HSU VTF-15H Subwoofer Review Supplemental Cont2.

OK, well, what about plugging both ports?  Is there an advantage to be had there?  Well, it seems in the audiophile community, this debate has been going on since I was born (1957) and still rages on today.  Perhaps this is as simple as different folks are sensitive to different things, but despite the vast majority of products released for consumption in the past 25 years being vented not sealed, it seems that sealed still has a lot of fans.  Let's take a look....

Blue and Red (1 port sealed & Q control varied from 0.7 to 0.3)
Green and Yellow (Both ports sealed & Q control varied from 0.7 to 0.3)

It seems the only advantage we have from this approach is the anti-phase from the ports is almost entirely gone (suggesting some small amount of air still leaking through the ports).  Below 100 Hz, however, the lack of porting has caused this low Q driver to have a droopy response which cannot be completely corrected by the Q control of the amplifier. 

Well then, what about the difference between side mounting measurements and normal (port on the bottom) orientation measurements?  Surely putting the sub on the side might introduce a difference which is more visible with one port plugged than both ports open.  After all, the system rolls off higher with the EQ switch set to position “2”, recommended for the both ports open mode.  What if we turn the sub back down on its side and look between 20 and 30 Hz?  See below....

Blue Curve becomes the top (purple) curve & Red curve becomes
the Gray curve when the sub is turned back onto its side
(Level adjustment for clarity of viewing only)

 

Well, it sure does look like putting the microphone closer to the ports heightens their contribution relative to the woofer, now farther away based on the quicker roll-off we see in the top two curves compared to the normal orientation blue and red curves.  Maybe that top port is contributing nothing at all to the output as has been suggested.  Well, lets do a comparison then.  Let's take the plug out of the top port (purple curve) and put it in the bottom port.  Then, we will remove both plugs, and look at the output with the EQ switch in both position 1 and 2.  Lets see below.....

A Whole bunch of stuff, all explained in the paragraph below.  Read on.

What we can tell about the port contribution difference is seen by comparing the orange to the purple curve above.  The orange curve is the ground plane measurement with the bottom port plugged, and the purple curve is the ground plane measurement with the top port plugged. (Remember the box is on its side for both of these measurements, meaning the port is closer to the mike in the purple curve because the orange curve shows the output of the system on the ground with the top port, which is off the ground open, and the bottom port on the ground, nearer the microphone.)   The actual difference appears to be about 0.8db at 20 Hz, and less above that frequency.

As you can tell from BOTH the notch depth at 250 Hz, as well as the bump near 30 Hz, the top two curves are made from the same drive voltage (see them overlay each other in the 100 to 200 Hz range) these curves were taken with BOTH ports open.  The difference between the top most curve (lets call this color Aqua) and the gray one just below it, is the quicker roll-off provided by the positioning of the “operating mode” switch at EQ2 (0/1/2 ports open).  The aqua curve was taken with the operating mode switch in postion EQ1, while the gray curve is taken with the switch in position EQ2. My use of this switch in position EQ2, based on the silkscreen on the amplifier panel suggesting that was the appropriate EQ for both ports open seemed then and now to be a logical choice.  I would suggest that this “operating mode” switch position is as much a determinant in the final curve as is the port configuration or the “Q” control.

It is important to remember when viewing these curves, that this is representative of the subwoofer performance in the long term, and not with a very short duration pulse. The Clio sweep used is 14 seconds in duration, and is a log sweep from 400 Hz to 10 Hz.  That means by the time the amp is at 100 Hz, the maximum output is stabilized, and the amp has reached its limitation.  If Hsu is claiming the amplifier is capable of 350 watts continuous, but 1400 peak, we are wondering a few different things.  First of all, how long is that 1400 watt peak, when the maximum power consumption of the amplifier is 430 watts (the fuse blows when pulling more than 600 watts, so presumably the amp might blow before the fuse does).  Also, if the speaker is already running out of excursion just below 40 Hz (as evidenced by the bump suggesting the BL of the speaker is dropping from the voice coil leaving the gap), why would you want to put 1400 watts into this speaker?  I would think at that point, it would have already bottomed, and possibly beat itself to death based on the mechanical noises it made below 20 Hz. Of course, I have only been designing powered subs for 23 years now.  I certainly don't know everything.  Maybe there is marketing value in the 1400 watt claim.  Not being a marketing expert, I will leave that to the readers to decide.