Velodyne Digital Drive DD-12 subwoofer
Moving a subwoofer around "until it sounds best" is pretty safe advice, but a sub's optimal position may be where people would keep tripping over it. Furthermore, even if you were able to place the sub in the optimal position, and assuming that its intrinsic acoustical performance is beyond reproach, there would still be peaks and valleys in the response because of the modal response of the room itself—and the resultant, unavoidable buildup of standing waves.
Digital Technology to the Rescue
Velodyne is probably the best-known manufacturer of subwoofers; their accomplishments include the use of servo control to reduce distortion, and the popular HGS series of highly compact subs, which produce bass totally out of proportion to their size. (I reviewed the HGS-10 in the September 1998 issue of SGHT.) Although driver technology continues to evolve, and digital-switching class-D amplifiers provide more power in a smaller size with less heat, these improvements do not address the fundamental problem of the effect of the room on subwoofer performance. Analog equalization can be useful, but only to a limited degree.
According to Velodyne, the solution is digital signal processing. In the Digital Drive series of subwoofers (which range from the DD-10 to the DD-18, the number indicating the diameter of the driver in inches), the response of the subwoofer is measured using a supplied microphone, and various parameters can be adjusted to match the subwoofer to the main speakers and equalize the response of the subwoofer to produce the best sound at the listening position. All of this is done in the digital domain: the analog signal is converted to digital using a 20-bit A/D converter, then the signal is processed using a powerful (31.6 million IPS) Texas Instruments TMS320LF2407 DSP chip, and finally the signal is converted back to analog by a 20-bit DAC and sent to the built-in amplifier. The amplifier itself is an Energy Recovery class-D digital model producing a whopping 1250W.
Yet another aspect of the DD-12 that has benefited from a redesign is Velodyne's patented High Gain Servo control, which is now all-digital. The new digital accelerometer checks the cone's motion up to 15,800 times per second—a big increase from the previous rate of 3500 times per second.
Setting up the DD-12 is quite a bit more complicated than setting up an ordinary subwoofer, which is likely to have no more than a couple of knobs for setting volume and crossover frequency, plus a switch for polarity reversal. For the DD-12, you first have to connect a video cable as well as an audio cable between the sub and the surround preamplifier-processor or receiver; you then connect the microphone to the appropriate input on the subwoofer. (The microphone, all cables, and a small microphone stand are supplied.)
With the microphone in place in the listening area, you turn down the subwoofer volume to zero and start the calibration process, viewing the results on your video screen. The DD-12 has a built-in signal generator; pressing the Test button on the remote control starts a sinewave sweep that cycles from 15 to 200Hz; the resulting frequency response is shown onscreen. You now have the system's frequency response in this range without the subwoofer (remember, the sub's volume is set to zero at this point).
The next step is to set the lowpass crossover frequency (I left it at the default 80Hz, which usually works well in my system), turn up the subwoofer volume, and switch back to Test mode to look at the frequency response that includes the contribution of the subwoofer. If the bottom end seems too weak or too strong, you switch to the setup mode, adjust the sub's volume accordingly, then switch back to Test to see the effect of these changes. Once the sub volume is in the right ballpark, you need to look at the effect of polarity reversal, leaving this control in the position that produces the smoother response in the crossover region. In my system, this was the minus (180° out of phase) position. With these basic settings done, the frequency-response curve will probably show extension down to 20Hz, albeit with various peaks and valleys.
Then things get a bit more complicated. The crossover slope (6–48dB/octave) and phase (0–180° in 15° increments) can be adjusted; the user is encouraged to experiment with these controls to produce the most linear response. This is probably the trickiest aspect of the setup procedure, because there are no set rules for adjusting these parameters; it's all a matter of try-it-and-see, and effects on the system frequency response are unpredictable. After fiddling with these controls a bit, I decided that default 24dB/octave and phase in the 180° position seemed to work as well as or better than any other settings, so I left them at that.
The really fun part comes with the parametric equalizers. There are eight of them, covering the range from 15Hz to about 120Hz, and each can be adjusted for frequency, level (+6 to –13dB), and Q (width of the frequency range affected). The idea is that, having looked at the peaks and dips in the unequalized frequency response, you move one of the parametric EQs to match the peak or dip, and decrease or increase the response at that frequency. Switching to the Test screen allows you to see the effect of the equalization.
In general, cutting a peak is fine, but boosting the response to compensate for a dip may not be a good idea—dips are often highly position-dependent, and boosting the output at a specific frequency may place too much stress on the subwoofer driver. For this reason, the DD-12's EQ adjustment range is 13dB in the negative direction but only 6dB in the positive. The manual warns the user not to bother trying to fix small deviations in frequency response, and suggests that an equalized response of +/-3dB is a reasonable goal.
But wait, there's more! Once setup is completed, with all parameters adjusted for a flat frequency response, there are five listening-mode presets, which modify setup parameters (subwoofer volume, crossover frequency/ slope, etc., but not the parametric EQ positions) and provide settings that are designed to tailor the DD-12's performance for different types of source material. The additional settings are Contour Frequency and Contour Level, which provide extra boost at a designated frequency, and a setting called Theater/Music. At the Music end of the scale, the amount of servo-control gain is set to the maximum of "8" (corresponding to the highest level of servo gain and therefore the least amount of distortion), while at the Theater end, it is set to the minimum of "1," which reduces servo gain and thus allows greater distortion but a more impressive effect with explosions. This control lets you adjust the level of servo gain anywhere between these values.
The five presets, labeled Action/ Adventure, Jazz/Classical, Movies, Pop/Rock, and Custom, can be selected from the remote control. A sixth preset defeats the EQ. Navigating the various onscreen menus and getting the cursor to the right parameter is simple enough once you know how everything works, but I wouldn't call it intuitive—you'd better keep the user's manual handy.
Dialing In the Performance
My home theater room allows very little flexibility in subwoofer placement: the only place I can put the sub that doesn't require inordinately long cables or that doesn't result in people tripping over it is in the right rear corner. Fortunately, this placement is also very good sonically, so it's where I placed the DD-12.
At first I just turned up the sub volume until it seemed about right, and used it that way for some time, watching movies and playing some music without trying in any way to be critical of the contribution of the subwoofer to the sound. Then it was time to go through the calibration/setup procedure.
The initial Test sinewave sweep, with the DD-12's volume at "0," showed that the response of my main speakers began to decline at about 80Hz to a plateau between 63 and 50, and then a steady decline below that. Turning up the DD-12's volume gradually while checking the effect onscreen in Test mode, I brought up the bass level so that the overall response became more full-range. The linearity of the bass response improved further when the DD-12's polarity was changed from positive to negative. At this point, the frequency response in the bass region actually looked pretty good, the most noticeable deviation in the Test sweep being a dip at around 160Hz—which is out of the subwoofer range, and was visible in the no-sub sweep as well.
Just eyeballing the system-response graph, I could see a little peak at about 23Hz, another at 55Hz or so, and a bigger one at 110Hz. I moved the parametric EQ frequencies to appropriate positions (easily done by aligning the indicators) and decreased the response at these frequencies. The graph in the Test sweep revealed that the effects were as expected: the peaks were reduced or eliminated.
Finally, keeping in mind that EQ boosts are less desirable than cuts, I brought up the response at 90Hz by a few dB, reducing the slight dip at that frequency. The response from a bit above 20Hz to about 130Hz was now impressively flat, the deviation from linearity in the neighborhood of +/-1dB—considerably better than the +/-3dB that Velodyne considers acceptable!
The setup and calibration procedure resulted in a frequency response that was quite linear at the microphone position, but the DD-12's manual points out that making the subwoofer sound right to you may involve increasing the subwoofer volume beyond flat response, even after the major peaks and valleys have been eliminated by the parametric EQ.
This was confirmed by my experience with the DD-12. Playing DVDs and CDs that have a lot of low frequencies, I found that the bass after calibration was very clean but a little weaker than I'm used to. Bringing up the subwoofer level by 2–3dB restored what I considered to be the proper balance.
At this point, the sound was truly excellent, the bass very deep and tight, without any bass-region frequencies being overemphasized or dropping in level. The many different drum sounds on Mickey Hart's Planet Drum maintained their unique characteristics, and the low synthesizer note that begins track 4 was super-clean and fully present in the room.
Another bass test that I like to use is the DVD of Jumanji: the scene early in the movie when the two children (including Kirsten Dunst, long before she got involved with that Spider-Man fellow) run upstairs to find the source of the drumming sound. Each drum stroke had a crisp character, with natural decay, and the loud bass drum that signals that they're about to find the Jumanji game was really loud—but, again, crisp and well-controlled, with a definite tone rather than being just an amorphous bass sound. The only bass test that the DD-12 failed was the 16Hz organ note in Saint-Sans' Symphony 3. I could hear something going on down there, but not the shake-the-furniture effect I'd heard with the much larger Bag End Infrasub-18.
The DD-12's listening-mode presets allow some selective tailoring of the subwoofer's response to match different source materials. These presets provide some worthwhile options, but I have a problem with labeling each preset with the name of a specific type of program material. Action/Adventure provides a boost at 35Hz, but most movies of this type already have overly boosted bass; in my opinion, selecting the corresponding preset that adds even more boost is not a good idea. Similarly, pop/rock is notorious for bass overemphasis; listening to this type of music, the last thing I'd want to do is to select the Pop/Rock preset, which introduces a boost at 45Hz.
Fortunately, the parameters of each preset are user-adjustable. My in-clination would be to set up just three presets: one with relatively flat response, another with some boost at, say, 35Hz, and another with greater boost at 40Hz. I would then select these only as the program material required it.
The Velodyne's listening-mode presets' Theater/ Music (servo gain) control is an interesting one, allowing a tradeoff between distortion and maximum subwoofer volume. I compared the effect of setting this control at 8—the "musical" end, corresponding to highest servo gain/least distortion—and 5, which is about halfway to the "theatrical" end of the scale. For music, I preferred a setting of 8, and for movies, I preferred a setting of . . . 8! At 5, the bass sounded looser, less well-controlled, and while the change was not great, I preferred the crisper sound at 8. Perhaps if I had a larger room, or wanted the bass to be ear-popping rather than necessarily accurate, then I might have gone for the lower servo gain setting; but, given my room and listening preferences, "musical" is also "theatrical."
What about the sonic effects of parametric EQ? The DD-12's remote lets you compare the equalized and unequalized sound, keeping other settings the same. I made some comparisons with music CDs as well as bass-intensive DVDs, and found it hard to consistently identify a difference between the sonic effects of the equalized (Classical/Jazz preset) and unequalized settings. The most I can say is that sometimes equalized string bass seemed a bit more "rounded," but it was very close. This is not all that surprising, given that the measured frequency response before EQ was already pretty flat. The parametric EQ is much more likely to be useful in situations where the subwoofer's in-room response shows much greater deviations to begin with.
Considering the Digital Drive DD-12's basic subwoofer performance—putting aside the digital calibration and setup options—it's clear that Velodyne has built on the high standards established in their HGS series. The package is still extremely compact, with a sleek, elegant appearance, and the combination of the new amplifier and driver, plus the enhanced servo control, enables the DD-12 to produce bass that goes deep with minimal distortion—bass that is tuneful as well as powerful, suitable for music as well as movies.
Its digital calibration and setup options allow the DD-12 to perform well in rooms where subwoofers without this sort of technology are unable to realize their potential. As it happens, the subwoofer placement in my home theater room is such that subwoofers don't need much help from equalization to sound good, but the DD-12's calibration and digital EQ performed exactly as claimed, virtually eliminating the minor response deviations in the bass range and producing an extremely linear frequency response.
At $2999, the Velodyne DD-12 might seem on the pricey side, but not when you consider that it includes a sophisticated digital measurement and parametric equalization system. With the DD-12 and other subwoofers in its Digital Drive range, Velodyne maintains its status as a leader in this highly competitive field.