Perlisten D215s: Is This the Most Accurate Subwoofer on the Planet?

We were left deeply impressed by Perlisten’s S7t in our recent review of that tower speaker, not just by the superbly neutral response but also by the fantastically low distortion combined with its wide dynamic range. This perked up our interest in their subwoofer line which features an innovative “push-pull” driver alignment. Aside from its state-of-the-art driver and amplifier technologies, the Perlisten D212s and D215s subwoofers also mount the drivers in such a way that can dramatically lower distortion. So how low can distortion be pushed down in a subwoofer? We decided to find out by taking a drive to Perlisten’s headquarters in Wisconsin and testing one for ourselves.

Before we get into our test results, let’s talk about these subs’ designs and what makes them so special. To begin with, all of Perlisten’s subwoofers are using the highest performing drivers in their size class. They have very lightweight carbon-fiber cones driven by an absolutely massive motor and held in place by a solid cast-aluminum frame. The drivers use multiple shorting rings to bring down nonlinearities that stem from inductance that would otherwise have been an issue given the large voice coils. Like the S7t drivers, the Perlisten subwoofers use aluminum voice coils instead of copper or copper-clad aluminum, and for the same reason.

To quote our review of the S7t:

“That might seem slightly counter-intuitive seeing as how copper is more conductive, but that extra electrical resistance can be made up by simply using a larger gauge wire, and the larger gauge wire has the added benefit of being better for thermal dispersion since it has a lot more surface area from which to radiate heat. What is more, even with the larger gauge wire, it is still lighter than copper, so using aluminum reduces moving mass, thereby making the driver more sensitive.”

Venting for the drivers is done under the spider in the basket. Perlisten claims an Xmax, which is one way of linear travel, to be more than 30mm for all of their subwoofer drivers. That is a tremendous amount for 12” and 15” drivers and is more like what you would expect to see from a high-excursion 18”. It’s worth mentioning that Perlisten designs and manufactures their drivers from the ground up and do not use an OEM to make their drivers.

One measure that Perlisten has taken in reducing distortion in the D215s and D212s has been to arrange the drivers in a push/pull configuration. This is a sealed dual-driver configuration where one driver is mounted with the cone facing outward and the other with the cone facing inward. The drivers are then wired in opposite phase of each other. So the cones of both drivers are moving inward and outward at the same time with respect to the cabinet, although they are moving inward and outward in opposite phase with respect to each other. The advantage of this is that the non-linear motion that only occurs in one way of travel is mitigated by the other driver since motion is restricted by the increase in air pressure by the inward motion of each cone. In other words, the cones are coupled together by the constant level of air pressure inside the sealed cavity, so the motion of one of the cones in one direction is always modulated by the other cone’s opposite direction of travel. If the cone has uneven excursion with respect to inward and outward oscillation (as all drivers do to some extent), that is balanced out by the interior air pressure change created by the opposite motion by the other driver. This reduces even-order harmonic distortions which are created by nonlinear motion in just one direction of the cone’s travel.  

Perlisten’s subwoofers are powered by 1,500-watt amps for the single driver models and 3,000-watt amplifiers for the dual driver models. These are extremely sophisticated class-D designs that feature app control (for iOS and Android systems) as well as a top-mounted touch-screen interface. Processing is done by a 32-bit ARM processor, so the computational number-crunching and speed are certainly there. The ARM Cortex Processor analyzes and adjusts the system performance at every millisecond to maintain sound quality and safe operating ranges.

Perlisten CEO Dan Roemer says of the system:

We have spent a lot of time creating adjustment algorithms for how to handle real-world scenarios. For example, if the AC line is lower, then all the limiter thresholds are adjusted accordingly.  This ensures the sub maintains similar sound quality regardless of AC line conditions whereas traditional designs would clip more under these conditions compromising performance and fidelity. Similarly, if the AC line is higher – the sub will take advantage of this as well. For thermals, we have created gradual adjustments to output power to prevent thermal shutdown (think of very long duration, high output scenario). So rather than full shutdown like normal amps, we pull back power temporarily as needed until thermals are cool enough.

Many other aspects of system operation are monitored in real time such as power supply output and current, amplifier temperature, power supply temperature, output voltage and current, short circuit faults, DC offsets, and AC undervoltage/overvoltage. I can’t think of any other subwoofer that has such a sophisticated monitoring system, and it should enable the sub to operate accurately and reliably in a wide range of conditions as well as save it from potentially damaging problems. 

One nifty thing about them is that a multiple Perlisten subwoofer system can be controlled as individual subs as well as grouped subs, so you can apply global changes to the entire gang of subs or just make individual changes to any single one of them. This is a great feature for those who prefer to apply EQ changes to all of the subs. What is more is that the Perlisten subs boast an incredible 10-band parametric equalizer, so they can be dialed-in to an extraordinarily fine degree; no external sub EQ is needed at all. The Perlisten subs have dual inputs and outputs for both balanced XLR and unbalanced RCA connectivity. The outputs will be convenient for those who want to daisy-chain subs in a multiple sub system or for those who want to send the signal out to some amplified speakers.     

The cabinets for the Perlisten subs are massive constructions using heavy-duty HDF instead of the standard MDF and feature an extensive network of internal bracing for a very inert enclosure. The front baffle is over 3” thick so the driver is mounted to a very sturdy surface. At this time, the only available finish is a gloss piano black, but custom finishes can be ordered for a surcharge for those who want something that better matches their decor and are willing to pay for it.

Perlisten D215s Subwoofer Measurements and Analysis, Conclusion

In order to secure the lowest noise floor for the most precise distortion measurements that we possibly could, we waited for a low-wind night for testing. Testing on the D215s was conducted at a 1-meter distance and then scaled back to 2-meters in our graphs by subtracting 6dB in output. The temperature was recorded at 45F degrees with 70% humidity. The subwoofers’ gain was set to maximum, phase was set to 0, and the low pass filters were left off.

The above graphs show the measured frequency responses for the Perlisten D215s subwoofer for each of its EQ modes. The “Large Room” mode yields the flattest response and is best used in larger rooms where not much room gain is likely to occur in low frequencies. The THX EQ response rolls off at a higher frequency but also at a more gradual slope, and this might be the best EQ mode for a medium-sized room. The “Small Room” mode is the most typical of a sealed subwoofer and is probably closest to that of the drivers’ native response in this enclosure. In a very large room, the lack of deep bass might be noticeable from this response, hence the need for a “Large Room” mode, but in a small room it should accrue a lot of gain and have a good deal of deep bass output. In all modes, the response stays flat to nearly 200Hz which is good news for those who want to use multi-sub systems to reduce room modes above the standard 80Hz crossover frequency.

CEA-2010 Data for Perlisten D215s (2 meter, RMS)

The above CEA-2010 measurements are short-term bursts that show the subwoofer’s clean peak SPL before heavy distortion sets in. Our measurements have been referenced to 2-meter RMS, which is 9 dB down from the standard requirement for the measurements to be shown at 1-meter peak. However most publicly available CEA-2010 measurements are shown at 2-meter RMS, so we followed that convention. The D215s’ puts out some massive numbers in this test, as would be expected from a sub of its spec set, but the full story of the sub is not told by raw output but by the measured distortion quantities. It is a sealed sub that can not be pushed past 10% distortion above 20Hz no matter how hard it is driven. It barely breaks 10% below 20Hz for that matter. All of these distortion quantities are well below anything audible, and no other sealed sub that we have tested can match this level of output with such low distortion. Except for the 10Hz measurement, all frequencies were only limited by the amp and not distortion, meaning the subwoofer simply could not touch the CEA-2010 distortion thresholds. Even our 10Hz measurement was limited by background noise rather than harmonics (it is always very difficult to get a passing 10Hz measurement due to environmental noise since the sub’s output at such a deep frequency is usually not very high with respect to the noise floor).

Of course, these numbers easily propel the D215s to our Bassaholics ‘Extreme’ Room Rating meaning it is sufficient to charge a room of over 5,000 cubic feet.

The above measurements show what happens when we turn down the gain a bit. This is a sampling from a few different gain levels. Distortion is phantasmically low. In upper frequencies, the noise floor is running up against what we can record from the sub itself, and the noise was relatively low for a subwoofer testing environment. These are still relatively loud levels, and from a perceptual standpoint, distortion is totally negligible. We posted these numbers to show just how low distortion falls merely by backing off the upper limits of the sub’s performance envelope. All subs have a dramatic decrease in distortion when not pushed to the maximum drive levels, and the D215s is no different, with distortion levels dropping next to nothing, even at these high levels of output.

Testing for long-term output compression was done by first conducting a 20-second sweep tone where 50 Hz hit 90 dB with the subwoofer 2 meters from the microphone. We then conduct further 20-second sweeps by raising the gain by 5 dB until no more output could be wrung out of the subwoofer. These tests show us the long-term continuous headroom that the subwoofer is capable of. The D215s is overall the most powerful sealed subwoofer I have tested. Above 40Hz, it can just about put out 120dB continuously. The compressed response shape is slightly altered from the nominal response but not by much for a sealed subwoofer. One thing to note is that the long-term output level does not differ that much from the maximum burst output, and that is very unusual. Most subs are able to produce a lot more output in a brief moment than over an extended period, but the D215s doesn’t vary its output a whole lot relative to the duration of the signal. This means it is more likely to stay true to the dynamic range of the input versus most other subs.

The above graphs show the corresponding total harmonic distortion to the long-term output graphs. Essentially, they depict how linear the subwoofer remains for the corresponding drive level seen in the long-term sweeps. The quantity being measured is how much of the subwoofer’s output is distortion and is shown here as a percentage. We can see here that the D215s can be made to produce some significant distortion quantities at high drive levels below 20Hz, so the D215s can’t really sustain the ultra-low distortion seen in the burst measurements for long periods of time, however, its distortion output above 20Hz is the cleanest I have seen on any sealed subwoofer and pretty much any subwoofer I have yet measured. At 100dB and lower drive levels, the D215s can’t surpass 5% THD at any frequency. While we have seen examples of subs that can maintain lower distortion below 20hz, the D215s puts up a remarkably good performance level, and it is pretty much untouchable for undistorted sound above infrasonic bass regions.

The above graphs depict measurements of the constituent harmonics from the long-term output sweeps and are what the total harmonic distortion measurements are composed of for the 2nd and 3rd harmonics. These individual harmonics can give us a clue as to what might be the cause of some quirk or non-linearity. We are only showing the 2nd and 3rd here because they more or less reflect the higher even-order and odd-order behaviors, although higher-order harmonics tend to be much further down as a percentage of distortion compared to the second and third.

The 2nd harmonic is vanishingly low, and we can see that the vast bulk of THD is comprised of the third harmonic. This is strong evidence of the push-pull design’s efficacy which theoretically would greatly reduce even-order harmonics. While odd-order harmonics are more audible in musical content, it is the mark of a balanced design to have primarily odd-order over even-order harmonics, and the reason is that even-order harmonics stem from non-linear motion that only occurs in one-half of the cone’s travel, whereas odd-order harmonics are from non-linear motion that occurs in both directions. When pushed hard enough, a moving assembly will inevitably run into non-linear motion as it reaches the full swing allowed by the motor or suspension. Having very little even-order distortion indicates that measures were taken to keep linear travel as great as possible before the driver inevitably runs into the end of its available excursion.

Group delay is the measurement of how much time it takes for individual frequency bands of an input signal to be produced by the speaker. It can indicate that some frequency components are developing slower than others or are taking longer to decay. It is generally thought that 1.5 sound cycles are needed for group delay to be audible at bass frequencies, although there is an argument that group delay should remain under 20 ms to be completely unnoticeable, but that is likely meant for mid and upper bass frequencies. Regardless, the D215s doesn’t surpass 20ms until below 30Hz where it will be inconsequential. We can see that the response shaping filters do hike up group delay a bit at lower frequencies, but it never comes close to 1 cycle, and the group delay in mid and upper bass frequencies are among the lowest I have ever seen. The time-domain behavior exhibited here is overall excellent.

Editorial Note on Measurements & Further Product Refinements:

Since the time that these measurements were conducted, Perlisten has made further refinements to the DSP control of the subwoofer’s behavior, and, through a firmware update that can easily be performed from their app, have reduced nonlinear distortion even more without sacrificing much output. Additional improvements can be expected throughout the lifetime of Perlisten’s subwoofers as firmware updates continue to fine-tune performance parameters and add features.

Conclusion

Taking stock of the overall performance seen here, I have to ask myself is the D215s one of the most accurate subwoofers that can be had at any price? I would need to test a lot more subs to answer that definitively, but I am leaning towards yes, especially above 20Hz. A subwoofer can only err in two ways with respect to audio performance: linear and nonlinear distortion. Linear distortion is a distortion that occurs independent of amplitude, and in our measurements, that is the frequency response and group delay. The D215s’ frequency response is perfect, and the group delay is terrific, although we have seen some subs with lower group delay below 30Hz and comparably low above that range. Still, for much of the audible bass frequency range, the D215s group delay is as good as it gets.

With respect to non-linear distortion, the D215s offers some of the best performance I have seen in a sealed sub above 20Hz. The only sealed subs that can challenge its distortion levels below 20Hz that Audioholics has dealt with is the JTR Captivator RS1, and even then only in long-term output. Above infrasonic ranges, however, the D215s keeps total harmonic distortion below 10% which is impeccable performance for a sealed sub. We should mention that there are low-tuned ported subs that offer more deep bass output with extremely low distortion levels that pretty much no sealed subwoofer can match, but they tend to be huge, and, as a consequence of their design, group delay will always surpass 1 cycle at the port tuning frequency, although the audibility of that extra group delay is very much debatable (Perlisten feels that having low group delay and time-domain performance is important to ensure fidelity, and this was a serious consideration in their design).

The performance is outstanding, but at $9k for a subwoofer, the user has a right to expect this level of performance. Perlisten delivers, and I suspect that a lot of other subs from high-end manufacturers can not touch this level of performance even for equally priced subs. The D215s enters the strata of audio products where performance is immaculate, much like their S7t loudspeakers, or ultra-low distortion amps such as the Purifi modules or the Benchmark AHB2 or ultra-low distortion DACs with a greater than 130dB range of clean signal output. These are all state-of-the-art products- the very best performance that money can buy for the purpose of sonic accuracy. A system where price is not an object and audio performance is not negotiable could certainly include Perlisten D215s subwoofers. The best of the best is not inexpensive, but the good news is that there are certainly subwoofers that are more expensive than the D215s that would not match its performance. If you want one of the most advanced and accurate subwoofers on the planet per decibel of output as well as per cubic foot of space, the D215s has to be my first suggestion. 

The Score Card

The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:

Performance × Price Factor/Value = Rating

Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

Audioholics Rating Scale

• — Excellent
• — Very Good
• — Good
• — Fair
• — Poor

Metric	Rating
Bass Extension
Bass Accuracy
Build Quality
Fit and Finish
Ergonomics & Usability
Features
Dynamic Range
Performance
Value