Test Bench: The Punktkilde AUGWL0016-JN02 6.5" Midbass Driver

June 17 2020, 15:20
The driver to be characterized in this Test Bench is the AUGWL0016-JN02 6.5” midbass driver, a new one from Eastech’s Punktkilde line that features a unique Magnesium-Lithium (Mg-Li) alloy cone. Mg-Li alloy is the same material used in the Eastech 1” dome tweeter (the AUGDL0003-JN03) featured in Test Bench in the November 2019 issue of Voice Coil.
 
Photo 1: The Punktkilde AUGWL0016-JN02 6.5" woofer.
Figure 1: Punktkilde AUGWL0016-JN02 woofer 1 V free-air impedance plot.
Table 1: Comparison data for Punktkilde’s AUGWL0016-JB02 woofer.

Features for the 6.5” AUGWL0016-JN02 include a proprietary eight-spoke cast-aluminum frame (basket) that is vented below the spider (damper) mounting shelf, and a lightweight, low-density heat-resistant Mg-Li alloy flat profile cone with a 2” diameter convex dust cap out of the same material. Compliance is provided by a fairly narrow 9 mm wide NBR (CBR) surround and a 3.5” diameter cloth elevated spider, all driven by a 49.5 mm diameter aluminum voice coil with five 0.25” diameter vents around the peripheral of the voice coil. These five vents are located below the spider mounting shelf.

The motor structure consists of a 119 mm diameter, 20 mm thick ferrite ring magnet with a milled and polished 6 mm thick front plate and a milled, shaped, and polished T-yoke. Additional cooling is provided by a 20 mm diameter pole vent with a curved outlet to minimize turbulence and a screen to keep out foreign material. Cosmetically, this is a very nice looking woofer.

I began to characterize the AUGWL0016-JN02 using the legacy LinearX LMS analyzer and VIBox to create both the voltage and the admittance (current) curves. Next, I clamped the driver to a rigid test fixture in free-air at 0.3V, 1V, 3V, 6V, 10V, and 15V. The curve fit on the 15 V was not adequate, and was discarded.

Following my established protocol for Test Bench testing, I no longer use a single added mass measurement and instead this month relied upon the Klippel laser measured Mmd. I post-processed the 10 550-point sine wave sweeps for each AUGWL sample, and divided the voltage curves by the current curves to generate impedance curves. Then, I derived the phase using the LMS calculation method. I imported the collected data, along with the accompanying voltage curves, into the LEAP 5 Enclosure Shop software. 

Because the 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 then 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 the transducer parameter derivation menu in LEAP 5 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 Punktkilde AUGWL0016-JN02 samples.

The AUGWL0016-JN02’s LEAP parameter calculation results correlated moderately well with the Eastech factory’s published T-S parameters. Eastech does use a somewhat less conservative Sd number, but the effect appears relatively minor here.
 
Figure 2: Punktkilde AUGWL0016-JN02 computer box simulations (black solid = sealed @t 2.83 V; blue dash = vented @ 2.83 V;
black solid = sealed @ 50 V; blue dash = vented @ 52 V).
Figure 3: Group delay curves for the 2.83 V curves shown in Figure 2.
Figure 4: Cone excursion curves for the 50 V/52 V curves shown in Figure 2.

As usual, 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 Butterworth (Qtc = 0.7 target) sealed enclosure alignment with a 0.13 ft3 volume (50% fill material) and the second was an Extended Bass Shelf (EBS) vented alignment with a 0.44 ft3 volume simulated with 15% fiberglass damping material and tuned to 50 Hz.

Figure 2 displays the results for the AUGWL0016-JN02 in the two simulated enclosures at 2.83 V and at a voltage level sufficiently high enough to increase cone excursion to Xmax + 15% (5.75 mm for the AUGWL0016-JN02). The closed box alignment produced a 3 dB down frequency of 93Hz (F6 =74Hz) and –3 dB = 54Hz (F6 = 46Hz) for the EBS vented box simulation.

Increasing the voltage input to the simulations until the maximum linear cone excursion criteria was reached resulted in 109dB at 28.5 V for the smaller sealed box simulation and 109dB at 25V input level for the larger vented enclosure. Figure 3 shows the 2.83 V group delay curves.

Figure 4 shows the 28.5 V/25 V excursion curves. In the case of the vented box simulations, excursion goes beyond the 5.75 mm Xmax + 15% number at frequencies lower than about 40 Hz, so a 30 Hz to 35 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 Punktkilde AUGWL0016-JN02.
Figure 6: Klippel analyzer Bl symmetry range curve for the Punktkilde AUGWL0016-JN02.
Figure 7: Klippel analyzer mechanical stiffness of suspension Kms(X) curve for the Punktkilde AUGWL0016-JN02.
Figure 8: Klippel analyzer Kms symmetry range curve for the Punktkilde AUGWL0016-JN02.

Klippel analysis for the AUGWL0016-JN02 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 performed by Pat Turnmire, owner of Redrock Acoustics and author of the SpeaD and RevSpeaD software.) The Bl(X) curve for the AUGWL0016-JN02 (see Figure 5) is moderately broad, with a rather small amount of asymmetry, and with a small amount of coil-out offset as well as trivial amount of tilt. Looking at the Bl symmetry plot (see Figure 6), the displacement is a meaningless 0.37 mm at 5 mm, which is this driver’s physical Xmax.

Figure 7 and Figure 8 show the Kms(X) and Kms symmetry range curves for the AUGWL0016-JN02. The Kms(X) curve is somewhat more asymmetrical in both directions than the Bl curve. Looking at the Kms symmetry range plot in Figure 8, the displacement converges at nearly zero offset at the driver’s 5 mm physical Xmax.

Displacement limiting numbers calculated by the Klippel analyzer for the Punktkilde AUGWL0016-JN02 were XBl at 82% Bl = 4.9 mm and for XC at 75% Cms minimum was 5.9 mm, which means that for the AUGWL0016-JN02, the Bl is the most limiting factor for prescribed distortion level of 10%, but both number were at or above the physical 5 mm Xmax. If we use the 20% criteria of Bl decreasing to 70% and compliance decreasing to 50%, XBl goes to 6.1 mm and XC to 72 mm, so even better.
 
Figure 9: Klippel analyzer L(X) curve for the Punktkilde AUGWL0016-JN02.
Figure 10: Punktkilde AUGWL0016-JN02 on-axis frequency response.
Figure 11: Punktkilde AUGWL0016-JN02 on- and off-axis frequency response (black = 0°, blue = 15°, green = 30°, purple = 45°).
Figure 12: Punktkilde AUGWL0016-JN02 normalized on- and off-axis frequency response (black = 0°, blue = 15°, green = 30°, purple = 45°).
Figure 13: Punktkilde AUGWL0016-JN02 180° horizontal plane CLIO polar plot (in 10° increments).
Figure 14: Punktkilde AUGWL0016-JN02 woofer two-sample SPL comparison.

Figure 9 gives the inductance curves L(X) for the AUGWL0016-JN02. Inductance will typically increase in the rear direction from the zero rest position as the voice coil covers more pole area, which is what happens here in classic fashion. However, the inductance range only varies from 0.046 mH to 0.082 mH, which is only 0.036 mH spread, which is good performance.

With the Klippel testing completed, I mounted the AUGWL0016-JN02 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 200Hz to 20kHz 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 AUGWL0016-JN02 on-axis response, indicating a moderately smooth rising response that is ±3.5 dB from 300 Hz to 6.5 kHz with major breakup modes located at 7 kHz and 13 kHz. Figure 11 displays the on and off-axis frequency response at 0°, 15°, 30°, and 45°, showing somewhat more directivity than most home audio 6.5” woofers, remembering that this is a flat rather than a curvilinear cone. The -3 dB at 30° with respect to the on-axis curve occurs at 2 kHz, so a cross point in that vicinity should be work well to achieve a good power response. Figure 12 gives the normalized version of Figure 11. Figure 13 displays the CLIO horizontal polar plot (in 10° increments). And finally, Figure 14 gives the two-sample SPL comparisons for the AUGWL0016-JN02, showing a close match (less than 1 dB) up to 5 kHz, and about 1 dB throughout the remaining range.
 
Figure 15: Punktkilde AUGWL0016-JN02 SoundCheck distortion plot.
Figure 16: Punktkilde AUGWL0016-JN02 woofer SoundCheck CSD waterfall plot.
Figure 17: Punktkilde AUGWL0016-JN02 woofer SoundCheck Wigner-Ville plot.

For the remaining series of tests, I fired up the Listen SoundCheck AudioConnect analyzer and SCM microphone (graciously supplied to Voice Coil magazine by the folks at Listen, Inc.) to measure distortion and generate time-frequency plots. I mounted the AUGWL0016-JN02 rigidly in free-air, and set the SPL to 94 dB at 1 m (5.29V), using a pink noise stimulus. Next, I measured the distortion with the Listen microphone placed 10 cm from the driver. This produced the distortion curves shown in Figure 15.

I then employed the SoundCheck software (V 17) to get a 2.83V/1m impulse response for the AUGWL0016-JN02 and imported the data into Listen’s SoundMap Time/Frequency software. The resulting cumulative spectral decay (CSD) waterfall plot is given in Figure 16. The Wigner-Ville plot (used for its better low-frequency performance) is shown in Figure 17.

Looking at all the data I collected for the AUGWL0016-JN02, the performance looks good for a 6.5” driver. Given the cone’s extraordinary looks, this Eastech transducer is definitely worth taking a closer look. For more information, visit www.eastech.com/punktkilde. VC

This article was originally published in Voice Coil, March 2020.
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