- Written by Chris Eberle
- Published on 27 February 2009
Subwoofer Operating Modes
There are three choices here: 20 Hz tuning with all ports open, 16 Hz with one port blocked, and Sealed with all ports blocked. There is also a 25 Hz setting on the amp, which can also be used with all ports open, and simply starts to filter the response a bit higher in the pass band to help compensate for smaller rooms which exhibit excessive amounts of low-end gain. I measured the 20, 16 and Sealed tunings for comparison. The graph below (fig. 1) shows the effects. This is the raw subwoofer response before any EQ or low pass filtering from the AV receiver. The effects of the three tunings can clearly be seen. Sealed mode rolls off smoothly at 40Hz, 16Hz mode stays relatively flat to 16Hz and 20Hz rolls off a bit more sharply at 20Hz.
Red Trace – Sealed
Blue Trace – 16Hz one port blocked
Green Trace – 20Hz no ports blocked
These are raw measurements with the test tones sent directly
to the sub and measured from the listening position.
Parametric Equalizer Control
I wanted to see the effects of the parametric EQ so I did some additional measurements. The three controls are Frequency (20-80Hz), Level and Q. Frequency controls the center point of the frequency range you want to adjust, Level controls the amount of cut, and Q controls the width of the change on either side of the target frequency. Like most PEQ controls, the level control is cut-only, as boosting nulls is never recommended and can result in subwoofer overload and amplifier damage. As the term literally implies, the higher Q setting produces a narrower control bandwidth (Low = wide, High = narrow). Once you understand the terminology, it's easy to tame frequency response peaks.
In order to use the PEQ control effectively, you will need to plot the frequency response of the subwoofer. You will have to use an SPL meter and discrete test tones (supplied by SVS or available from other on-line sources). An even better choice is Room EQ Wizard. This free software is available from the Home Theater Shack website (www.hometheatershack.com). With a soundcard equipped laptop and a Radio Shack SPL meter, you can graph your room response with reasonable accuracy and see exactly what your sub is doing.
One thing that makes using this control tricky is the Frequency dial is not marked with any numbers between 20 and 80Hz (12 o'clock is 50Hz). This makes zeroing in on the target frequency a trial-and-error affair. When you're trying to tame a room peak, it may take several measurements before you achieve the right combination of settings. It also would be nice if there were more than one band available. Still, having this level of flexibility at this price point really sets the PB12-Plus apart from the competition.
Since my room response was decent to begin with I simply changed the controls in order to illustrate the different response curve (fig. 2).
The red trace shows the uncorrected response. The blue trace shows the effect of setting the Frequency to around 30Hz, the Level to max and the Q to low. The green trace shows the Q set to high. Notice the difference in the bandwidth of the correction.
In-Room Equalized Measurements
Since I have Audyssey MultiEQ XT available in my Onkyo TX-805 receiver, I wanted to see what effect it would have on the sub's in-room frequency response. On the SVS, I disabled the PEQ control, set the subwoofer gain at the half-way point, disabled the low pass filter, and then ran the Audyssey setup for each operating mode in my usual six measurement positions. After completing the auto-setup I changed the main speaker size to small and set the crossover to 80Hz. The sub was measured with the mains disconnected. The results were quite nice as you can see from the below graphs. Red traces are with Audyssey on, blue traces are uncorrected.