Test Bench: SB Acoustics’ Satori MW16TX-8 Textreme Woofer

In this Test Bench, we characterize the SB Acoustics MW16TX-8 (Photo 1). This Textreme cone 6.5” woofer is the third Textreme diaphragm transducer to be explicated in Test Bench. The first was the Eminence N314X-8 compression driver, characterized in the May 2020 issue, followed by the SB Acoustics TW29TXN-B-4 in the September 2020 issue.

TeXtreme diaphragms are definitely finding acceptance among loudspeaker manufacturers over the last several years. In the two-channel high-end market Rockport Technologies uses Textreme woofer and midrange cones in all four of its models starting at $26,500/pair to $169,500/pair. In the studio monitor market, Ex Machina Soundworks uses a Textreme midrange cone in a coax driver for both of its monitors priced from $8000 to $11,500. And recently, THX certified, the Perlisten Audio’s $15,000/pair S7t tower, which uses four 6.5” Textreme cone woofers. Given the array of products, it would appear that this relatively new diaphragm material has gained a fair amount of attention and acceptance rather quickly, somewhat like Beryllium did years back.
  The SB Acoustics MW16TX-8 is from the SB Satori line of high-end transducers and has a substantial feature set that begins with a proprietary six-spoke cast-aluminum frame, comprised narrow (about 9mm) tapered spokes, and a completely open below the spider (damper) mounting shelf for cooling. Additional cooling for this driver is provided by six 4mm diameter vents in the voice coil former, as well as a 12mm diameter pole vent.

The cone assembly consists of a curvilinear inverted single piece “bowl” shaped cone. The cone is glued a conventional-looking conic section of Textreme joining the cone to a normal looking voice coil neck joint (Photo 2). Compliance is provided by a NBR surround that has a mostly shallow transition to the cone attachment, with the remaining compliance coming from a 3.25” diameter flat BIMAX spider (damper).
  The motor is an FEA-optimized neodymium magnet-type with milled plates and extended copper sleeve shorting rings (Faraday Shields) on the pole piece. Driving the cone assembly is a 36mm (1.4”) diameter voice coil wound with round copper-clad aluminum wire (CCAW) on a non-conducting fiberglass former. Last, the IEC 268-5 power handling is rated at 60W and the voice coil silver lead wires are terminated to gold-plated solderable terminals located on opposite sides of the former to discourage rocking modes.

I began testing the SB Satori MW16TX-8 woofer using the LinearX LMS analyzer and the Physical LAB IMP Box (the same type of fixture as a LinearX VIBox) to create both voltage and admittance (current) curves with the driver clamped to a rigid test fixture in free-air at 0.3V, 1V, 3V, 6V, and 10V, with the oscillator on time between sweeps to simulate the actual thermal process over time. The 10V curves were too nonlinear to get a sufficient curve fit and were discarded.

Following my established protocol for testing, I no longer use a single added mass measurement and instead use the physically measured Mmd data (11.85 grams). The collected data, in this case the eight 550-point (0.3V to 6V) sine wave sweeps for each SB Acoustics sample, were post-processed and the voltage curves divided by the current curves to generate impedance curves, with the phase derived using the LMS calculation method, which I imported, along with the accompanying voltage curves, to the LEAP 5 Enclosure Shop software.

Figure 1 shows the 1V free-air impedance curve. I selected the 1V TSL data in LEAP 5’s transducer parameter derivation menu and created the parameters for the computer box simulations. Table 1 compares the LEAP 5 LTD/TSL TSP data and factory parameters for both the MW16TX-8 samples.
  LEAP LTD and TSL parameter calculation results for the MW16TX-8 woofer appear to correlate reasonably well with the factory published data. As normally I do in this column, I followed my established protocol and proceeded to set up computer enclosure simulations using the LEAP LTD parameters for Sample 1. I programmed two simulated enclosures into the LEAP 5 software—one Butterworth Qtc=0.7 sealed box with 0.45 ft3 air volume with 50% damping material (fiberglass); and a vented Quasi Third-Order Butterworth (QB3) alignment with a 0.84 ft3 volume with 15% damping material tuned to 33Hz.

Figure 2 displays the box simulation results for the MW16TX-8 woofer in the sealed and vented enclosures at 2.83V and at a voltage level that achieves excursion equal to Xmax + 15% (6.9mm for the MW16TX-8). This resulted in a F3 of 58 Hz (-6dB = 47Hz) with a Qtc = 0.70 for the closed box and a -3dB for the vented simulation of 43 Hz (-6dB = 36Hz). Increasing the voltage input to the simulations until the Xmax + 15% excursion was reached resulted in 102dB at 15.5V for the sealed enclosure simulation and 104dB with a 16V input level for the larger vented box. Figure 3 shows the 2.83V group delay curves. Figure 4 shows the 15V/16V excursion curves.
    Klippel analysis for the MW16TX-8 was performed this month by Redrock Acoustics (Patrick Turnmire) with the Klippel DA2 analyzer (courtesy of Klippel GmbH). The Bl(X) curve for MW16TX-8 seen in Figure 5 is moderately broad, but with an obvious degree of “tilt” coil-in to coil-out. The Bl symmetry curve in Figure 6 shows a 1.9mm Bl coil-out (forward) offset once you reach an area of reasonable certainty (about 4mm), progressively decreasing to a 1.1mm coil-out offset at the 6.9mm excursion point.
  Figure 7 and Figure 8 show the Kms(X) and Kms symmetry curves for the MW16TX-8 driver. The Kms stiffness of compliance curve shown in Figure 7 is reasonably symmetrical and with a small degree of forward (coil-out) offset. The Kms symmetry range curve in Figure 8 exhibits small 0.69mm coil-out (forward) offset at a region of high certainty (1mm to 2mm) that is fairly constant out to 8mm.
  Displacement limiting numbers calculated by the Klippel analyzer using the full-range woofer criteria for Bl was XBl @ 82% (Bl dropping to 82% of its maximum value) equal 4.3mm for the prescribed 10% distortion level. For the compliance, XC @ 75% Cms minimum was only 3.5mm, which means that for the MW16TX-8, the compliance is the more limiting factor for getting to the 10% distortion level. However, if we use the less conservative 20% distortion criteria, XBl @ 70%=5.5mm and XC @ 50%=5.7mm, both numbers are closer to the driver’s physical Xmax.

Figure 9 gives the inductance curve Le(X) for this transducer. Motor inductance will typically increase in the rear direction from the zero rest position as the voice coil covers more of the pole in a conventional motor, which is what you see in the graph. More important, the inductive “swing” from maximum inductance to minimum inductance from 6mm coil-in to 6mm coil-out is a very small 0.05mH, which is very good.
  Next, I mounted the SB MW16TX-8 woofer in a foam-filled enclosure that had a 17” × 8” baffle. I then measured the device under test (DUT), using the LoudSoft FINE R+D analyzer and the GRAS 46BE microphone (courtesy of LoudSoft and GRAS Sound & Vibration) both on- and off-axis from 200Hz to 20kHz at 2V/0.5m, normalized to 2.83V/1m using the cosine windowed Fast Fourier Transform (FFT) method. All of these SPL measurements also included a 1/6 octave smoothing.

Figure 10 gives the MW16TX-8’s on-axis response, indicating a fairly smooth rising response with no break-up modes or peaking out to about 4kHz, with a peak in the response at 5.2kHz, beginning its low-pass roll-off above 10kHz. Figure 11 displays the on- and off-axis frequency response at 0°, 15°, 30°, and 45°, -3dB at 30° with respect to the on-axis curve occurs at 2.2kHz, so a cross point in that vicinity or lower should be work well to achieve a good power response. Figure 12 gives the normalized version of Figure 11, while Figure 13 displays the CLIO pocket horizontal polar plot (in 10° increments) for the MW16TX-8. And finally, Figure 14 gives the two-sample SPL comparisons for the MW16TX-8, showing a close match (≤ 0.5dB).
    For the remaining series of tests, I employed the Listen, Inc. SoundCheck AudioConnect analyzer and SCM microphone to measure distortion and generate time frequency plots. (The SoundCheck AudioConnect analyzer and SCM microphone were graciously supplied to Voice Coil by the folks at Listen, Inc.) For the distortion measurement, I mounted the MW16TX-8 driver rigidly in free-air, and set the SPL to 94dB (my criteria for home audio transducers) at 1m (5.74V), using a SoundCheck pink noise stimulus. Then, I measured the distortion with the Listen microphone placed 10cm from the driver. This produced the distortion curves shown in Figure 15, which is distinguished by very low third-harmonic content throughout its operating range.
  I then engaged the SoundCheck software to get a 2.83V/1m impulse response for this driver and imported the data into Listen’s SoundMap Time/Frequency software. Figure 16 shows the resulting Cumulative Spectral Decay (CSD) “waterfall” plot. Figure 17 shows the Wigner-Ville plot (chosen for its better low-frequency performance).
  Looking at all the various data I collected for the new SB Acoustics MW16TX-8 Textreme cone 6.5” midbass driver and knowing the timbral desirability of the Textreme diaphragm material as well the overall design and build quality, I can say this is a well-crafted product specifically intended for the high-end hi-fi or studio monitor market. For more information, visit www.sbacoustics.com. VC

This article was originally published in Voice Coil, June 2021.