Surround Sound Speaker Systems

Hsu Research ULS-15 Subwoofer and HB-1/HC-1 MK2 Speakers


The ULS-15

The ULS-15 is a departure from prior Hsu models. First, as the model number indicates, it has a front-firing, 15” driver, the first time Hsu has used anything larger than a 12” speaker. It uses a patented motor (the voice coil and surrounding magnet constitute a "motor") that Hsu claims is linear over a much wide range than typical motors, which becomes especially important in large drivers.

Hsu ULS-15 Subwoofer HB-1 HC-1 Speakers

However, the most obvious difference between the ULS-15 and other Hsu subs is that the ULS-15 is a sealed design. In other words, there are no ports. Enthusiasts debate the relative merits of sealed vs. ported designs with gusto. To oversimplify, there are only so many ways to produce loud, deep bass. The size of the driver, driver excursion (movement of the cone), amplifier power, and size of the enclosure pretty much tell the story. One cost-effective way to increase low frequency output is to design the enclosure with ports so that the cabinet resonates at certain frequencies. The size and placement of the port ducts and enclosure dictate the resonant frequencies. It’s a way to increase output and extension without having to build a bigger cabinet or use a larger driver.

Of course, there is no such thing as a free lunch, and there are downsides to ported designs. The most commonly observed one is that a ported speaker has poorer transient response than a sealed design, particularly at low frequencies. The technical term is “group delay,” which means that the low bass response starts later and ends later than the rest of the sound. I recently read a technical article that claimed studio monitors exhibiting group delay were a bigger problem than speakers with uneven frequency response. And in fact, most recording and mixing studios use sealed monitors.

But whether group delay is a significant issue for the typical home theater user is less clear. On the one hand, it can be argued that when a depth charge explodes in U-571, the consumer is more interested in loud, room shaking bass than whether the signal is perfectly aligned in the time domain. On the other hand, when listening to Hey Nineteen from Steely Dan, my musician brain wants to hear the tight, locked-in kick drum and bass guitar in perfect sync with the rest of the song.

Unlike Hsu’s VTF series (Variable Tuning Frequency) which allows the user to configure the sub for either maximum extension into lower frequencies, or maximum output by plugging the ports and selecting a switch on the sub, the ULS-15 is a sealed design, designed for maximum extension. Hsu’s published specs list the ULS-15 as being down only 1dB at 15 Hz.

The ULS-15 is driven by an onboard BASH amplifier capable of 1000 watts RMS (short term). The back panel is similar to that of the VT3-HO, with line and speaker level inputs. However, the ULS-15 also sports two balanced XLR inputs. There is a defeatable low pass filter (30 Hz - 90 Hz, at 24 dB/octave) for use with the speaker level inputs, variable phase control from 0-180 degrees, and a ULF trim dial (adjustable from 16 Hz - 60 Hz) designed to compensate for standing waves caused by room boundaries.

Hsu ULS-15 Subwoofer HB-1 HC-1 Speakers

The ULS-15 also offers a wireless (RF) connection to the receiver or pre-pro, using a separate transmitter powered by a wall wart.  The transmitter connects to the subwoofer-out RCA jack on the receiver. The transmitter actually has two mono RCA inputs, and Hsu recommends using a Y-cable splitter (not provided) so that the signal is sent through both inputs. The transmitter has a four-way DIP switch, which provides four different channels to minimize interference with other wireless signals, and also to allow you to use more than one discrete channel subwoofer, such as front left/right.

Hsu ULS-15 Subwoofer HB-1 HC-1 Speakers

The default channel 1 worked fine without interruption from my home’s Wifi network, but occasionally picked up interference from my son’s Wii when he connected it to my main listening system. Flipping the DIP switch resolved the issue. The transmitter has a small antenna that can be adjusted both vertically and horizontally to maximize reception. I placed the transmitter on a shelf inside my equipment rack, but there were no reception issues. Note that you have to match the dip switch setting on the transmitter to the setting on the receiver.

The subwoofer has a matching receiver antenna that plugs into the back of the unit, along with a switch to set the sub to wireless (rather than wired) operation. Wireless subs seem to be the next big thing. Several manufacturers were showing them at CEDIA, and after spending a little time with the ULS-15, I can see why. Subwoofers are very sensitive to room placement, and moving them even a foot or two in one direction can seriously alter the output at various frequencies. So while sub placement is still tethered to finding conveniently located AC power, you can now put the sub anywhere without wondering how you are going to run cable from the sub to the receiver. Wireless connections also eliminate one more possible source of ground loop hum.

Hsu markets the ULS-15 in single, dual, or quad configurations. The DualDrive consists of two ULS-15’s at a discounted price of $2,199 (satin black), and claims up to 6 dB more output than a single ULS-15.

Subwoofer aficionados know that multiple subwoofers are generally the best way to increase output, in addition to combating room modes with careful placement. The QuadDrive system has four ULS-15’s. Hsu states that a QuadDrive can provide up to 16 times the output of a single ULS-15 (a whopping 12 dB). The QuadDrive system sells for $3,999 (satin black finish). Considering that a single ULS-15 costs $1,299, that’s a volume discount of almost $1,200.