Morel Ultimo PowerSlim PS104D

Overall summary

On this sample and under these test conditions, the Ultimo PowerSlim PS104D behaves as an ultra-shallow 10 with modest motor strength and a higher Qts that point to limited low frequency extension and small sealed enclosure performance. The 70% BL standard xmax is 10.36 mm one way and 50% CMS is 10.64 mm one way on this sample, so the practical linear stroke is set by BL and CMS at roughly the same window, just below the manufacturer rated 11 mm. Inductance behavior is a strength on this driver, with Excellent Le(x) and no Le-17 percent limit reached before the protection ceiling, which helps prevent odd order H3 distortion from dominating as level increases inside the usable stroke.

At 1 volt of input, distortion is already high and rising in the lower frequencies below 50Hz, with H3 distortion dominant below about 25 Hz and H2 distortion taking over above that, which means you are starting from a not-super-accurate baseline before level goes up to realistic volumes. At higher drive, distortion is very high in lower frequencies with mostly even order H2 distortion, about 25 percent H2 at 20 Hz with roughly 28 percent total harmonic distortion, about 14 percent total harmonic distortion at 30 Hz, and around 5 percent by 40 Hz. If accurate low frequency reproduction (below 50hz) is the goal, do not lean on this driver for heavy output and accuracy. Based on the data, I would only suggest using it as a front sub from the ~45Hz to ~110 Hz range, and hand the deep bass off to a larger sub that can hold down low distortion reproduction where it matters if accuracy is your primary goal.

Given the alignment realities, a 0.50 ft³ sealed box lands at a high Qtc of 1.122 on this sample, and a classic 0.707 alignment would require 23.0 ft³, which is not practical. Thankfully, this driver is not as sensitive to an undersized enclosure as most others. Again, my suggestion based on this data in these tests would be to use this driver as a front subwoofer crossed in the 45 to 110 Hz region that hands off the deep work to a larger rear sub. If you keep stroke demands modest below about 40 Hz and cross it appropriately, it integrates cleanly and behaves predictably for its format."

Manufacturer's suggested use case

Morel pitches the Ultimo PowerSlim as a compact sub for tight spaces that still delivers real bass and takes power. They credit the large 5.1 inch External Voice Coil and Hexatech windings for thermal headroom and linearity, plus the Uniflow frame, PFS suspension, and flat HexaCore cone for controlled excursion and a quick, clean feel. The intended use is a small, sealed enclosure where depth is limited, like behind trim or in truck cabs, and where front-stage integration matters. Their claim is that it excels when you want credible, accurate low end without a big box, and it earned an EISA award that they use to reinforce that positioning.

Our suggested use case

Use it sealed as a front sub from about 45 Hz to 110 Hz to fill the gap between a larger rear subwoofer and the front midbass drivers. Do not target a true low-Q sealed system on this sample since a 0.707 alignment requires 23.0 ft³. A small sealed front placement with a higher Qtc is fine as long as you can cross it over high enough (about 45Hz). Set the crossover and target curve so this unit is not asked for large stroke below about 40 Hz and you will get clean, tight upper and mid bass with relatively low distortion. Using it as a regular subwoofer to cover down to 20hz is not a good idea, as distortion is very high below 40Hz.

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: 10 volts

Approximate electrical power at that limit at 20Hz: 55 watts volts

Rated power (published): 700 watts

Power used to hit the standardized limits in free air, relative to their xmax rating free air: 55 watts, which is about 8 percent of the 700 watt rating to reach the standardized near-limit of approximately 9.4 mm one way at 20 Hz in free air, just under the BL-70 percent limit of 10.36 mm.

Claimed Xmax vs. measured at BL 70%: 10.36 mm, only 94 percent of the claimed 11mm

Xmax @ 50% Cms: 10.64 mm, only 97 percent of the claimed 11 mm

Xmax @ 17% Le: >10.69 mm

Manufacturer suggested sealed enclosure size (and its resulting QTC): 0.5 ft³, which would net a Qtc of 1.122 on this sample

Required sealed enclosure for 0.707 QTC: 23.0 ft³

Xmax @ 50% Cms: 10.64 mm, only 97 percent of the claimed 11 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 microphone positioned at one tenth the cone diameter plus two inches, on axis. Response measured to 1 kHz and THD to 500 Hz, both with 1/6-octave smoothing. One measurement at 1 V for baseline, one measurement at the standardized high level set just under the BL-70 point derived from LSI. Each measurement was ran 3 times to verify consistency. On this sample, the near-limit sweep was 10.5 V, approximately 55 W into the nominal 2 Ω configuration, producing approximately 9.4 mm one-way excursion at 20 Hz in free-air.

At 1 volt - baseline

Distortion is already high at 10 percent at the bottom of the band around 20Hz. In this region below about 25 Hz, H3 distortion is the dominant driver of the rise, and there are narrow spikes near 28 Hz and 35 Hz that add extra roughness. Once you get past the mid 20s, H2 distortion becomes dominant and total distortion drops as frequency increases, reaching low single digits by the 40 to 50 Hz range. Net, you are starting from an already high distortion baseline at very low frequency before level goes up.

At high level voltage (10 volts)

At drive levels that put xmax at 9.4mm, below its 70% BL limit of 10.36mm, distortion becomes very high from 20 to 35 Hz, led mostly by H3 distortion, and it stays clearly audible. Distortion level falls with frequency rise but is still several percent through about 40 to 50 Hz, with H2 distortion dominant and H3 distortion not far behind. No new narrow spikes show up at this level aside from the expected low-end rise.

Delta - 1 volt distortion vs. high level distortion

In this test on this sample, the distortion shape does not change, it just gets louder. At 1 V the bottom of the band is already distortion heavy with H3 distortion dominant below about 25 Hz, then H2 distortion takes over as frequency rises. At 10.5 V the same pattern is pushed harder, with H3 distortion remaining in charge through roughly 30 to 35 Hz before H2 distortion becomes dominant above that. The small spikes you see near 28 Hz and 35 Hz at 1 V do not introduce new narrow defects at 10.5 V, aside from the expected rise at the lowest frequencies. Net, level increases raise magnitude and extend the region where distortion is clearly audible, but they do not create a new kind of problem.

What this means in practice

In this test on this sample, below about 35 Hz, this driver produces very high harmonic distortion no matter the drive level, with H3 distortion dominant at realistic drive levels from roughly 20 to 35 Hz and elevated H2 distortion above that. From about 40 to 50 Hz the distortion level is lower but still higher than what you would want from a driver that is being used in a sound system who’s main goal is accurate reproduction. This behavior lines up with the LSI data, where BL symmetry is biased toward coil-out, which raises even order H2 distortion as stroke increases, while Le(x) remains well controlled, which helps keep odd order H3 distortion from dominating once you move higher in frequency. In short, expect audible harmonic coloration below 35 Hz at both low and high drive, with comparatively cleaner, more usable results for accurate reproduction in the 40 to 100 Hz band.

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

Overall, not a bad shape or limit. BL-70 occurs at 10.36 mm one way on this sample. The usable shelf declines progressively rather than forming a flat plateau, so force drops pretty gradually as stroke increases, but it isn’t necessarily bad.

Bl(x) symmetry

The symmetry point is offset toward the outward stroke by about 1.25 mm. On the inward half cycle, motor force drops sooner, so the inward stroke reaches a given BL value earlier and at a lower magnitude than the outward stroke. This bias reduces clean headroom on the inward half cycle, introduces waveform asymmetry that increases even order distortion as excursion grows, and makes the driver hit its BL-70 percent limit sooner on the inward stroke at the bottom of the 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

The CMS 50 % limit is 10.64 mm one way excursion on this sample. The compliance curve is progressive with no sharp knee in the observed window and matches shallow geometry expectations. Within the measured window there is no early compliance cliff or extreme non-linearities.

Cms(x) symmetry

CMS symmetry is not perfect, but is relatively good in the resolved range, so the suspension itself is not adding a significant bias to what BL(x) already sets. In practice, the even order distortion trend is primarily a motor symmetry effect rather than a suspension asymmetry.

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 and its position dependence are well controlled. The Le-17 percent threshold is not reached before the protection ceiling of the test. This indicates that position-dependent inductance is far from the first performance limiter in this motor within the stroke we can use.

Current dependence

Changes of Le with current are also very well controlled. This helps prevent additional odd order growth higher in frequency, but it does not suppress the low-frequency H3 that is already present, most likely from the symmetric parts of BL(x) and CMS(x).

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 on this sample is on the higher side for a driver intended for small sealed use, and it trends upward with excursion and temperature in a relatively normal way. The Qts(x) trend is not the primary constraint on clean output. The enclosure size limits are set by the shallow geometry and physics limiting motor force and the sealed volume realities, and the BL and CMS windows rather than by any unusual Qts(x) instability.

LSI takeaway

On this test, this sample’s usable one-way stroke is set by BL-70 at 10.36 mm and CMS-50 at 10.64 mm, so practical linear travel is about 10.5 mm. BL symmetry is offset about 1.25 mm toward coil out, which makes motor force fall earlier on the inward half cycle, reduces clean headroom on that side, and pushes the BL-70 percent limit sooner at the bottom of the band. At the 1 V baseline, odd order H3 distortion dominates below roughly 25 Hz, consistent with symmetric nonlinearities, and as excursion grows into roughly the 25 to 50 Hz range the BL bias shows up as elevated even order H2 distortion. Inductance with position and current is well controlled and is not the first limiter here, so the distortion profile is governed by the BL asymmetry and the shallow suspension geometry more than by Le effects. Qts is on the high side and trends upward normally, reinforcing that a small sealed, low-Q target is unrealistic on this sample. In practice, expect audible harmonic distortion below about 35 Hz at both low and high drive, with comparatively cleaner, more usable results in the 40 to 100 Hz band where this driver makes the most sense.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 0.5 ft³, which nets a Qtc of 1.122 on this sample

Sealed volume required for 0.707 QTC on this sample: 23.0 ft³

Applicable for infinite baffle? It does technically have the QTS and enclosure requirements to behave well for it, but considering the low xmax and high low-end distortion, this is not a good driver for IB use, unless being used as a front subwoofer where it will be high-passed.

T/S parameters