The SB Acoustics Satori MW13TX-4 TPCD cone 5” woofer (Photo 1) is the fourth TPCD diaphragm transducer to be explicated in Test Bench. The first was the Eminence N314X-8 compression driver in the May 2020 issue, followed by the SB Acoustics TW29TXN-B-4 in the September 2020 issue, and the MW16TX-8 woofer in the June issue.
TPCD diaphragms are definitely finding acceptance among loudspeaker manufacturers over the last several years. In the two-channel high-end market, Rockport Technologies uses TPCD woofer and midrange cones in all four of its models, which range in price from $26,500/pair to $169,500/pair. In the studio monitor market, Ex Machina Soundworks uses a TPCD midrange cone in a coax driver (based on the SEAS King Coax) for both of its monitors priced from $8,000 to $11,500. And, THX recently certified Perlisten Audio’s $15,000/pair S7t tower, which uses four 6.5” TPCD market. So, 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 MW13TX-4 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 of narrow (about 9mm) spokes, 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 an 8mm diameter pole vent.
The cone assembly consists of a curvilinear inverted single piece bowl-shaped cone (Photo 2). The cone is glued a conventional looking conic section of TPCD joining the cone to a normal-looking voice coil next joint. Compliance is provided by a NBR surround that has a moderately shallow transition to the cone attachment, with the remaining compliance coming from a 2.8” diameter flat BIMAX spider (damper)
The motor is an FEA-optimized neodymium magnet type with milled plates and extended copper sleeve shorting ring (Faraday Shield) on the pole piece. Driving the cone assembly is a 30.5mm (1.2”) 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 40W 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 to test the MW13TX-4 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. The driver was 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 temperature increase over time. I discarded the 10V curves were too nonlinear to get a sufficient curve fit.
Following my established protocol for Test Bench testing, I no longer use a single added mass measurement and instead use the physically measured Mmd data (7.06 grams for the MW13TX). I post-processed the measured data, in this case the eight 550 point (0.3V-6V) sine wave sweeps for each sample, and divided the voltage curves by the current curves to generate impedance curves, with the phase derived using the LMS calculation method. I imported the data, along with the accompanying voltage curves, to the LEAP 5 Enclosure Shop software. Figure 1 shows the 1V free-air impedance curve. I then selected the 1V TSL data in the transducer parameter derivation menu in LEAP 5 and created the parameters for the computer box simulations. Table 1 compares the LEAP 5 LTD/TSL TSP data and factory parameters for both MW13TX-4 samples.
LEAP LTD and TSL parameter calculation results for the MW13TX-4 woofer appear to correlate well with the factory published data. I followed my established protocol and set up computer enclosure simulations using the LEAP LTD parameters for Sample 1. Two simulated enclosures were programmed into the LEAP 5 software — one Butterworth Qtc=0.7 sealed box with 201 in3 air volume with 50% damping material (fiberglass); and a vented Quasi Third-Order Butterworth (QB3) alignment with a 294 in3 volume with 15% damping material tuned to 50Hz.
Figure 2 displays the box simulation results for the MW13TX-4 woofer in the sealed and vented enclosures at 2.83V and at a voltage level that achieves excursion equal to Xmax + 15% (5.75mm for the MW13TX-4). This resulted in a F3 of 90.4Hz (-6dB = 71.4Hz) with a Qtc=0.68 for the closed box and a -3dB for the vented simulation of 71.4Hz (-6dB = 28.8Hz). Increasing the voltage input to the simulations until the Xmax +15% excursion was reached resulted in 103dB at 15V for the sealed enclosure simulation and 104.5dB with a 16.9V input level for the larger vented box. Figure 3 shows the 2.83V group delay curves. Figure 4 shows the 15V/16.9V excursion curves.
Klippel analysis for the MW13TX-4 woofer was performed this month by Warkwyn’s Jason Cochrane with the Klippel KA3 analyzer. The Bl(X) curve for MW13TX 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 1mm Bl coil-out (forward) offset once you reach an area of reasonable certainty (about 3mm) and progressively decreasing to a 0.48mm coil-out offset at the 5mm physical Xmax excursion point.
Figure 7 and Figure 8 show the Kms(X) and Kms symmetry curves for the MW13TX driver. The Kms Stiffness of Compliance curve seen 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 0.6mm coil-out (forward) offset at a region of high certainty (1.6mm) that decreasing to a trivial 0.25mm the 5mm Xmax position.
Displacement limiting numbers calculated by the Klippel analyzer for the full-range woofer criteria for Bl was XBl @ 82% (Bl dropping to 82% of its maximum value) equal to 4.7mm for the prescribed 10% distortion level. For the compliance, XC @ 75% Cms minimum was only 2.9mm, which means that for the MW13TX-4 woofer, 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% = 6mm and XC @ 50% = 5.1mm, both numbers are beyond the physical Xmax of the driver.
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 pole in a conventional motor, which is what you see in the graph. More importantly, the inductive “swing” from maximum inductance to minimum inductance from 6mm coil-in to 6mm coil-out varies between 0.0mH to 0.04mH, which is very good.
Next, I mounted the MW13TX 5” woofer in a foam-filled enclosure that had a 15” × 7” 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 2.0V/0.5m, normalized to 2.83V/1m, using the cosine windowed FFT method. These SPL measurements also included a 1/6 octave smoothing to approximate the 100-point LMS gated sine wave sweeps used in Test Bench for more than 20 years.
Figure 10 gives the MW13TX-4’s on-axis response, indicating a fairly smooth rising response with no break-up modes or peaking out to about 4 kHz, with peaks in the response at 6.7kHz, 10.2kHz, and 16kHz, beginning its low-pass roll-off above 20kHz. Figure 11 displays the on- and off-axis frequency response at 0°, 15°, 30°, and 45°, and -3dB at 30° with respect to the on-axis curve occurs at 2.7kHz, 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. Figure 13 displays the CLIO pocket horizontal polar plot (in 10° increments). And finally, Figure 14 gives the two-sample SPL comparisons, showing a close match (≤ 0.5dB).
For the remaining tests, I employed the Listen, Inc. SoundCheck AudioConnect analyzer and 1/4” SCM microphone (graciously supplied by the folks at Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I mounted the MW13TX-4 woofer rigidly in free-air, and set the SPL to 94dB (my criteria for home audio transducers) at 1m (5.24V) using a SoundCheck pink noise stimulus. Next, I measured the distortion with the Listen microphone placed 10cm from the driver. This produced the distortion curves shown in Figure 15.
I then engaged the SoundCheck software to get a 2.83V/1m impulse response and imported the data into Listen’s SoundMap Time/Frequency software. The resulting cumulative spectral decay (CSD) “waterfall” plot is shown in Figure 16. Figure 17 shows the Wigner-Ville plot (chosen for its better low-frequency performance).
Looking at all the data I collected for the new SB Acoustic MW13TX-4 TPCD cone 5” midbass driver and being familiar with the timbral desirability of the TPCD diaphragm material as well the overall design and build quality, I can say this is a well-crafted product specifically intended the high-end hi-fi or studio monitor market. For more information, visit https://sbacoustics.com. VC
This article was originally published in Voice Coil, January 2022.
