Test Bench: The New 7.5” MT19CP-8 Coincident Coaxial Driver from the SB Acoustics Satori Line

Editor’s Note: Before beginning this driver explication, I want to address a reader’s letter I recently received regarding a past Test Bench article. In the letter the reader was obviously confused about the purpose of the Test Bench column, but I won’t go into details. The original “mission statement” for Voice Coil’s Test Bench was to present a comprehensive objective measurement data set that would enable a speaker systems engineer (like myself) to decide whether or not to get a sample of the driver, which is why I always refer to them as explications. Test Bench articles were never intended as reviews…to do that I would have to design the transducer into a system, and perform a subjective evaluation, which is far too time consuming for the purposes of monthly Test Bench explications.

All the drivers featured in Test Bench are OEM catalog drivers (virtually 99% of the drivers used in my consulting business over the last 30 years were developed by myself and my transducer engineering team), and I will have to say that pretty much all of the companies I deal with in Test Bench have both competent engineering and competent manufacturing (if they are not, they do not end up in Test Bench), so whether or not you agree with the design tradeoffs made by these manufacturing transducer engineers is really a matter of opinion. Whether a particular driver ends up sounding good or bad to a large extent depends on the application and the skill set of the systems engineer designing the product. Seriously, I have made some pretty pedestrian (read that severely mediocre) transducers I was required to incorporate into a system design come out sounding very very good…a silk purse from a sow’s ear if you will. That said, please keep in mind that when you read Test Bench, what you are getting is an objective outline of the performance of the DUT, and not my opinion of the usefulness of the driver in question…that is up to you!

 

Coincident coax speakers with the tweeter or compression driver mounted at the apex of the cone, are an interesting loudspeaker category. In home audio, they were originally dominated by KEF and Tannoy, with KEF having a patent for its KEF Uni-Q technology, which the company defended vigorously for several years (US5548657A “Compound Loudspeaker Drive Unit”). Eventually, the patent defense subsided, and the coincident coax driver became common in car audio, pro sound, and home audio. While popular, there really are not that many OEM catalog coax drivers available when compared to separate woofers and tweeters. However, over the years, I have explicated several of these coincident types of coax transducers in Test Bench, including the 6.5” SEAS “King Coax” E0060-08/06 (June 2017), the 6.5” B&C 6CXN36 (November 2022), the 5” Faital Pro 5HX140 (May 2024), and the 4.5” TB Speakers W4-2315 (May 2024).

For this Test Bench, I received the new 7.5” MT19CP-8, another high-end home audio/studio monitor coincident coax driver from the SB Acoustics Satori line (Photo 1). The industry is currently seeing several three-way speakers on the market using coaxial mid/hi sections in both home audio and studio monitors. For the home audio market, the new KEF R5 and R7 Meta series are great examples. Examples of three-way studio monitors using coincident coaxial for the woofer/midrange and tweeter include Genelec’s The Ones product line (Genelec 8331A, 8341A, 8351A, and 8361A), Antelope Audio (Atlas i8), Ex Machina (Pulsar MKII, Ganymede, Quasar MKII, and Arcturus), and Kali Audio (IN-8 V2 and IN 5). These speakers all incorporate a small diameter coincident coax driver such as the MT19CP-8. Given the MT19CP-8’s 7.5” diameter woofer, it works well as a standalone coax product, but also in applications where it could be used as a mid/hi driver.

In terms of features, the MT19 woofer is built on a rather stylish and aerodynamic-looking proprietary six-spoke injection aluminum-cast frame that minimizes reflections back into the cone. As can be seen from Photo 2, the frame is totally open below the spider (damper) mounting shelf, which allows substantial air flow past the voice coil over the front plate. What appears to be a second part of the frame is actually an injection-molded rubber trim piece that is attached to the motor assembly.

 

The woofer cone assembly comprises a light and stiff curved profile proprietary Egyptian Papyrus material, which is made at the SB factory and not sourced from a cone vendor. Instead of the usual NBR-type surround used on the other Satori woofers, the MT19 uses a flat pleated three-roll coated cloth-type surround. The remaining compliance is provided by a 3.5” diameter flat cloth spider (damper) with the tinsel leads attached 180° on opposite sides of the driver to discourage “rocking” modes. The cone assembly is driven with a 38.6mm (1.52”) diameter voice coil wound with copper-clad aluminum wire (CCAW) on a non-magnetically conducting fiber glass former.

The motor assembly is made up of a neodymium ring magnet sandwiched between a 6mm thick milled front plate and milled back plate plus a copper sleeve-type shorting ring (Faraday shield) on the pole piece. With a 13.2mm long voice coil winding height, the MT19CP-8 has a physical Xmax=3.6mm. Sensitivity for the woofer is rated at 90dB 2.83V/1m with an IEC 268-5 power handling rating of 60W.

The high-frequency device is mounted on a pole inside the long woofer voice coil with a plastic housing that includes a 6mm long horn-loading ring. The tweeter is comprised of a 25.4mm (1”) diameter coated cloth-type dome diaphragm suspended by a cloth surround. Powered by a neodymium slug, the tweeter motor incorporates a resonant back chamber design that gives the tweeter a 1.2kHz resonance. Like the woofer motor, the voice coil for the tweeter is also wound with a CCAW and includes a copper cap-type shorting ring (Faraday shield).

Terminals for the 1” soft dome tweeter are located in the center of the back of the woofer motor return cup. Applications for the MT19CP-8 are primarily as a high-performance small two-way speaker or as the midrange/tweeter in a three-way home or studio monitor design.

Testing commenced with the woofer section of the SB Satori MT19 coincident coax driver using the LinearX LMS and Physical Lab IMP Box (the same type of voltage/current fixture as a LinearX VI Box) 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, 10V, and 15V with a gradually increasing oscillator on time between sweeps to simulate the actual thermal temperature increase over time, allowing the voice coil to reach the third time constant. The 15V curves were not sufficiently linear enough to get a reasonable curve fit and were discarded.

Following my established protocol for Test Bench testing, I no longer incorporate a single added mass or sealed box measurement and instead use the company’s supplied Mmd data (11.02 grams for the MT19CP-8). The collected data for the MT19CP-8 included the 10 550-point (0.3V-10V) voltage and current sine wave sweeps for each SB sample, which were post-processed and the voltage curves divided by the current curves to generate impedance curves, with the phase derived using the highly accurate LinearX phase calculation method. That information, along with the accompanying voltage curves, were imported to the LEAP 5 Enclosure Shop software. Figure 1 shows the woofer’s 1V free-air impedance curve (solid black curve), along with the 1” soft dome tweeter’s free-air impedance curve (dashed blue curve). Table 1 compares the LEAP 5 LTD/TSL Thiele-Small Parameters (TSP) data and factory parameters for both SB Acoustics MT19CP-8 samples.

 

Parameter measurement results for the SB Acoustics MT19CP-8 correlated moderately well with the SB Acoustics factory published data. One thing I almost never see is a factory sensitivity level that is lower than my measurements, which in this case was 90dB factory vs. 91.2dB to 92dB from my TSP calculation routine. I checked the factory parameters against the measured parameters in the same sealed box volume, and the F3/Qtc for the factory data was 86Hz Qtc=0.55 vs. 76Hz Qtc=0.7 for the measured data, so not all that close. However, over the years is it not uncommon for the preliminary release of factory data to be updated once in production. Regardless, I proceeded to set up two computer enclosure simulations using the LEAP LTD parameters for Sample 1, a Butterworth alignment sealed box with a 0.53ft3 volume and 50% fiberglass fill material, and a vented QB3 alignment with a 0.90ft3 box volume tuned to 59Hz with 15% fiberglass fill material.

Figure 2 displays the results for the MT19CP-8 woofer, simulated in the sealed and vented enclosures at 2.83V and at a voltage level sufficiently high enough to increase cone excursion to Xmax+15% (4.1mm for the MT19CP-8). This produced a F3 frequency of 76Hz (-6dB=61Hz) for the closed enclosure with a Qtc=0.70, and a –3dB=51Hz (-6dB=45Hz) for the QB3 vented box. Maximum linear excursion (Xmax+15%) resulted in 107dB at 19V for the sealed box simulation and 108.5B for the same 19V input for the vented enclosure. Figure 3 shows the 2.83V group delay curves and Figure 4 shows the 19V excursion curves.

 

Klippel analysis for the woofer half of the MT19CP-8 was performed this month by Jason Cochrane at Warkwyn using the Klippel KA3 distortion analyzer. The Bl(X) curve for MT19CP-8 woofer (Figure 5) is moderately symmetrical and reasonably broad for a small diameter 7” driver, but also with an obvious degree of “tilt” and coil outward offset. The Bl symmetry curve (Figure 6) shows a rather small 0.57mm forward offset once the graph reached a place of certainty at the 2mm point, staying mostly constant to 0.64mm forward (coil-out) offset at the driver’s physical Xmax at 3.6mm.
 

Figure 7 shows the Kms(X) and Figure 8 shows Kms symmetry curves for the MT19CP-8 coincident coax. The Kms Stiffness of Compliance curve (Figure 7) is also relatively broad and symmetrical, again for a small diameter woofer, and with a degree of forward offset and tilt. The Kms symmetry range curve (Figure 8) indicates a fairly constant forward offset of 0.60mm forward offset at the 1.0mm point of reasonable certainty and decreases slightly to 0.58mm at the 3.6mm physical Xmax point.
 

Displacement limiting numbers calculated by the Klippel analyzer using the full-range woofer criteria for Bl was XBl at 82% (Bl dropping to 82% of its maximum value) was 3.55mm (about the same as the physical Xmax for the MT19) for the prescribed 10% distortion level. For the compliance, XC at 75% Cms minimum was only 2.14mm, which means that for the MT19CP-8 woofer section of this coax driver, the compliance is the more limiting factor for getting to the 10% distortion level.

Figure 9 gives the inductance curves Le(X) for the MT19CP-8. Inductance will typically increase in the rear direction from the zero-rest position as the voice coil covers more pole area, which is what is happening here. The maximum inductance swing for this driver from Xmax in to Xmax out is a low 0.015mH, which is good inductive performance for this neodymium woofer motor with a copper pole cap.

 

With the Klippel analysis completed, I proceeded to mount the SB Acoustics MT19CP-8 in an enclosure, which had a 15”x9” baffle, filled with foam-damping material. I then measured both the woofer and the high-frequency driver on- and off-axis from 200Hz to 40kHz at 2V/0.5m, normalized to 2.83V/1m using the Loudsoft FINE R+D analyzer and the GRAS 46BE microphone (courtesy of Loudsoft and GRAS Sound & Vibration). Figure 10 gives the SB Acoustics MT19CP-8 woofer’s on-axis response along with the tweeter’s on-axis response. This response graph shows the woofer to be very well behaved with a gently rising response ±1.5dB out to 3kHz. The response rises 6dB above 3kHz followed by several break-up modes out to 10kHz, where it begins a typical second-order low-pass roll-off.
 

For the tweeter, the response is reasonably flat from 3kHz out to 30kHz with an 8dB dip centered on about 9.5kHz. This “dip” is the typical on-axis cancellation from cone reflection that goes away off-axis.

Figure 11 depicts the woofer’s on- and off-axis frequency response at 0, 15, 30, and 45 degrees. The -3dB at 30 degrees with respect to the on-axis curve occurs at about 2kHz, which puts it at the low-frequency performance range of the 1” tweeter dome. Figure 12 gives the normalized version of Figure 11. Figure 13 depicts the woofer’s CLIO 180° polar plot (in 10° increments). And last, Figure 14 gives the two-sample SPL comparisons for the SB Acoustics MT19CP-8 woofers, both samples matched within 0.5dB to 1.2dB within the woofers operating range.

 

For the 1” cloth dome tweeter, Figure 15 gives the on- and off-axis horizontal frequency response out to 45 degrees, with the normalized version shown in Figure 16. The CLIO 180° polar plot (processed in 10° increments) is shown in Figure 17. Figure 18 depicts the two-sample SPL comparison for the dome tweeter half of the coax, which is within 0.5dB to 1dB out to 15kHz.
 

The last batch of test procedures was performed using the Listen AmpConnect analyzer and SC-1 microphone (courtesy of Listen, Inc.) along with the SoundCheck software to measure distortion and generate time-frequency plots. Setting up for the distortion measurement consisted of mounting the driver rigidly in free air and setting the voltage level determined to raise SPL to 94dB for both the SB Acoustics MT19CP-8 woofer and tweeter at 1m using a pink noise stimulus. I measured the distortion with the Listen microphone placed 10cm from the woofer dust cap and tweeter dome. This produced the distortion curves shown in Figure 19 for the woofer (at 3.24V) and in Figure 20 for the tweeter dome (at 3.4V).
 

With the distortion test completed, I set up SoundCheck to produce a 2.83V/1m impulse response for both the woofer and dome tweeter and imported the data into Listen’s SoundMap Time/Frequency software. Figure 21 shows the resulting cumulative spectral decay (CSD) waterfall plot for the SB Acoustics MT19CP-8 woofer and Figure 22 shows the CSD plot for the SB Acoustics MT19CP-8 soft dome tweeter.
 

 

For the final SoundMap graphic output, Figure 23 gives the Wigner-Ville logarithmic surface map for the woofer and Figure 24 displays the Short Time Fourier Transform (STFT) graph for the coated cloth tweeter dome.
 

Looking over all the data presented, the SB Acoustics MT19CP-8 is a well-designed and configured coincident coax transducer. SB Acoustics’ products are consistently well engineered by the folks at Danesian Audio in Denmark, combined with the manufacturing expertise of the Sinar Baja Electric factory in Indonesia…with the MT19CP-8 representing another good example of the company’s craft. For more information, visit the SB Acoustics website at www.sbacoustics.com. VC

This article was originally published in Voice Coil, November 2024