Component Reviews

Hsu Research ULS-15 Subwoofer System

Its presence was a welcome addition to my system. I never had any problems and was never dissatisfied with its performance.

Published on October 18, 2010

Hsu Research ULS-15 Subwoofer System

Hsu Research ULS-15 Subwoofer System
$1300 Black/$1400 Rosenut each, $2200 Black/$2350


The system I reviewed consisted of two ULS-15 15” woofers; rated extension -1 dB at 15 Hz; BASH 1000W short term amplifier power each; bypassable 24 dB/octave with 30-90 continuously variable low pass filter; 0/180 switchable phase; Ultra Low Frequency (ULF) trim control; Inputs: stereo balanced XLR, stereo RCA line level, speaker level and wireless with supplied transmitter; 93 pounds each; 18.875” H x 18” W x 20.25” D (19.25” w/o grill); 7-year warranty on speaker, 2-year warranty on electronics.

Hsu Research
985 N. Shepard Street
Anaheim, CA 92806
800-554-0150 (voice)
714-666-9261 (fax)

Audio/Video Equipment

Meridian 568.2MM Preamplifier, Sunfire Cinema Grand Amplifier, Bowers and Wilkins Signature 8NT and DS8 Speakers, Oppo BDP-80 Blu-ray Player, Dish Network Vip622 HD DVR, Roku HD Netflix Player, Popcorn Hour A-110 Media Player, PS Audio Power Plant Premier, Audioquest, DeCorp and Belden cabling.

Measurement Equipment

Dell Studio 1537 running Windows XP, Room EQ Wizard Software V5 (REW for short), Radio Shack 30-2050 SPL Meter, spreadsheet with correction numbers for low frequency response, Tascam US-122MKII External Soundcard/Mixer, Behringer ECM8000 microphone and calibration file from Cross Spectrum Labs.

I took raw measurements with the SPL meter at first and entered the data into an Excel spreadsheet that I downloaded from the Internet (there are multiple tables available, but make sure the table is the correct one for the meter that you have).  Here is a link for calibration files for some of the Radio Shack meters (for REW) and downloadable spreadsheets as well.  Later I received a calibrated microphone from Herb at Cross Spectrum Labs.  The link in the paragraph above goes to the company’s information page and pricing for his microphone calibration services.  If you don’t already have a microphone then the ECM8000 is a good deal with the calibration file.  This allows you to load a file directly in the REW software (and others) to produce flat frequency response within 1 dB from 5 – 25,000 Hertz when using the calibrated microphone.

For the frequency measurements I took readings at the listening position with both the Radio Shack meter as well as the microphone.  For the SPL/distortion measurements I took readings within two inches of the front baffle of a single subwoofer.

Setup and Description

Hsu offers the ULS-15 single, dual or quad configurations.  As you can see (above) there are price breaks if you buy more than one.  My room dimensions are roughly 8.5’ high x 14’ wide x 22’ long with an opening at the front left side (~2800 cubic feet).  I did testing with a single and a dual woofer to hear the benefits of using two subwoofers while checking for differences in the amount of bass and evenness of bass in the room.  The woofers were placed in the back left and back right corners to start.

I had wires run to both these locations, so it was not necessary for me to use the wireless feature.  As my theater system is in the living room and I did not opt for an in-wall woofer solution, I searched for a woofer that would produce quality bass yet integrate with my living room décor (i.e. no black).  There are a host of other woofer options for those who do not have any aesthetic or space issues.  Although the ULS-15 is not exactly small, it did manage to pass the designer and WAF (Wife Acceptance Factor) in the Espresso finish.  As of this writing there are still subs available in this finish as well as Walnut for a slight up-charge.

I wouldn’t consider myself a bass-freak by any means and had Hsu offered a 12” woofer in a smaller enclosure I would have requested it.  Frankly, I thought I was going to have a heart attack after lifting these babies up the stairs.  By the time I got everything connected I was prepared to be “moved” by the sound of a pair of 15” woofers.


The ULS-15 is designed and assembled in the USA.  My wife and I actually visited the factory (as I live in Los Angeles) and were shown their showroom and had all my questions answered by Pete.  There is a miniature golf course and arcade next door (for those who need an excuse to drag the family along).

After getting the woofers out of the box I put the rubber feet on the bottom.  The side corners of the speakers are rounded, so there is little chance of hurting oneself and it makes the woofers look less boxy.  The grills are black, fairly heavy and sit onto the front baffle magnetically.  In the long term I would get them recovered in a cloth that matched the finish of the woofer or possibly leave them off and orient the woofer so the driver isn’t visible.

The woofer can connect via XLR, wirelessly, or via RCA inputs for low level connections (with or without a low pass crossover).  Additionally, it is possible to run speaker wires (high-level connections) direct into the sub for those who have wiring already in place and/or don’t want to connect wirelessly.  I connected the subs with a wired RCA connection.

The subwoofer allows for a 0/180 phase adjustment, crossover from 30-90Hz and a ULF trim.  The ULF trim is a compensation control for very low frequencies to account for room gain.  The manual recommends setting the volume control at 11 o’clock and bypassing the crossover when using a subwoofer output (as I did).  The manual was a bit mysterious about the best way to determine where to set the ULF trim (i.e. for small rooms set it to 50, for very large rooms set it to 16, but other rooms set it in between).  I did frequency response measurements that helped me set the trim as detailed later in the review.  To set phase, play music (or tones) in the crossover region (between the main set of speakers and sub) and set the switch to whichever produces more level.  This is easily accomplished with a test CD/DVD.

The Meridian processor I used during the review has extensive bass management options, so I settled on using its internal electronic crossover and bypassing the switchable crossover in the Hsu.  My front speakers have a rated -3 dB point at 33 Hz, but I chose to set the crossover at 45 Hz.  I could have used the microphone and measured the distortion in the low frequencies at various levels, but I chose to listen and run the fronts as full-range as I could without any obvious distortion.  The manufacturer has gone to great lengths to design the passive crossover and make sure the drivers integrate well, so why make it harder on myself?  Based on data I’ve read THX Labs believe 80 Hz is low enough in frequency to prevent the woofer from being located in the room.  Other people have suggested that this lower limit is more like 60.  In any case my choice was well below these “suggestions.”

The manual is short and easy to digest.  If something isn’t working as it should there are two pages devoted to troubleshooting.


Proper subwoofer placement is one of the biggest pains in audio.  It is seldom a matter of “set and forget” as many would have you believe.  Hsu’s recommendations for placement are: (1) Avoid placing the sub (or yourself) in the center of the room; (2) Place the subwoofer close to the listener unless the room is less than 15’ back; (3) If (2) is not an option, then place the woofer in the corner and away from openings into the room—as I did; (4) Alternatively, you could put the woofer in a piece of furniture, but allow 1” of spacing all around, make sure nothing can rattle and that the woofer fires out into the room; and lastly, try to maintain line of sight to the woofer with the wireless transmitter (if utilized).

Since my location options were fairly limited I didn’t have much room to move the subwoofers around.  I tried moving the subs away from the back and side walls a bit as well as rotating them in various directions.  Happily there wasn’t a great deal of difference in sound quality and the single sub sounded good right out of the box.  Due to the cables sticking out the back panel (and the moulding on the floor) I wasn’t able to get the woofer right up against the wall and it sounded fine within 4-8” away (and looked better this way as well).


I had never used the measurement equipment before so there was a bit of an adjustment period.  The mic/mixer was especially hard to work with as the controls didn’t always seem to do what I expected them to.  In any case I got the soundcard calibrated (almost ruler flat), the microphone calibration file loaded (within 1 dB), and calibrated SPL with the Radio Shack meter (according to the meter within a fraction of a dB).  All the measurements I took were in the listening room, so keep this in mind.  Every room and system will be different.  I include them here for illustrative purposes (and for future comparisons with other subs).

My first attempt at a frequency response curve for one sub using the Radio Shack meter at the listening position was not the best.  I used a spreadsheet with a correction for my meter but the frequency points were slightly off based on the test discs I had, thus the resulting graph wasn’t entirely accurate.  Also, the graph interpolates the results so there is a curve rather than connected points.  Without any measurement equipment this would have been a start at least.

When I got the microphone up and running I was more comfortable with the accuracy of my readings.  The first task was to decide on the ULF setting.  I ran two charts with the ULF all the way up and all the way down.  I found flatter response with the control down.  The REW software offers the ability to smooth the graph and I used a 1/3 octave setting.  The four results are below.

The graph y-axis shows output level and the division is in 5 dB.  I connected the computer’s soundcard directly into the woofer to obtain these results.  The measurement was from 10 – 80 Hz even though the graph does not show up that high.  The smoothed curve with the ULF all the way down shows response within +/- 2.5—a 5 dB range from 10 – 50 Hz!  The unsmoothed curve has a variance of 10 dB.  Measurements away from the listening position (which unfortunately is centered left to right in the room as most are) were even better.

The most notable change when adding the second subwoofer to the system was level.  There was a slight improvement in response, but it was not significant at the primary position.  The listening tests (below) proved that for my listening taste (and level) that a single subwoofer was sufficient.  However, that didn’t deter me from keeping the second subwoofer connected and running during normal listening!  Below is a similar graph to the one above but taken with both subwoofers in use.

From a convenience standpoint these measurements were taken with the subwoofers connected to the output of the Meridian (thus the more restricted measurement range).  As can be seen, the smoothing shows a response within +/- 2 dB while the unsmoothed curve varies by about 8-9 dB across the range.

To be honest I think a lot of distortion measurements of speakers can be somewhat misleading, however I know there are a bunch of readers who are going to be disappointed if I don’t present them here.  So, here goes:

I took measurements for 80, 50, 31.5 and 17 Hz tones.  For the first three I increased the level until I was in the 5% distortion range (not including noise).  For the 17 Hz tone I went up to the point of 12.5% distortion.  See above and below.

Due to the limited range of the graph, you can barely see the evidence of the 2nd harmonic at 100 Hz above.

Notice that the single subwoofer is able to produce 105 dB with only 5% distortion.  2nd and 3rd harmonics are about 30 dB down from the intended signal.

As mentioned above, the 17 Hz tone registered 12.5% distortion at about 98 dB.  Here’s where having a larger driver would have made a difference.  2nd and 3rd harmonics are 20 dB down from the main signal.

Performance of the ULS-15 with actual material was impressive considering the cost.  Compared to many big name brand woofers you’d have to spend a lot more to get this type of performance in a sealed box (that looks nice).

One thing you find running tones through a big sub is all the sources of rattling, vibration and noise in the room.  First of all, the subwoofer did generate its own rattle with material from 20 – 30 Hz at high levels.  It was clearly audible with no other material playing though the system but with real-world material I didn’t notice anything amiss.  What was clearly heard are my old windows and a sunburst mirror on the wall in my living room.  A particular range of really low frequencies would excite the window (that just has two latches).  With some sound dampening I would be able to make this go away.  At high levels I could hear other parts of the house shaking and rumbling.  The mirror is another story…


Although I didn’t need to use the wireless feature I thought it would be important to test this feature.  A single transmitter will support up to 4 subwoofers and there are four frequency settings to eliminate interference.  The transmitter offers stereo RCA inputs while there is nothing to add to the subwoofer except a small antenna that screws into the back plate (included).  When this subwoofer was introduced this feature was very uncommon.  Nowadays other manufacturers are starting to jump on the bandwagon.  For those wishing their sub had wireless capability see right here–it can for $160.

I connected my laptop and external soundcard to the transmitting device and used a long extension cord so I could move the transmitter to different locations in the room.  Anywhere in the listening space (at about a maximum of 25 feet) I was able to get a working signal.  It was only when I went outside the room and behind an outside wall (at more than 30 feet) that I started to lose signal.  If the wireless feature is important I would verify that it will work as expected before committing.


Unlike most equipment under review, subwoofers are just begging to be used and abused.  By this I mean the reviewer, in addition to trying to coax the best performance out of the sub, tries to find its weaknesses by playing it too loud and looking for each and every recording that will make it raise the white flag in surrender.

The first thing to consider with true subwoofers is that they are not only designed to produce extended low bass that you can hear, but bass you can feel as well.  I wasn’t expecting to feel the sound like a large array of 18” woofers, but I wasn’t disappointed in the sub’s ability to create room shaking bass below the realm of audibility (i.e. below 20 Hz).  I don’t typically sit and listen to music with very low bass (like organ), so I pulled out Pink Floyd, the Sheffield Drum and Track Record, Telarc’s 1812 Overture with cannon blasts, Kruder Dorfmeister’s K&D Sessions and some old school rap.

The digital cannons were the only thing that I could get the woofer to flap like crazy (aside from the test tones) and it didn’t break up audibly at the more than comfortable level I listened.  Another thing to consider with a subwoofer designed to only play really low frequencies is that most of the bass in music exists at higher frequencies.  Almost all the sounds of drums and bass (including the rap music) were coming out of the front speakers.  This was fine because the really low frequency bass was still happening out of the subs.  The direction of the sound (aside from the rattling in the room) never came from the back of the room (where the woofers were).  This is very important.  Nothing will bring you out of your enjoyment like hearing where speakers are.  And if you think it is typical that a subwoofer just naturally blends in with main speakers and sounds natural then think again.

I ran additional test tones to verify that the response was not exaggerated in any particular frequency range.  There was a slight gap centered around the crossover, so I flipped the phase switch to eliminate it.  Overall, the combination with the HSU ULS-15 and the Bowers & Wilkins front speakers is one of the best matched I’ve had in my room in years—the only other one that comes to mind is the REL woofer I had a long time ago, but it was much more limited in low bass capability.

Based on my measurements the Hsu has more than enough range to work effectively above the 45 Hz crossover I used in the review.  This means that it isn’t essential to match it with speakers that have significant low bass output.  Placement might have been impacted more by running it at a higher frequency, so I was still happy with my choice.


I might be wrong, but I’m still under the impression that most people buy subwoofers to help out their system with low bass extension for movies.  The LFE channel is specifically designed for a dedicated subwoofer and there is no shortage of super low bass in modern films.  There is a thread on the AVS forum that documents movies that have tremendous amounts of low bass energy.  Many of the films have the exact locations (by timing and chapter) where the bass occurs as well as waterfall plots showing distribution of low bass in a graphic format.

Perusing the list I pulled out some from my video collection including: Lord of the Rings: Fellowship of the Ring, Minority Report, Frequency, Star Wars: Attack of the Clones, and Harry Potter and the Order of the Phoenix.  Just about every action flick these days has at least a few scenes with something blowing up or pounding/throbbing bass in which to envelope the viewer.  The movies above were perfect examples from the exciting opening sequence in Frequency to the powerful low bass that occurs when the large spaceship flies past in Attack of the Clones.

One of the elements of good bass is the ability to start and stop as needed.  Loud bass that just rumbles on incessantly is not likely what was intended.  Proper woofer design means playing the right amount of bass for only as long as it is called upon to do so.  This was one of the strengths of the Hsu woofers—the ability to disappear and not call attention to itself except when the material dictated it should.


There is no doubt in my mind of the necessity of a good subwoofer for use in a home theater system, but subwoofers can be a just as beneficial to a music system.  In many ways a subwoofer has a more difficult job than other speakers and is one of the reasons it may cost more than some of the other speakers in a typical system.  It has to provide deep bass with low distortion and definition, with enough level to fill the space and meet the needs of the material, it needs good transient response and impact, it needs to blend in well with the other speakers, while still producing prodigious amounts of bass and (in my case and maybe yours) it needs to be physically attractive.  Okay, the last one is not essential, but it is sure nice to have.

A single HSU ULS-15 handily accomplished these goals in my listening room.  With the addition of a second subwoofer I was able to play bass at levels that could get downright uncomfortable and clearly outpace my front channels—in other words there was no reason to play the system under those conditions.  However, there was an improvement in bass distribution throughout the room with the second woofer, so a benefit did exist in my system.  Larger rooms can just add more subs for additional output and added room response flattening.

My time with the ULS-15(s) was longer than most of the review samples I get (thanks Pete) and its presence was a welcome addition to my system.  I never had any problems and was never dissatisfied with its performance.  There is always room for improvement and the goals of louder and deeper would probably only be a possibility with a larger driver in a larger enclosure.  One wish I’d have is for some sort of onboard bass EQ with a calibration microphone.  Many enthusiasts employ outboard equalizers to help tame issues of bass response in the room and it would be nice to have this ability “right in the box.”  That and the low price would be hard to beat in the market and go right up against the “big boys.”  Recommended!

Brian Bloom   

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