Hertz MPS 250

Overall summary

In this sample and under these test conditions, the Hertz MPS 250 S2 measured like a shallow sealed-box subwoofer with useful mechanical stroke, but with inductance becoming the earliest clean-stroke concern by a wide margin, at only 6.4mm. At 1 V, distortion through the main 20 to 120 Hz subwoofer band is low, with the 20 to 30 Hz region slightly higher than the midbass. H2 distortion is generally the dominant component through the useful passband at low level, while H3 distortion stays lower and does not define the overall shape.

At 20 V, distortion rises broadly and significantly through the desired subwoofer passband. The entire 20 to 180 Hz band is high in distortion, with H2 distortion carrying most of the rise. The LSI data explains part of that behavior: BL is reasonably wide but slightly offset, CMS does not become the limiting factor inside the protected window, and Le(x) reaches the 17 percent variance limit much earlier than BL or CMS. That means the driver has more mechanical stroke available than its clean inductance behavior suggests. However, the motor data alone does not fully account for the elevated distortion. The cone and dust cap design are also likely contributing factors, particularly given how broadly the distortion rises across the passband. Structural behavior, modal activity, or other diaphragm-related effects can increase harmonic distortion even when the motor and suspension remain within their primary operating limits. This is not a guarantee that this is the exact cause, but this is my personal suspicion.

For enclosure use, the manufacturer’s compact sealed recommendation lines up with the intended shallow-subwoofer use case. The 0.5 ft³ sealed enclosure produces a Qtc of 0.806 on this sample, while 0.85 ft³ is needed for 0.707 Qtc. The measured behavior points toward compact sealed use first, not infinite baffle use.

Manufacturer's suggested use case

Hertz positions the MPS 250 S2 as a Mille PRO shallow 10 inch subwoofer for installations where enclosure depth and volume are limited. The manufacturer lists a 3.25 inch mounting depth, 500 watts continuous power handling, 1000 watts peak power handling, a single 2 ohm voice coil, a 38 mm six-layer copper voice coil, polypropylene cone, die-cast aluminum frame, and a ferrite motor. Published use is sealed enclosure operation, with an ultra-compact suggested sealed volume of 14.2 L, or 0.5 ft³, and a published Fc of 49 Hz and Qtc of 0.81.

The published positioning is clearly compact sealed truck-box and shallow-install oriented. Hertz also lists 16.8 mm Xmax, 20 mm Xmech, 82.5 dB sensitivity, 25 to 400 Hz frequency response, and an internal support structure intended to reduce depth while maintaining excursion.

Our suggested use case

Based on the data, this sample is best treated as a compact sealed subwoofer first. The manufacturer’s 0.5 ft³ sealed recommendation produces a higher-Qtc result, while 0.85 ft³ is required for 0.707 Qtc on this sample. That gives the user a practical choice between the compact box the driver was clearly designed for and a larger sealed enclosure with lower system Q.

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: 20 volts volts

Approximate electrical power at that limit at 20Hz: ~165 watts. Real power varies with frequency and impedance. volts

Rated power (published): 500 watts

Power used to hit the standardized limits in free air, relative to their xmax rating free air: ~42.0 percent. Hits 15.7 mm of excursion in free air with 210 watts of power. Real power varies with frequency and impedance.

Claimed Xmax vs. measured at BL 70%: 15.26 mm, only 90.8 percent of the manufacturer’s claim of 16.8 mm.

Xmax @ 50% Cms: >16.8 mm, greater than 100 percent of the manufacturer’s claim of 16.8 mm.

Xmax @ 17% Le: 6.37 mm, only 37.9 percent of the manufacturer’s claim of 16.8 mm.

Manufacturer suggested sealed enclosure size (and its resulting QTC): Suggested 0.5 ft³ nets a Qtc of 0.806.

Required sealed enclosure for 0.707 QTC: 0.85 ft³ nets a 0.707 Qtc.

Xmax @ 50% Cms: >16.8 mm, greater than 100 percent of the manufacturer’s claim of 16.8 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 1/10th the cone diameter plus 2 inches, on-axis. Response measured to 1 kHz and THD to 500 Hz. 1/6-oct smoothing. Two drive levels, 1 V baseline and a high level set at 20 V per the under BL 70 rule derived from LSI for this unit. This sample in this test at this voltage level hit 13.4 mm at 20 Hz in free air, below the accepted standard of 70 percent of BL. Distortion reported both as percent and by harmonic.

At 1 volt - baseline

The 1 V baseline is mostly low through the desired subwoofer passband, with only mild jagged behavior from 20 to 120 Hz. THD is roughly 1 to 2 percent near 20 Hz, then stays around or below roughly 1 percent through most of the 40 to 120 Hz band. H2 distortion is generally dominant below ≈25 Hz, while H2 distortion and H3 distortion are both low from ≈30 to 120 Hz. The only major spike is far above the normal subwoofer passband, around 430 to 470 Hz, where the plotted THD rises sharply.

At high level voltage (20 volts volts)

At 20 V, distortion rises broadly across the useful passband instead of staying near the low-level baseline shape. THD is about 10 percent at 20 Hz, dips near 30 Hz, then rises into the roughly 10 to 12 percent range through much of the 40 to 100 Hz region. H2 distortion is the dominant harmonic below ≈25 Hz and remains the dominant harmonic through most of ≈30 to 120 Hz. H3 distortion stays much lower than H2 distortion through this band, so the high-level acoustic result is not primarily H3 distortion dominated in the main subwoofer range. The 50 to 120 Hz region matters here because the distortion remains elevated there, not just at the bottom octave. Above the desired passband, a stronger spike appears around 430 to 470 Hz, but that is outside the range most subwoofers will be used to cover.

Delta - 1 volt distortion vs. high level distortion

Compared with 1 V, the 20 V sweep shows a broad increase through the 30 to 120 Hz region rather than a single isolated low-frequency spike. The harmonic balance also shifts toward dominant H2 distortion across most of the passband, while H3 distortion remains lower in the acoustic sweep. That means the higher-level behavior is not simply the low-level trace scaled upward, and the distortion does not scale in the especially predictable way often seen when a single large-signal mechanism is dominating. The LSI data still matters because Le(x) is the earliest standardized limit, but the visible 20 to 120 Hz distortion shape is mainly H2 distortion dominated on this sweep. It is also possible that cone or diaphragm behavior is contributing to the elevated distortion profile, since the broad rise through the passband and the way the distortion develops with drive level do not point cleanly to only BL, CMS, or Le nonlinearity. While the data cannot directly confirm cone flex, the acoustic behavior suggests that additional structural effects may be involved.

What this means in practice

At low drive, this sample stays relatively clean through the desired subwoofer passband. At realistic drive levels with the 20 V high-level sweep, the main concern is the broad H2 distortion rise through roughly 20 to 180 Hz, not only the lowest frequencies. In practical terms, this is a relatively high-distortion subwoofer when used within its limits, with distortion remaining elevated across a large portion of the usable subwoofer range rather than being confined to the lowest frequencies. The driver has useful mechanical excursion in sealed use, but the practical clean xmax ceiling is limited by Le behavior before BL or CMS become the first limit.

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 occurs at 15.26 mm one way. The BL curve is rounded and somewhat peaky rather than a flat plateau, but it remains usable close to the published Xmax figure. The coil-in side tapers more gradually, while the coil-out side shows a slightly different loss shape near the edge of the window. This can contribute to the H2 distortion rise when the driver is pushed.

Bl(x) symmetry

BL symmetry at xprot shows xsym at -0.96 mm with 9.10 percent BL asymmetry. That is not severe, but it is not perfectly centered either. The small inward bias is consistent with the high-level sweep becoming H2 distortion dominant through much of the useful passband.

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

CMS 50 percent was not reached inside the protected window, with xC listed as >16.77 mm one way. The LSI note says CMS only resolved to 70 percent, so the exact compliance shape should be treated cautiously. In practical terms, the suspension is not the standardized stroke limiter in this test window, which is usually favorable.

Cms(x) symmetry

The stiffness symmetry result shows akms at 54.77 percent, but this needs to be read with the CMS resolution note in mind. The Kms/CMS behavior does not show a conventional early stiffening wall inside the usable stroke range. I would treat the suspension result as non-limiting in this window, with the exact symmetry less reliable than the BL and Le results.

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 at rest is 2.19 mH, and the 17 percent Le variance criterion occurs at 6.37 mm one way. Le(x) changes strongly with position, falling from roughly 3.1 mH on the coil-in side to roughly 1.5 mH on the coil-out side. Based on the inductance behavior, there is no indication of inductance management measures in this subwoofer. This is the earliest standardized clean-stroke limit.

Current dependence

Le(i) rises in a relatively drastic manner with current, from roughly 2.0 mH at low current to roughly 2.5 mH at the high-current end of the plot.

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 is 0.48 cold near center, 0.52 warm near center, and 0.56 in the small-signal column. As expected, Qts follows a generally U-shaped trend with excursion, rising on both sides of center. What matters here is that the rise is not perfectly symmetrical. The coil-out side increases more strongly, reaching above 1.0 near the edge of the protected window, while the coil-in side remains lower. This indicates damping changes noticeably with displacement and can contribute to shifting response behavior, increased distortion, and compression as drive level increases.

LSI takeaway

The large-signal data shows a driver with substantially more mechanical stroke available than its inductance behavior would suggest. The earliest standardized limit is Le 17 percent at 6.37 mm one way, occurring well before either the BL or CMS thresholds. BL remains usable out to 15.26 mm one way and stays reasonably wide, though it is slightly offset and rounded rather than flat. CMS 50 percent was not reached within the protected window, indicating that suspension compliance is not the limiting factor in this test range. Le(x) varies strongly with position and is the dominant linearity concern, while Le(i) shows moderate current dependence. Qts also becomes slightly unstable due to asymmetry toward the outer portions of excursion. Taken together, the data indicates that inductance variation, not motor force or suspension compliance, is the primary factor limiting clean excursion in this sample.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 0.5 ft³ nets a Qtc of 0.806.

Sealed volume required for 0.707 QTC on this sample: 0.85 ft³

Applicable for infinite baffle? Not recommended.

T/S parameters