it up changes the magnetic conditions the coil sees, which nudges the response shape around and pushes odd-order content like H3 higher at larger strokes on this sample.

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

In this sample, effective damping stays more stable on the inward stroke but rises on the outward stroke as excursion increases. That outward drift means less control on that half-cycle, which would most likely add further to the already elevated and rising low-frequency distortion trend observed in the TRF measurements for this unit, as we only measured to just below 9mm at 20hz, and the major QTS swings start at just after 9mm.

LSI takeaway

In this sample and under these test conditions, the motor, suspension, and inductance behavior together set practical headroom well before the manufacturers stated xmax and show major contribution to distortion components, especially at higher stroke. Stereo Integrity states 14mm one way as linear xmax, but using the common BL 70 percent standard this sample falls below the standard at about 9.3mm, the suspension reaches the CMS 50 percent standard at only 6.8mm, and inductance behavior puts a practical clean one-way limit near 4.7mm. BL symmetry shows an outward offset that makes the inward side collapse sooner, and the suspension is very strongly biased outward, which favors even-order growth on inward peaks and compresses earlier on that half cycle. Inductance is unusually high at rest and varies strongly with position and current, which aligns with level-dependent response drift and rising odd-order content in the TRF sweeps.

Taken together, these factors most likely explain the unusually high distortion observed, including about 45 percent at 25 Hz near our 17 volt test (which achieved just shy of only 9mm of excursion at 20Hz). We are not asserting intent or speaking for other units, but under our definitions and setup, the measured linear stroke on this sample is substantially lower than the 14 mm figure and should not be treated as clean headroom.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 0.40 ft³ which nets a Qtc of 1.275. Yikes, that is very high.

Sealed volume required for 0.707 QTC on this sample: Not possible. A QTC of 0.800, known as an "equal ripple response" still requires 6.5 cubic feet of airspace.

Applicable for infinite baffle? Applicable for nothing in my opinion. I honestly don't even know what to say about this driver.

T/S parameters