Test Bench - Wavecor SW223BD02 8.75” Subwoofer

March 2 2016, 10:00

The SW223BD02, is a new 8.75” subwoofer from high-end OEM manufacturer Wavecor (see Photo 1). The SW223BD02 is part of a Wavecor’s rather extensive line of subwoofers — 20 models in all, ranging in size from 2.75” to 12”. Accompanying this lineup of subwoofers is a broad range of passive radiators with sizes from 6.25” to 12”. Wavecor’s SW223BD02, which is the 4-Ω version, has a generous feature set that includes a proprietary eight-spoke cast-aluminum frame that has minimal reflection surfaces and is completely open below the spider mounting shelf. Other features include the incorporation of a very stiff flat black anodized aluminum cone, further stiffened by a 3.5” convex black anodized aluminum dust cap. Suspension is provided by a low loss (high Qm) nitrile butadiene rubber (NBR) surround plus a 5” diameter black flat cloth spider (damper).
 
Photo 1: Wavecor’s SW223BD05 subwoofer.

All this is driven by a 51-mm diameter (2”) voice coil wound with round wire on a black fiber glass nonconducting former. The motor system powering the cone assembly utilizes two 20-mm thick, 125-mm diameter ferrite magnets sandwiched between a black plated 6-mm thick front plate and a black plated and shaped T-yoke that does not use a pole vent. This format drives more air out the gap area and across the front plate below the spider mounting shelf for enhanced cooling of the motor system. The SW223BD02 also uses an aluminum shorting ring (Faraday shield) that reduces distortion caused by eddy currents. The braided voice coil lead wires terminate to a pair of gold terminals.

I used the LinearX LMS analyzer and the VIBox to measure the SW223BD02’s voltage and admittance (current). Sweeps were generated in free air at 1, 3, 6, 10, 15, and 20 V. Wavecor provided the measured Mmd (an actual physical cone assembly measurement with 50% of the surround and spider removed), which I used rather than a single 1-V added (delta) mass measurement. 

It should also be noted that this multivoltage parameter test procedure includes heating the voice coil between sweeps for progressively longer periods to simulate operating temperatures at that voltage level (i.e., raising the temperature to the first and second time constants).
 
Figure 1: Wavecor SW223BD05 subwoofer 1-V free air impedance plot.
Table 1: Wavecor SW223BD02 subwoofer.
Figure 2: Wavecor SW223BD05 computer box simulations (black solid = sealed at 2.83 V; blue dash = vented at 2.83 V; black solid = sealed at 36 V; blue dash = vented at 34.5 V).

The 12 sine wave sweeps for each woofer were further processed with the voltage curves divided by the current curves to produce the impedance curves. I used the LEAP phase calculation routine to generate the phase curves. Then, I copy/pasted the impedance magnitude and phase curves plus the associated voltage curves into the LEAP 5 Enclosure Shop software’s Guide Curve library. I used this data, along with the LEAP 5 LTD transducer model, to calculate parameters.

Because most manufacturing data is produced using either a standard transducer model or, in many cases, the LEAP 4 TSL model, I also used the 1-V free air to generate LEAP 4 TSL model parameters that can also be compared with the manufacturers data. Figure 1 shows the SW223BD02’s 1-V free-air impedance plot. Table 1 compares the LEAP 5 LTD and LEAP 4 TSL Thiele-Small (T-S) parameter sets for the Wavecor SW223BD02 driver samples along with the Wavecor factory data.

From Table 1’s comparative data, you can see that all four parameter sets for the two samples were reasonably similar and correlated well with the factory data. Following my normal Test Bench protocol, I used the Sample 1 LEAP 5 LTD parameters and set up two computer box simulations, one in a 0.4-ft3 Butterworth type sealed enclosure with 50% fill material (fiberglass) and a second vented box EBS (a lower tuning of a traditional quasi third-order Butterworth) alignment in a 0.89-ft3 box with 15% fill material and tuned to 21.5 Hz.
 
Figure 3: Group delay curves for the 2.83-V curves shown in Figure 2.
Figure 4: Cone excursion curves for the 36 V and 34.5 V curves shown in Figure 2.

While I did this alignment with a simulated port tube, this low of a tuning in this small of an enclosure would be well suited to using a passive radiator. Figure 2 shows the SW223BD02’s results in the sealed and vented enclosures at 2.83 V and at a voltage level sufficiently high enough to increase cone excursion to 13.5 mm (XMAX + 15%). This resulted in a F3 of 46 Hz (–6 dB = 36 Hz) with a Qtc = 0.68 for the 0.4-ft3 closed box and –3 dB for the EBS-vented simulation of 33.8 Hz (–6 dB = 25.6 Hz). Increasing the voltage input to the simulations until the approximate XMAX +15% maximum linear cone excursion point was reached resulted in 106.8 dB at 36 V for the sealed enclosure simulation and 107.2 dB with a 34.5-V input level for the larger vented box. Figure 3 shows the 2.83-V group delay curves. Figure 4 shows the 36 V/34.5 V excursion curves.
 
Figure 5: Klippel analyzer Bl(X) curve for the Wavecor SW223BD05.
Figure 6: Klippel analyzer Bl symmetry range curve for the Wavecor SW223BD05.
Figure 7: Klippel analyzer mechanical stiffness of suspension KMS(X) curve for the Wavecor SW223BD05.
Figure 8: Klippel analyzer KMS symmetry range curve for the Wavecor SW223BD05.

The Wavecor SW223BD02’s Klippel analysis produced the Klippel data graphs shown in Figures 5–8. If you do not own a Klippel analyzer and would like to generate this type of data, Redrock Acoustics (www.redrockacoustics.com) can provide Klippel analysis for most drivers.

The Wavecor SW223BD02’s Bl(X) curve shown in Figure 5 is moderately broad and mostly symmetrical, typical of a driver with a relatively high XMAX. The Bl symmetry curve in Figure 6 shows a small and not significant 0.9-mm Bl coil-in (rearward) offset at rest, which transitions to less than 0.5 mm offset close to the physical 11.7 mm XMAX position.

Figure 7 shows the KMS(X) curves. Figure 8 shows the KMS symmetry curves. Like the Bl curve, the KMS stiffness of compliance curve (see Figure 7) is symmetrical, with only a minor offset. The KMS symmetry range curve exhibits a minor 0.8-mm offset at rest that stays constant throughout the driver’s operating range, suggesting a slightly rearward of magnetic center positioning, but close enough to be within manufacturing tolerance. 

The SW223BD02’s displacement limiting numbers — calculated by the Klippel analyzer using the subwoofer criteria for Bl — was XBl at 70% (Bl dropping to 70% of its maximum value) equal to 5.6 mm for the prescribed 20% distortion level (the criterion for subwoofers). For the compliance, crossover (XC) at 50% CMS minimum was 6.7 mm, which means the Bl is the more limiting factor for getting to the 20% distortion level.
 
Figure 9: Klippel analyzer Le(X) curve for the Wavecor SW223BD05.
Figure 10: Wavecor SW223BD05 on-axis frequency response.
Figure 11: Wavecor SW223BD05 on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot).

Figure 9 gives the SW223BD02’s inductance curve Le(X). Motor inductance will typically increase in the rear direction from the zero rest position and decrease in the forward direction as the voice coil moves out of the gap and has less pole coverage. However, that doesn’t happen here. Instead, we get a lower inductance variation from full-in to full-out travel, which is the goal. It’s easy to see the benefit of the aluminum shorting ring with inductance only varying from 0.9 to 0.5 mH, which is not bad for such a large motor.

Next, I mounted the driver in an enclosure filled with foam damping material and that had a 15” × 9” baffle area. I measured the SW223BD02’s sound pressure level (SPL) on- and off-axis using the LMS gated sine wave technique. Figure 10 gives the on-axis response measured 300 Hz to 20 kHz at 2.83 V/1 m. The response is a smooth ±1.95 dB out to 2 kHz, with a breakup mode peak up 10 dB at 2.8 kHz. Figure 11 shows the on- and off-axis to 45°. Even though this is a subwoofer, it would be possible to use it in a three-way configuration with a low-pass crossover frequency as high as 1.6 kHz or so to a midrange driver and tweeter, or as a two way to a large planar transducer.
 
Figure 12: Wavecor SW223BD05 two-sample SPL comparison.

Finally, Figure 12 gives the two-sample SPL comparison showing, the drivers to be well matched. Next, I used the Listen SoundCheck analyzer to perform distortion and time-domain analysis. I set the voltage level with the driver rigidly mounted in free air. I increased the voltage until it produced a 1 m SPL of 94 dB (9.6 V). I took the distortion measurement with the microphone placed near-field about 10 cm from the dust cap (see Figure 13). This actually includes two plots, the top graph being the standard fundamental SPL curve with the second and third harmonic curves, and the bottom graph the second and third harmonic curves plus the total harmonic distortion (THD) curve with an appropriate X-axis scale. It is almost impossible to interpret the subjective value of conventional distortion curves; however, looking at the relationship of the second to third harmonic distortion curves is of value.

I then used SoundCheck to get a 2.83 V/1m impulse response (again with the driver mounted in the enclosure with the 15” × 9” baffle) and imported the data into Listen SoundMap Time/Frequency software. Figure 14 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 15 shows the Wigner-Ville plot (for its better low-frequency performance). As can be inferred from all the above data, the SW223BD02 is another well-crafted transducer from the engineers at Wavecor.
www.wavecor.com
 
Figure 13: Wavecor SW223BD05 SoundCheck distortion plots.
Figure 14: Wavecor SW223BD05 SoundCheck CSD waterfall plot.
Figure 15: Wavecor SW223BD05 subwoofer SoundCheck Wigner-Ville plot.

This article was originally published in Voice Coil, April 2015
 
related items