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 at $8000-$11,500, plus THX certified the Perlisten Audio’s $15,000/pair S7t tower, which uses four 6.5” TPCD market, so it’s obvious that this relatively new diaphragm material has gained a fair amount of attention and acceptance rather quickly.
In terms of features, the 5” MR13TX-4 is built on a proprietary two-piece 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. The cone is glued to a conventional-looking conic section of TPCD joining the cone to a normal looking voice coil next joint. Compliance is provided by an inverted NBR surround, 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 30W (obviously also dependent upon whatever high-pass filter frequency is incorporated) 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.
Since this midrange is likely be crossed over at a minimum of 200Hz, I chose not to use the multi-voltage LEAP 5 LTD TSP testing protocol and instead established parameters with a single 1V TSL admittance (current)/voltage sweep using the Physical Labs IMP Box (the same type of fixture as a LinearX VI Box). The goal here was to establish an enclosure volume to ascertain the impedance resonance and Q. This information is useful if a passive high-pass network is employed, and of course, not relevant if using active filters.
Following my established protocol for Test Bench testing, I no longer use a single added mass measurement and instead use the company supplied Mmd data (6.87 grams for the MR13TX-4). The single impedance sweep for each driver along with the Mmd data was used to produce the TSL parameters for the two MR13TX-4 samples. Figure 1 shows the 1V free-air impedance curve. Table 1 compares the single sweep TSL data and factory parameters for both SB Acoustics MR13TX-4 samples.
LEAP parameter calculation results correlated well with the SB Acoustics factory published Thiele-Small parameters (TSP), except for the 2.83V/1m sensitivity, which was higher than the TSP calculated numbers and likely taken as an average SPL. As usual, I followed my established protocol and proceeded setting up computer enclosure simulations using the LEAP LTD parameters for Sample 1. Since this driver will likely have a high-pass filter of at least 200Hz to 250Hz, I only want to establish the F0 and Q for a box volume as reference for a passive network design. Using the LinearX LEAP 5 legacy software’s Quick Design utility, I came up with two volumes—129in3 sealed enclosure and 192in3 vented box, however the upper impedance peak in the vented box was at the exact frequency of the sealed box resonance (111Hz), so would not provide any passive network advantage over the sealed box and was discarded.
Figure 2 displays the frequency response result for the SB Acoustics TPCD midrange in the single simulated sealed enclosure at 2.83V. The closed-box alignment produced a 3dB down frequency of 111Hz, Qtc=0.68, while Figure 3 and Figure 4 give the box impedance group delay curves, respectively. Here the F0 and Q data could be used to design a LCR resonance conjugate filter to facilitate a 200Hz to 400Hz passive high-pass filter. For the purposes of this explication, Klippel analysis was not appropriate since this device is not being used in its piston range.
I next mounted the SB Acoustics MR13TX-4 TPCD midrange in a foam-filled enclosure that had a 12”×7” baffle and 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 (one of the really outstanding tricks FINE R+D can do), using the cosine windowed FFT method. All these SPL measurements also included a 1/6 octave smoothing (this is done to match the resolution of the 100-point to 200-point LMS gated sine wave curves used in the column for a number of years).
Figure 5 gives the MR13TX-4’s on-axis response, indicating a moderately smooth rising response that is ±2.5dB from 200Hz to 5kHz with breakup mode at 5.9kHz and a response that extends out to 20kHz. Figure 6 displays the on- and off-axis frequency response at 0°, 15°, 30°, and 45°. The -3dB at 30° with respect to the on-axis curve occurs at 3.5 kHz, so a low-pass cross point in that vicinity should be work well to achieve a good power response.
Figure 7 gives the normalized version of Figure 6. Figure 8 displays the CLIO horizontal polar plot (in 10° increments with 1/3 octave smoothing). And, Figure 9 gives the two-sample SPL comparisons for the MR13TX-4, showing a close match between 0.5dB to 1dB up to 5kHz.
Next, I initialized the Listen SoundCheck AudioConnect analyzer and ¼” 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, the 5” driver was mounted rigidly in free-air, and the SPL set to 94dB at 1m (4.75V), using a 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 10.
I employed the SoundCheck software (V21) to get a 2.83V/1m impulse response for this driver and imported the data into Listen’s SoundMap Time/Frequency software. Figure 11 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 12 shows the Wigner-Ville plot.
Looking at all the TSP and response data I collected for the new SB Acoustics MR13TX-4 TPCD cone midrange, the performance looks good and provides the flexibility of a TPCD midrange to SB Acoustics’ Satori line of TPCD transducers. For more information, visit the SB Acoustics website at www.sbacoustics.com. VC
This article was originally published in Voice Coil, September 2024
