Test Bench: Faital Pro 6RS140 Midwoofer / Midrange

September 25 2019, 14:10
The driver I tested this month is the 6” 6RS140 from Italian pro sound OEM Faital Pro’s extensive line of seven high power handling mid-bass woofers (see Photo 1). A recent Test Bench featured a similar driver from the same line-up, the 6PR160 — but this woofer, the 6RS140, has an even higher power handling rating of 200 W (AES).
 
Photo 1: Faital Pro’s 6RS140 midwoofer/midrange.
Figure 1: Faital Pro 6RS140 midwoofer/midrange 1 V free-air impedance plot.
Table 1: Comparison test data for the Faital Pro 6RS140.
Figure 2: Faital Pro 6RS140 computer box simulations (black solid = sealed at 2.83 V; blue dash = vented at 2.83 V; black solid = sealed at 56 V; blue dash = vented at 56 V).

Applications are the same as the 6PR160, which suggest use as a midbass or midrange in three-way stage or studio monitors, as well as midrange arrays in large PA systems. As with the 6RP160, features for the Faital Pro 6RS140 are also fairly extensive. The frame is a proprietary four-spoke cast-aluminum frame, and as with many small diameter drivers, it does not have a venting below the spider mounting area. Cooling for this driver is provided by a 20 mm diameter pole vent with a 22 mm flared exit.

The cone assembly consists of a curvilinear paper cone (without the front side waterproof hydrophobic coating, which was applied to the 6PR160) and a 3.4” diameter concave paper dust cap. Compliance is provided by a pressed and treated polycotton half-roll surround, which is not common in pro sound with the remaining compliance coming from a 4” diameter flat black cloth spider with constant height waves.

The motor design on the Faital Pro 6RS140 utilizes a neodymium ring magnet fitted with an aluminum shorting ring (Faraday Shield). The neodymium ring magnet motor was FEA designed, using a 44 mm (1.73”) diameter voice coil wound with round aluminum wire on a nonconducting glass fiber former. The neodymium ring magnet is sandwiched between a milled and polished front plate and a cast shaped T-yoke with a 15 mm bump-out. Last, the voice coil is terminated to an injection-molded terminal block with chrome-plated color-coded push terminals.
 
Figure 3: Group delay curves for the 2.83 V curves in Figure 2.
Figure 4: Cone excursion curves for the 72 V curves in Figure 2.

I began characterizing the 6RS140 using the LinearX LMS analyzer and VIBox to create both voltage and admittance (current) curves, with the driver clamped to a rigid test fixture in free-air at 0.3 V, 1 V, 3 V, 6 V, 10 V, 15 V, and 20 V. The Faital Pro midwoofer remained acceptably linear enough for LEAP 5 to curve fit at 20 V, which is a big deal for this small a diameter transducer.

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 (14.5 grams for the 6RS140). I post-processed the collected data — the 14 550-point sine wave sweeps for each 6RS140 sample — and divided the voltage curves by the current curves to generate impedance curves.

I derived the phase using the LMS calculation method and imported the information, along with the accompanying voltage curves, into the LEAP 5 Enclosure Shop software. Because Thiele-Small (T-S) data provided by the majority of OEM manufacturers is generated using either the standard model or the LEAP 4 TSL model, I additionally created a LEAP 4 TSL parameter set using the 1 V free-air curves. I selected the complete data set, the multiple voltage impedance curves for the LTD model, and the 1 V impedance curve for the TSL model in LEAP 5’s transducer parameter derivation menu and created the parameters for the computer box simulations. Figure 1 shows the 1 V free-air impedance curve. Table 1 compares the LEAP 5 LTD and TSL data and factory parameters for both of the Faital Pro 6RS140 samples.

LEAP parameter calculation results for the 6RS140 correlated reasonably well with the factory published data, except for the Xmax and Vas numbers. Faital uses its own definition of Xmax, which is why it’s different. Standard practice and what you will find in my book (The Loudspeaker Design Cookbook) is Xmax = (Hvc-Hg)/2, so with a voice coil length of 13.2 mm and a gap height of 6 mm, Xmax is 3.6 mm. Faital Pro uses Xmax = (Hvc-Hg)/2 + Hg/3, so it would be 3.6 mm plus 2 mm (Hg/3), which is why its published Xmax for this driver is 5.6 mm. This actually is a somewhat similar fringe-field compensation factor to what I do in finding a driver’s maximum SPL in a LEAP 5 box simulation when I raise the input voltage until the excursion maximum is Xmax + 15%.

This comes from Mark Gander’s work at JBL Pro some years ago, and is based on excursion beyond Xmax producing 3% THD, which for the 6RS140 is 4.14 mm, but even that is more conservative than Faital Pro’s Xmax definition. Faital also uses a more conservative Sd number of 122.6 cm2 compared to the 128.7 cm2 used in my work, which is derived from the cone diameter plus 50% of the surround width on each side of the assembly. This difference in methodology somewhat affects the Vas calculation.

However, as usual 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 computer box simulations into LEAP 5—one a sealed box with a 81 in3 volume (50% fill material) and the second a vented enclosure a modified Extended Bass Shelf (EBS) alignment with a 182 in3 volume simulated with 15% fiberglass damping material and tuned to 100 Hz.

Figure 2 displays the results in the two enclosures — at 2.83 V and at a voltage level sufficiently high enough to increase cone excursion to Xmax + 15% (4.2 mm). This produced a F3 frequency of 166 Hz (F6 = 133 Hz) with a Qtc = 0.65 for the sealed enclosure and –3 dB = 110 Hz (F6 = 100 Hz) for the vented box simulation.

Increasing the voltage input to the simulations until the maximum linear cone excursion criteria was reached resulted in 116 dB at 56 V for the closed box simulation and the 117.6 dB at the same 56 V input level for the vented enclosure. Figure 3 shows the 2.83 V group delay curves. Figure 4 shows the 56 V excursion curves. In the case of vented box simulation, excursion goes beyond the 4.2 mm Xmax + 15% number, around 75 Hz, so a 60 Hz to 70 Hz steep (24 dB/octave) high-pass filter would be a useful addition and prevent over excursion and distortion.
 
Figure 5: Klippel analyzer Bl(X) curve for the Faital Pro 6RS140.
Figure 6: Klippel analyzer Bl symmetry range curve for the Faital Pro 6RS140.
Figure 7: Klippel analyzer mechanical stiffness of suspension Kms(X) curve for the Faital Pro 6RS140.
Figure 8: Klippel analyzer Kms symmetry range curve for the Faital Pro 6RS140.
Figure 9: Klippel analyzer L(X) curve for the Faital Pro 6RS140.

Klippel analysis for the 6RS140 produced the Bl(X), Kms(X) and Bl and Kms symmetry range plots given in Figures 5-8. (Our analyzer is provided courtesy of Klippel GmbH and the analysis is conducted by Patrick Turnmire of Redrock Acoustics.) The Bl(X) curve for the 6RS140 (Figure 5) is moderately broad and symmetrical, with a very small amount of coil-in offset. Looking at the Bl symmetry plot (Figure 6), the displacement is a trivial 0.05 mm at the 2 mm position, and increasing to 0.12 mm at the physical Xmax of 3.6 mm.

Figure 7 and Figure 8 show the Kms(X) and Kms symmetry range curves for the 6RS140. The Kms(X) curve is also fairly symmetrical in both directions, as well as having small amount of coil-out offset. Looking at the Kms symmetry range plot shown in Figure 8, there is a likewise trivial 0.18 mm coil-out offset 2 mm increasing to 0.13 mm the physical Xmax 3 mm position.

Displacement limiting numbers calculated by the Klippel analyzer for the 6RS140 were XBl at 82% Bl = 3.8 mm and for XC at 75% Cms minimum was 4 mm, which means the Bl is the most limiting factor for prescribed distortion level of 10%. However both numbers are greater than the physical Xmax of this driver. Figure 9 gives the inductance curves L(X) for the 6RS140. Since Faital Pro has installed an aluminum shorting ring in the 6RS140, the inductance swing is a maximum of 0.03 mH, which is very good inductive performance.
 
Figure 10: Faital Pro 6RS140 on-axis frequency response.
Figure 11: Faital Pro 6RS140 on- and off-axis frequency response (black = 0°, blue = 15°, green = 30°, purple = 45°).
Figure 12: Faital Pro 6RS140 normalized on- and off-axis frequency response (black = 0°, blue = 15°, green = 30°, purple = 45°).
Figure 13: Faital Pro 6RS140 horizontal CLIO polar plot.
Figure 14: Faital Pro 6RS140 woofer two-sample SPL comparison.

With the Klippel testing completed, I mounted the 6RS140 in a foam-filled enclosure that had a 15” × 8” 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 200 Hz to 20 kHz at 2 V/0.5 m normalized to 2.83 V/1 m (one of the really outstanding tricks FINE R+D can do) using the cosine windowed Fast Fourier Transform (FFT) method. All of these SPL measurements also included a 1/12 octave smoothing.

Figure 10 gives the 6RS140’s on-axis response indicating a fairly smooth rising response with no break-up modes or peaking out to about 5.5 kHz, peaking about 7.6 kHz where the driver begins its low-pass roll-off. Figure 11 displays the on- and off-axis frequency response at 0°, 15°, 30°, and 45°, showing somewhat more directivity than most home audio 6” woofers. The -3 dB at 30° with respect to the on-axis curve occurs at 2.3 kHz, so a cross point in that vicinity should work well to achieve a good power response.

Figure 12 gives the normalized version of Figure 11, while Figure 13 displays the CLIO horizontal polar plot (in 10° increments). And finally, Figure 14 gives the two-sample SPL comparisons, showing a close match (less than 0.5 dB) throughout the operating range. For the remaining series of tests on the Faital Pro 6RS140, I employed the Listen, Inc. SoundCheck AudioConnect analyzer and 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 6RS140 rigidly in free-air, and set the SPL to 104 dB at 1 m (12.5 V) using a pink noise stimulus. I measured the distortion with the Listen microphone placed 10 cm from the driver. This produced the distortion curves shown in Figure 15.

Next, I employed the SoundCheck software (Version 17) to get a 2.83 V/1 m impulse response 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 (used for its better low-frequency performance). 
 
Figure 15: Faital Pro 6RS140 SoundCheck distortion plot.
Figure 16: Faital Pro 6RS140 woofer SoundCheck CSD waterfall plot.
Figure 17: Faital Pro 6RS140 woofer SoundCheck Wigner-Ville plot.

Examining the various data I collected for the 6RS140, even without doing any kind of subjective evaluation, it looks like a very well-crafted product. The driver readily portrays the kind of engineering integrity and high build quality you would expect from Faital Pro. For more information, visit www.faitalpro.com VC

This article was originally published in Voice Coil, July 2019.
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