In 2016, Oxeon successfully applied its TeXtreme thin-ply carbon reinforcements to create a new type of speaker diaphragm. The significant technical and commercial success led Oxeon to spin off its audio-related business to a dedicated, focused company—Composite Sound. The correct terminology for these cones is Thin-Ply Carbon Diaphragms (TPCDs). The February 2022 issue of Voice Coil also features a great article about TPCD cones written by Composite Sound’s Martin Turesson.
Given that information, the SB Acoustics MW19TX-8 TPCD cone 7.5” woofer is the fourth TPCD diaphragm transducer to be explicated in Test Bench. The first was the Eminence N314X-8 compression driver (May 2020), followed by the SB Acoustics TW29TXN-B-4 (September 2020), the SB MW16TX-4 woofer (June 2021), and the SB MW13TX-4 (January 2022).
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 starting at $26,500/pair to $169,500/pair; while 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 $8000 to $11,500; plus THX recently certified the 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 MW19TX-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 of narrow (about 12mm × 9mm) spokes, completely open below the spider (damper) mounting shelf for cooling. Additional cooling for this driver is provided by a 13mm diameter pole vent with a flared exit.
The cone assembly consists of a curvilinear inverted single piece bowl-shaped cone (Photo 1). The cone is glued to a conventional-looking conic section of TPCD joining the cone to a normal-looking voice coil neck joint (Photo 2). 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 39mm (1.53”) diameter voice coil wound with round copper-clad aluminum wire (CCAW) on a non-conducting vented fiberglass former. Last, the IEC 268-5 power handling is rated at 70W 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 MW19TX-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. I clamped the driver to a rigid test fixture in free-air at 0.3V, 1V, 3V, 6V, 10V, and 15V with oscillator turned on at 200Hz between sweeps to simulate the actual temperature increase over time in order to reach the third time constant. The 15V curves were too nonlinear to get a sufficient curve fit and were discarded.
Following my established protocol, I no longer use a single added mass measurement and instead use the physically measured Mmd data (14.45 grams for the MW19TX). The measured data, in this case the eight 550 point (0.3V-10V) 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. 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 of the MW19TX-4 samples.
LEAP LTD and TSL parameter calculation results for the MW19TX-8 woofer correlate well with the SB Acoustics’ published data. I set up the 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 0.52ft3 air volume with 50% damping material (fiberglass); and a vented Quasi Third-Order Butterworth (QB3) alignment with a 0.77ft3 volume with 15% damping material tuned to 38Hz.
Figure 2 displays the enclosure simulation results for the MW19TX-8 woofer in the sealed and vented enclosures at 2.83V and at a voltage level that achieves excursion equal to Xmax + 15% (7.7mm for the MW19TX-4). This resulted in a F3 of 67Hz (-6dB = 52Hz) with a Qtc=0.69 for the closed box and a -3dB for the vented simulation of 51Hz (-6dB = 42Hz). Increasing the voltage input to the simulations until the Xmax +15% excursion was reached resulted in 108.2dB at 26V for the sealed enclosure simulation and 109.6dB with the same 26V input level for the larger vented box. Figure 3 shows the 2.83V group delay curves. Figure 4 shows the 26V excursion curves. As with all vented enclosures, the MW19TX starts to over excurse below 30Hz, so it would benefit from a steep high-pass filter below the tuning frequency.
Klippel analysis for the SB Acoustics 7.5” woofer was performed by Warkwyn’s Jason Cochrane, using the Klippel KA3 analyzer. The Bl(X) curve shown in Figure 5 is moderately broad, but with an obvious degree of “tilt” coil-in to coil-out and is somewhat asymmetrical. The Bl symmetry curve in Figure 6 shows a 1.4mm Bl coil-out (forward) offset once you reach an area of reasonable certainty (about 4mm) and progressively decreasing to a trivial 0.59mm coil-out offset at the 6.7mm physical Xmax excursion point.
Figure 7 and Figure 8 show the Kms(X) and Kms symmetry curves. The Kms stiffness of compliance curve shown in Figure 7 is reasonably symmetrical with a small degree of forward (coil-out) offset. The Kms symmetry range curve shown in Figure 8 exhibits a small 0.13mm coil-out (forward) offset at a region of high certainty (1.6mm) that increases to 0.51mm at the 6.7mm Xmax position.
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 6.1mm for the prescribed 10% distortion level. For the compliance, XC @ 75% Cms minimum was only 3.8mm, which means that for the MW19TX driver, the compliance is the more limiting factor for getting to the 10% distortion level.
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 6.7mm coil-in to 6.7mm coil-out is varies between 0.081mH to 0.020 mH, which is very good and at least in part due to the copper sleeve shorting ring (Faraday shield) on the pole piece.
Next, I mounted the MW19TX 5” woofer in a foam-filled enclosure that had a 16” × 7” baffle and 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 40kHz at 2.0V/0.5m, normalized to 2.83V/1m, using the cosine windowed FFT method. All of 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 MW19TX driver’s on-axis response, indicating a fairly smooth rising response with no break-up modes or peaking out to about 4kHz, with peaks in the response at 5.2kHz and 11kHz, beginning its low-pass roll-off above 12kHz.
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 1.9kHz. This means a cross point in that vicinity or lower should be work well to achieve a good power response (2kHz or is generally an appropriate crossover frequency for an 8” diameter woofer, and the MW19TX is close to being an 8” driver).
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 for the MW19TX, showing a close match (≤ 0.5dB) in the operating range up to about 3kHz.
For the remaining series of tests, I employed the Listen, Inc. SoundCheck AudioConnect analyzer and 1/4” SCM microphone (graciously supplied to Voice Coil by the folks at Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I mounted the MW19TX driver rigidly in free-air, and set the SPL to 94dB (my criteria for home audio transducers) at 1m (3.85V) 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. Note that the third-order harmonic content stays below 0.4% throughout the operating range of the driver.
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).
Combining the beneficial subjective performance of the TPCD TeXtreme cone with the SB Acoustics Satori motor system makes this driver a good candidate for high-end, two-channel home theater or studio monitor system designs. For more information, visit www.sbacoustics.com. VC
This article was originally published in Voice Coil, February 2022.
