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HSU VTF-15H Subwoofer Review Supplemental

by February 16, 2011

This article is an addendum to our original research and testing of the Hsu Research VTF-15H subwoofer.  One of the statements we have read online since publication of the original review, is about our failure to use the sub with a bass management system, which can cause it to sound muddy if the internal crossover was bypassed.  Well, we were silent on the issue, but in point of fact during my listening tests with the Hsu VTF-15H, I did in fact use a DSP based loudspeaker management system, the Xilica XP-4080. (http://www.xilica.com/?c=78&cat=2&id=5) This is a $1200 system meant for professional use, and I have one in my laboratory.  I failed to mention its use because frankly it is overkill for a $1000 subwoofer.  For the record, when I bypassed the Hsu's internal low pass filter, which is limited to a top cutoff frequency of 90 Hz, I used the DSP system and varied the crossover point up as high as 120 Hz.  I listened to the Hsu with the crossover set as both a third and fourth order (18 or 24 db/octave) low pass.  My complaints from my listening tests with the Hsu were in NO WAY based on its high frequency performance which I found excellent for a subwoofer.  It was based on the poor damping near 30 Hz which is evident at or near maximum output levels of the system.  NO BASS MANGAGMENT SYSTEM can cure that ill, it can only limit the output of the Hsu sub to its linear maximums.

Because many of the shootout entrants have multiple controls allowing multiple settings which can result in a nearly infinite number of potential results to be fairly and accurately measured, each shootout entrant was given instructions to send me ONE SET of settings which I would use for CEA testing to determine the maximum output of the subwoofer.  Dr Hsu was there in person for testing, and was able to tell me himself which settings we would use for our test.  Despite that fact, we have received considerable criticism for failing to test the alternate mode of the subwoofer, stuffing one of the ports with a provided foam plug.  This foam plug will lower the tuning substantially and at the cost of maximum output at higher frequencies.  We have read and heard that this extended mode offers great advantages. 

So, because of the numerous requests by both our readers and readers on the AVS Forum, to review and perform additional testing on the HSU VTF-15H subwoofer,   we did this.  Not only did we perform more CEA 2010 standard maximum short burst SPL tests, we repeated the CEA tests with the supplied unit three additional times.  First repeat test measured the result with a single burst.  The second round of testing was performed as per the first except using 8 bursts, repeated at one second intervals and averaged by computer to lower the “random noise” floor.  This method improves the signal to noise ratio in order to gather more reliable results in the event random noise like that generated by passing trucks is entering the measurement space and interfering with the SPL results.  Finally, a third test was run later in the test day, after the ambient temperature increased.  Prior to the third test, the microphone was re-calibrated at this new temperature and humidity.  (The morning was cold and damp, and the afternoon, sunny and mild).

In the morning the amp was cold, and in the afternoon, it had been on (without playing signals) and had become warmer.  The same tests were once again performed at this higher temperature and lower humidity (both of which can cause variations in the pressure and microphone performance.)  All four sets of results are shown at the end of this article, followed by a short summary for those of you who want to get right to the max SPL numbers without reading through the many steps on the way which helped to lead us to our conclusions. 

In the current CEA 2010 standard, the majority of systems with ports on the bottom will appear to have more output at the lowest frequencies (where the port takes over from the woofer) relative to the frequencies over which the woofer is the primary or sole radiator producing acoustic power for the sole and simple reason that the mike will be closer to the port, than the center of the speaker.  In plain English, the current standard should cause the curves to tilt, in favor of greater output in the region where the port contribution is highest. (Which is almost always the lowest frequency in the range of the system).  This is not a situation where manufacturers, always fighting to get lower and lower frequency performance in smaller and smaller boxes, are likely to be standing in line to complain about for what we had hoped, were quite obvious reasons. (Till we started reading the blogs that is).  We addressed this issue in our Audioholics 2010 Subwoofer Shootout Measurements overview document but it apparantly escaped the attention of some readers.  In the near future we will be addressing CEA directly in hopes of improving this standard.

Unfortunately, the CEA standard does not presently take into account these orientation issues, and since the measurements we have received from Hsu Research have some discrepancies relative to ours, we reran the tests with the ports on the bottom as per the CEA standard as well as Hsu Researches current practices (I base this statement on the agreement between curves they submitted, and those we generated independently, not from personal knowledge). We were hopeful that any discrepancy between our findings and that of Hsu Research would become obvious by additional measurements. 

hsu back.jpg 

Before proceding we felt it necessary to point out that the HSU VTF-15H has two switch positions on the amplifier back panel called "EQ 1" and "EQ 2", respectively.  EQ 1 essentially runs the sub fullrange while "EQ2" employs a 30Hz second order HPF filter for better mechanical protection of the subwoofer driver. HSU recommends running "EQ"2 with both ports open for maximum output while they recommend running the sub in EQ1 for 1 or both ports plugged for maximum extension mode.  In this report, we test both positions and show their respective tradeoffs associated.

The first curve (below) was taken in November 2010.  In comparison to the second curve, it shows some slight differences due to the side (horizontal) vs standing (vertical) orientation.


Original November Measurement Displayed with 40db dynamic range, EQ2

HSU Research's argument after our original review of the VTF-15H published was that due to their port shape and location, placing the sub on the side would put their product at a disadvantage so we decided to see what kind of merit there was in the argument, and expanded our testing at great expense of time and effort.   In the retest, we placed their sub upright.   The curve taken under the same conditions save for box orientation is shown below.


Same system, same location, same measurement equipment, Normal Box Orientation
(Meaning ports at the bottom nearer the mike than the woofer)  & “Q” = 0.7 & EQ2


HSU supplied Graph of the VTF-15H showing the effects of rotating
the “Q” control knob on frequency response – Note 10db notch at 250 Hz

The more choppy appearance of the Hsu curve is likely a result of two things.  First, the dynamic range shown is less, 35 not 40 db as the Clio curves are.  Second, the number of points used to generate the curve is likely a small fraction of those gathered by Clio, which smooths the data to 1/24th octave precision, about 8 times what the human ear is capable of.   The most salient feature we see in the second and third curve  is the increase in the depth of the notch due to out of phase information from the speaker and port at about the 250 Hz mark. With the ports on the bottom closer to the microphone, (changing both the phase and amplitude relationship between the port and woofer) the depth of the notch at low level inputs, where the system is linear, has increased from about 6db (side orientation) to 10 db (normal box orientation). 

What is curious of course, is the difference in the low end.  While my measurements show it rising, in the Hsu Research measurements, it is not. 

In both instances, the Q control was set to 0.7, (Implying the filter it controls is set to the “maximally flat” position) and the High Pass switch (which for some reason is labeled “operating mode”) was set to “EQ2” position, which I had supposed works best when both ports are open since these are the settings Dr. Hsu recommended I use for acquiring data during his November visit. (Later measurements will show this is the WRONG setting for both ports open). Still, all in all, this is pretty good agreement, especially with regard to the point at which the system rolls off.  (Of all the entrants, Dr Hsu was the only one present for testing, and this is not surprising as he is located in our neighboring county and personally delivered the VTF-15H to my laboratory.)

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Recent Forum Posts:

meheather33 posts on February 27, 2013 23:45
Help!!! I am trying to decide between hsu-vtf-15, svs pb16 ulta and martin logan descent. They will be going with my ml vistas,stage and vignettes. Any suggestions?
ooobbbccc posts on February 26, 2013 08:02
Thanks for the help. Just found out VTF-15h is certified “Large” with a bit of searching

2010 Subwoofer Shootout Room Size Recommendation Comparison — Reviews and News from Audioholics
gene posts on February 25, 2013 13:51
kmpurc56, post: 944810
From a layman's perspective, the biggest problem I have is how the measurement data is not the same for each review. If you look at all your reviews together post CEA, the presentation of data varies. The data Gene provides shows a significant more screen shots of test results than than others. The data presented in other reviews will vary as well. I have enough of a background that all testing criteria be exactly the same for all subwoofers for evaluation purposes. I should see the exact number and types of screen shots of your test measurements in each subwoofer review. I also wish that explanations be in such way that I don't understand a review for a SVS subwoofer that indicates that a dbspl range of higher htz 95/93 db to lower htz 87/85 db is rated for big rooms and plenty of low output would not be more limited than this review which seems to indicate output range from dbspl 100db to 95/93 db is limited.

I know price point is a real factor, but I believe a range should be developed for example $500 to $1000 grouping. It is hard to translate the term this product compares to other products at much higher price points then see above issue. I did some research since I have a background in noise exposure and found a 90 db level at below 80 htz is still restricted by OSHA. A 4 hour exposure limit is required. Even at this limitation, continual exposure will result in hearing loss. It seems to me that a flatter range with a lower db output in the higher range and higher output in the lower range would be better. One consistency I do see is not matter what the sub is (brand and expense), there is always a substantial drop near, at or below 20 htz. Since we feel more than hear below 20 htz, I have read were the vibration is observed by the listener at lower db rates anyway. Between 30 and 80 htz is where a more continual output will occur, higher db exposure at 100 or more db will potentially cause more hearing loss issue. A logical view might be that performing numerous evaluations over hears might negatively effect listening part of reviews.

You're obviously new to the site and not aware of the following:

Powered Subwoofer Testing: Outline and Procedures Overview — Reviews and News from Audioholics

Audioholics Subwoofer Room Size Rating Protocol — Reviews and News from Audioholics

Audioholics Subwoofer Measurement Data Compilation & Report — Reviews and News from Audioholics
(this link not only teaches you how we measure and what it means, but it also has a PDF with all data tabulated in a very consistent manner).

I challenge you to find another A/V magazine that even remotely does this, and at this level of completeness and accuracy.
zhimbo posts on February 25, 2013 13:41
The Bassaholics rating is relatively new-ish and was not, for most of the history of the site, a part of subwoofer reviews.

There is a spreadsheet of all rated subs here:

Audioholics Subwoofer Measurement Data Compilation & Report — Reviews and News from Audioholics

The link to the most recent spreadsheet is at the bottom of that page. The Bassaholic ratings are on page 4.
ooobbbccc posts on February 25, 2013 00:34
Why left out the Bassaholics rating again? Is this “Medium” or “Large”?
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