Acoustic Elegance SBP 15 with Apollo Motor Upgrade

Overall summary

The Acoustic Elegance SBP 15 with the Apollo Motor upgrade is, from a distortion and linearity standpoint, about as close to perfect as a 15"" subwoofer gets. In the 20 to 120 Hz range that matters most for car audio, especially for infinite baffle and large sealed applications, it stays dead clean across the board, both in TRF measurements and in my own personal subjective use. Harmonic distortion remains extremely low even as you approach its mechanical limits, and the response stays stable and uncolored through the entire passband, extending cleanly up toward 500 Hz. There are no unexpected artifacts, no compression issues in band, and no signs of instability even as you start to push it. On top of that, the motor shows near-perfect BL symmetry within its intended range, and exceptionally well-controlled inductance behavior, both with respect to position and current. It behaves exactly like you'd hope a purpose-built low-distortion driver would.

That said, output capability related to mechanical power handling is the one major caveat. For a subwoofer of this size/depth, its xmax and xmech leave a good bit to be desired for automotive installations. This is not a sub you throw into a single-sub setup and expect to get anywhere near concert-level bass out of. Our testing confirmed what we have personally experienced in real installs over the years—this driver bottoms out easily and reaches its useful excursion limit with surprisingly little input. In this particular test, it took just 10 volts, or roughly 25 watts at 4 ohms in free air, to reach the 70% BL excursion threshold. That doesn’t leave much room if your system goals involve realistic and desirable volumes, especially with heavy content down low. And again, for the not-so-hefty amount of linear xmax, its depth of over 8 inches is a bit of an eyebrow raiser. This depth makes it even more of a challenge in our already tight car audio environments. Then again, Acoustic Elegance primarily focuses on home audio, where that tradeoff isn’t nearly as much of a concern.

The BL profile is wide and symmetrical in its intended stroke. It’s the limited headroom and excursion efficiency that will be the limiting factor in most real-world installs, not its linearity.

For most SQ-focused systems that don't need as much output as most desire and have plenty of room and depth to spare, these tradeoffs can be worth it. If you're building an infinite baffle setup and care more about low distortion and clarity without the need for near-concert volume levels, this driver is one of the best options available. For those who want something that hits hard and can achieve the volume levels that most of us SQ guys desire on its own, or who don’t have the space or layout to support multiple 15s in IB, a pair of sealed 12s (or even some 15's) with more mechanical headroom will make more sense. But if the install allows for it and the output requirements are reasonable, this sub delivers the kind of clean, accurate low end that most other drivers in its class simply can’t match.

In short, the SBP15 Apollo isn’t a “do-everything” driver, but what it does do, it does exceptionally well. This is a serious tool for serious sound quality builds. Just be realistic about what you expect from it in terms of output and its large packaging, and make sure to plan your system around that and its constraints aren't limiting for your needs and installation. If you do, it’ll be one of the best sounding subwoofers you’ve ever worked with. Just make sure it will supply you enough output for your needs at low frequencies.

Manufacturer's suggested use case

Acoustic Elegance positions their SBP15 as a low-distortion, high-Xmax woofer for sealed enclosures, with wide usable bandwidth and very low, linear inductance from its Lambda motor and full copper Faraday sleeve, with an Apollo upgrade option (which this unit features). Claimed use cases include dedicated subwoofer duty, bass or midbass in 2-way or 3-way systems, and automotive infinite baffle. Guidance calls out sealed boxes in the 3.0 to 8.0 cubic foot range and notes that the SBP series is also intended to work well in automotive IB. Claims emphasize clean extension past 500 Hz, high excursion capability, and application in high-end studio, home theater, and hi-fi systems. This particular sample includes the Apollo Motor upgrade.

Our suggested use case

In this sample and under these test conditions, the AE SBP15 with Apollo Motor is clearly best suited for large sealed enclosures or infinite baffle setups, especially in car audio where IB is more common among our crowd. Within the 20 Hz to 120 Hz range that we focus on for subwoofer testing, its performance is excellent. Distortion and response remain impressively clean all the way up to just under 500 Hz. The BL(x) curve is wide and symmetric out to ±16.5 mm, and inductance is extremely well controlled, with Le(x=0) coming in at just 0.08 mH.

Important IB note for car audio: this is not a good match for “trunk baffle” installs that use the trunk as the rear volume. It belongs in a true through-vehicle IB implementation where the rear wave vents to the outside and the exterior air volume acts as the rear space. In a trunk baffle the effective rear volume is too small, so the driver reaches excursion limits early. If high output below roughly 35 Hz is a goal, plan on multiples or choose a different approach. See next section to get an idea as to why I am saying this.

That said, power handling and output are its clear weak points. It only took 10V (about 25 watts at 4 ohms) to hit just below the 70% BL threshold in our TRF testing. That lines up with past real-world use—these hit xmax and bottom out earlier than most drivers we’ve tested in similar applications. For sound-quality-focused systems where you're not pushing high output, this is one of the best sounding subs we've ever used. But if you're after accurate low end at high SPL, you’ll likely need a pair to get the extension and output that makes an IB install worthwhile. Otherwise, a sealed pair of 12s might be the more practical move for similar performance with more headroom and an easier install.
Bottom line, this is a near-perfect subwoofer when it comes to sound quality and linearity. Just don’t expect it to keep up in single-driver high-output builds. Hats off to John at Acoustic Elegance—this thing is seriously well engineered.

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: ~25 watts if treated as a 4 ohm 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: About 5 percent. This sample hit 14 mm of excursion in free air with 65 watts of power.

Claimed Xmax vs. measured at BL 70%: 14 mm manufacturer claim vs 16.5 mm measured, about 118 percent of claim.

Xmax @ 50% CMS: Not reached within our ±17 mm evaluation window on this sample

Xmax @ 17% Le: No practical in-band constraint observed on this sample due to very low and stable Le(x) and Le(i)

Manufacturer suggested sealed enclosure size (and its resulting QTC): Manufacturer suggests 3.0 to 8.0 ft³ sealed. On this sample, Qtc = 1.13 at 3.0 ft³, Qtc = 0.80 at 8.0 ft³

Required sealed enclosure for 0.707 QTC: 12.0 ft³ nets a 0.707 QTC.

Xmax @ 50% CMS: Not reached within our ±17 mm evaluation window on this sample

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: TRF testing was performed with a nearfield microphone positioned 1/10th the diameter of the cone plus two inches from the center of the cone, pointed directly at it. Sweeps were run at 1 volt 3 times to prove consistent, and 3 times for consistency again at 10 volts, which was the level needed to approach the 70% BL threshold per LSI results and it reached 8.7mm of excursion at 20Hz. Distortion is shown as the fundamental with the harmonic components, as well as relative percentage, all with 1/6-oct smoothing applied.

At 1 volt - baseline

This driver shows extremely clean baseline behavior in the 20 to 120 Hz range, with a smooth fundamental response and no odd peaks or resonances. H2 is present in the lower octave as expected but falls off quickly, and H3 stays very low throughout the passband. Above 120 Hz, distortion remains impressively low up to our 500 Hz testing limit, confirming the low Le and stable motor. This is about as clean as you’ll find in a 15" subwoofer.

At high level voltage (10 volts)

Distortion increases are *extremely* minimal, but also without any negative surprises. Below 30–40 Hz, both H2 and H3 hardly rise if at all, and there are no sharp in-band artifacts or new peaks introduced. The overall response shape in the usable range holds together well, and upper bass stays clean. There is essentially nothing to pick out here. From a distortion perspective, this driver is nearly perfect. This aligns well with what we see in the BL(x) and Le(x) curves as it is very flat and well controlled throughout the stroke.

Delta - 1 volt distortion vs. high level distortion

Again, great performance in this category. Nothing concerning appears when jumping from the 1V to 10V sweep. The main change is a smooth and progressive increase in low-end harmonic content, with no major loss of composure in the 20 to 120 Hz range that we are focusing on for car audio use cases. No instability, breakup, or resonance shows up, which is what you’d expect from a motor this linear and an inductance profile this flat. Overall, the driver behaves exactly how a low-distortion design should when pushed close to its limits. Just don’t expect to go much beyond this level without running into mechanical limits and the added distortion that comes with it, as noted earlier.

What this means in practice

In this sample and under these test conditions, the SBP15 Apollo is simple to integrate because the distortion signature stays clean and the response shape holds with level. You will not be chasing driver-caused peaks, so set the crossover where it fits the system and focus EQ on the car, not the woofer. Plan gain around excursion, not the 500 W rating, since the xmax limit is hit at approximately 90 watts at 20Hz in free air. In sealed or true through-vehicle IB it will sound very clean, it just runs out of stroke if you ask for more output than the system can support.

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

The usable BL window reaches the 70 percent criterion at about ±16.5 mm, but the plateau is not especially flat (this is common for overhung topology motors), but is still excellent overall. BL rolls off progressively from center rather than forming a broad mesa that is common with a perfect underhung motor, but symmetry is excellent. Remember, the flatter and more symmetrical the BL curve is, the less distortion the motor is adding.

Bl(x) symmetry

Very minimal offset at the evaluation limit; symmetry is a non-issue in band. The shape, not symmetry, is the constraining factor and even that is great.

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

Stiffness rises gradually toward stroke limits, without an abrupt knee inside the intended operating region.

Cms(x) symmetry

The shape is a bit lopsided, and a modest increase in asymmetry appears near the outer window, consistent with higher excursion. It did not seem to produce new in-band distortion artifacts on the TRF test 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

Le(x=0) ≈ 0.08 mH and stays very low with position with little variance, aligning with the low distortion.

Current dependence

Le stays seemingly perfectly flat with current applied. Le(i) stability is a strength of this motor and matches the TRF observations.

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 tracks consistently through the useful stroke window, with only minor drift at the edges, consistent with the KMS rise noted earlier.

LSI takeaway

This unit pairs excellent BL symmetry and very low inductance, but with a BL curve whose plateau is not especially flat, it is still a great BL curve. In practice, this sample reached the 70 percent BL limit at 10 volts in free air, so excursion-driven headroom per driver is the practical constraint, not in-band linearity.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: Manufacturer suggests 3.0 to 8.0 ft³ sealed. On this sample, Qtc = 1.13 at 3.0 ft³, Qtc = 0.80 at 8.0 ft³

Sealed volume required for 0.707 QTC on this sample: 12.0 ft³

Applicable for infinite baffle? Yes. Perfect candidate if you do not need high volume as it is relatively limited on excursion.

T/S parameters