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Bass is the most difficult part of the frequency spectrum to reproduce in audio systems. Itís difficult because at low audio frequencies the wave lengths are very long. The result is, first, that to achieve reasonable efficiency in creating these long wavelength signals one must use a large mechanical transducer with a large enclosure supporting it. The second result is that these long wave lengths will interact significantly with the room in which they are created, because the typical home theater room is not large enough to allow these long waves to fill the room. The result is standing waves, where one listening position may have greatly enhanced bass at certain frequencies but diminished bass at other frequencies, and a different listening position a few feet away will completely change the frequencies that are enhanced or attenuated.
The traditional method of addressing these problems is the philosophy that "bigger is better." Thus, subwoofer manufacturers use large drivers Ė often 15" or 18" Ė and large enclosures of several cubic feet. This makes the traditional subwoofer big, bulky, heavy, and, in the view of many, ugly. Also, while larger drivers allow more low bass response, they can often become slow and muddy in the mid to upper bass, lacking the detail and articulation needed to follow fast bass lines and the starting and stopping of the bass.
Bob Carver of Sunfire Corporation pioneered a new approach with the Sunfire True Sub. Use a small enclosure about a cubic foot, put a 10" driver in it that has long excursion so it can move more air, and then to make up for the high inefficiency of this system move the driver with a massively powerful but efficient amplifier that can overcome these obstacles. The Sunfire has the reputation of putting out an amazing quantity of bass, but not necessarily the highest quality bass, due to fairly high distortion levels (which some people like in bass, though it is not accurate). Now Velodyne has developed its own implementation of the mini-sub with its HGS ("high gain servo") line of subwoofers, available in 10" and 12" driver sizes. These units are about one cubic foot, and have attractive, high-gloss black cabinets.
In the HGS series, Velodyne uses its patented servo mechanism to help reduce distortion and provide more accurate bass. An accelerometer sensor on the voice coil generates a signal that corresponds to the actual cone movement. This signal is fed back to a circuit that compares the cone movement to the input signal 3500 times each second, and adjusts the input signal as needed to make cone movement accurately track the signal.
Velodyne uses a transformerless, switching power supply to efficiently generate large amounts of power (3000 watts peak) to overcome the high inefficiency of a small woofer in a small enclosure.
So with all this clever technology, how does it work in practice?
The HGS-10 has excellent transient response, starting and stopping very quickly. It has top-notch articulation in the mid to upper bass. This makes it a very musical sub. The superb bass playing of David Pilch on Holly Coleís "Donít Smoke in Bed" CD is displayed in good form, and integrates well with the rest of the music. Also, the HGS-10 can energize the entire room with bass, so that it is impossible to localize the source. The HGS-10ís fast transient response yields an interesting benefit: much better imaging of the entire system throughout the frequency range, with more sense of the original acoustic environment (real or artificial) in which the recording was made. Apparently the ear and brain use the timing of low frequency impulse signals to help localize sound, and having this information accurately conveyed by a fast subwoofer such as the HGS-10 improves all the sound, not just the bass.
Also, the HGS-10 can be surprisingly dynamic when called upon, adding to the overall sense of dynamics of the system.
In my large room the output was quite flat down to 25 Hz, with some drop-off at 20 Hz. Using warble tones, I could achieve 84 db sound levels at 20 Hz before the HGS-10ís built in limiting circuit took over and precluded higher output levels, although maximum level at 35 Hz was over 104 db.
The low bass does not have the impact and authority of an 18" sub (some would say that 18" woofers provide too much bass). Still, it was good enough to vibrate the walls and vigorously shake the hanging light fixture. Itís not surprising that the small woofer and cabinet result in some degree of compromise in the low bass; however, the compromise is relatively minor considering these factors. In addition, in home theater applications nearly all the low frequency effects are at 30 Hz and above. The real strength of the HGS-10 is in the mid and upper bass, where it is very quick and authoritative when called for. In other words, there is no perfection in life, and all products involve trade-offs and compromises.
The cross-over in the HGS-10 is flexible. It is a 12 db per octave low pass for the bass, with a continuously adjustable turnover frequency from 40 to 120 hz. There is also a simple, passive, first-order high pass cross-over at 80 Hz to reduce the bass in the upper frequency drivers, though not adjustable in any way. Because the high-pass cross-over is in the sub, if the sub is some distance from the preamp it is necessary to run a long length of interconnect from the preamp to the sub, and then a long length to the amplifier for the upper frequency drivers. Iím not crazy about this arrangement, because long interconnects run the risk of degrading the delicate line level signal and picking up interference. However, it is the most common approach, except for those rare subs that remove the cross-over from inside the sub and place it in a separate chassis that can be located near the preamp. For home theater use the surround processor provides its internal cross-over and there is no need to run the interconnect back to the amplifier for the high frequency drivers. Also, Velodyne allows the subís low-pass cross-over to be easily switched out of the circuit, relying on the cross-over in the surround processor. This feature is often left out of subwoofers, resulting in two cross-over filters in the circuit that compete with each other and compromise the frequency response and timing of the bass. The HGS-10 includes a phase reversal switch, and a power switch to choose always on or signal-sensing auto-power on.
To achieve the high power required to drive the cone, the unit uses a transformerless switching power supply. This supply does generate some radio frequency interference that radiates outside the enclosure. When the unit was placed too close to the television (closer than about 3 feet) it produced some video interference on the lower VHF cable channels. The unit should not be placed close to the tv set.
In sum, Velodyne has created a small, attractive, excellent subwoofer
that is very fast, is extremely musical, has low distortion, excels in
the mid and upper bass where most bass occurs (and where many would say
large subwoofers fall down), and can be amazingly authoritative considering
its size. Great work, Velodyne. Highly recommended.