PB12-Plus/2 Measurements and Analysis

Figure 5 shows the PB12-Plus/2's amplitude response curves for the 25 Hz (Blue), 20 Hz (Purple) and 16 Hz (Green) system tunings. The drive level was 2.828 Vac and measurements were taken nearfield, subsequently scaled to 1m. For these measurements the crossover network and parametric EQ were both disabled, phase = 0°. All measurements were done indoors. Keep in mind that the nearfield measurement technique effectively excludes the measurement-skewing effects of the room within which the unit is being measured, therefore actual dB spl levels attained in practice are much higher, owing to room gain.

For each of the three tunings we see a respectably flat amplitude response which to a degree explains how the PB12-Plus/2 did so well in the music playback section of the critical listening sessions. Finessing subtle acoustic details and integrating with the rest of the system as well as the PB12-Plus/2 did is dependent on the ability to generate amplitude response curves like these.

You'll note that at the 16 Hz tuning the system exhibited a slightly lower dB spl output than that seen in either the 20 Hz or 25 Hz. In my listening experience this really wasn't a shortcoming as the system can dissipate so much acoustic power into your listening space in the first place that even the heartiest of bass freaks will find the system satisfying. Indeed, the PB12-Plus/2 is so capable, if in your mind you have any question as to the ability of the floor or other nearby structures making up your listening space to withstand the rigors of a resident sub in this class, by all means make the effort to strengthen or otherwise protect those structures and the room's contents.

We've seen thus far the PB12-Plus/2's ability to maintain respectably flat amplitude response curves at modest levels. But where it comes to how & why the PB12-Plus/2 performed as well as it did for the shock and awe portion of the listening sessions, respectably flat amplitude response curves only tell part of the story. For more insight we need to look at the sub's power compression behavior.

Power compression curves tell us to what degree and at what frequencies the system shifts from a linear relationship - between electric power in to acoustic power out - to a nonlinear one, the higher the sub's acoustic output goes before sliding into nonlinear behavior, the greater the dynamic "punch" the system will deliver.

For the power compression test I moved the PB12-Plus/2 from the main listening room to my home theatre. My home theater is purpose built to withstand outrageous amounts of acoustic power dissipating within and not vibrate, jiggle etc and sports a concrete sub-floor structure. My main listening room, on the other hand, sits on a wooden floor, has lots of stuff that can jiggle, vibrate, sway, etc and I just didn't want to take the risk of damaging anything. If you'd like to recreate these tests, I highly recommend you (and anyone else present) wear a good pair of earplugs. During those tests that resulted in the curves presented in Figure 6b, the PB12-Plus/2 would at times dissipate enough acoustical power into the room sufficient to cause blurred vision! That 舗 s pretty loud.

Figure 6a provides a relative look at the PB12-Plus/2's compression characteristics. For the series of curves shown, measurements were taken nearfield, then normalized to a reference level, referred to as 0 dB) in the graph. As can be seen, power compression is evident, but the PB12-Plus/2 does a fine job of keeping it to a reasonable minimum.

Figure 6b is a second, entirely separate set of curves. Rather than nearfield measurement conditions, I positioned the measurement microphone at the listening position, located ~ 2m away from the PB12-Plus/2 and placed at ear height..A series of sweeps were run, each time incrementally increasing the gain approximately 3dB. The parametric EQ (note the respectably flat 16 舑 30 Hz segment of the system's amplitude response) and requisite high-pass filter were used.

The PB12-Plus/2 could easily reach 106 dB @ 20 Hz at the listening position (~ 2m distance), with nary a hint of compression. Excellent! Previously, I 舗 ve had the unit cranking at levels > 110 dB, (with expected, but nevertheless, realistically low compression levels). For this series of tests I did not take it higher than 106 dB simply because at that level I had so much stuff vibrating across the 2 floors above my home theater that I figured the PB12-Plus/2 had admirably proved its point.

Indeed, for the last 3 sweeps (101.7, 103.2, and 106 dB sweeps) I left the home theater and walked around my house to observe the effects of the vibrations produced on it and its contents. At the 106 dB sweep, I watched a telephone vibrate off a table top and fall to the floor, 2 stories up! Like I said, it had proved its point.

One particularly noteworthy feature found in the measurements is that the amplitude response in the 16 - 30 Hz segment of interest is that it grew flatter as the dB spl output increased.

As we can see from the power compression curves, the system did indeed diverge increasingly from linear operation at the + 9 dB and + 12 dB drive levels. For comparison purposes, if the PB12-Plus/2 remained linear at each test drive level, then Figure 6 would show a collection of straight lines.

The valleys we see in the + 9 and + 12 dB curves, can be caused by ducts going into non-linear operation, built-in electronic limiting in action or a combination of both. In any case, power compression is evident and illustrates that even a plaster-busting behemoth like the PB12-Plus/2 has its limits.

With the power amp disconnected I ran a series of impedance scans. Here we see the double Z-peak typical of a vented cabinet. Minimum impedance was just over 11 ohms, which wasn't unexpected as the drivers are connected in series.

Impedance curves are useful in diagnosing pathologies such as strong mechanical resonances, defective drive units, etc. They're also useful for getting a fairly accurate view of the systems tuned resonance frequency. In this case, the impedance minima appearing between the two z peaks were within a couple of Hz of the stated tuning frequencies listed in the manual (16 Hz, 20 Hz and 25 Hz).

In the case of the PB12-Plus/2, the z curves are also useful for determining what affect (if any) the baseplate, situated as close as it is to the front of the system's two drivers has on system tuning.