HSU VTF-15H Subwoofer Review Supplemental Cont.

So far so good.  As far as getting repeatable data, there does not seem to be any significant errors in my data.  Now to repeat the measurement with the Q control set to the opposite end, 0.3. (Maximum damping).  (See below)

Same as previous conditions (EQ2), except the “Q” control is set to “0.3” (Maximum damping)

Well, this curve is certainly flatter than the first one, and when driven at low levels (red curve) it again, does not roll off till about 30 Hz.  At very high input levels, where the system is running out of power or excursion or both (Top gray curves) we can see a hump caused either by a lack of current available from the amp, or perhaps the loss of BL that occurs when the VC leaves the gap.  Seems to me this system was designed expressly to allow for the higher roll-off of 30 Hz, perfectly suitable for music in 99% of the time, and with the Q control set to minimum, at the ground plane it looks perfectly flat! 

Time to see what happens when the system is set to a lower tuning using the available triangular shaped port plug.  Let's see....

One port Plugged, EQ1 position & Q Control = 0.7 (flat)

As is clear from the red curve, the system is now in an extended mode.  What is also clear is that the maximum output relative to the both port open modes has been compromised between 20 and 30 Hz.  The reality is, there is far more music from 30 to 40 than there is from 20 to 30 Hz, and it is a heck of a lot easier to reproduce at high SPL's.  To make matters worse of course, we see all this choppy response going on below 20 Hz?  Is it ambient or background noise?  No.  How can I tell?  If it were ambient/background noise, it would be worst at the red curve, and least objectionable at the top gray curve, because the more output the sub makes, the less influence ambient/background noise will have on our measurement.  What was happening (and I could clearly hear this, as I was sitting outside with a clear path between the measurement rack, and the sub) was there were loud mechanical noises being produced by the subwoofer in trying to reproduce very high levels of signal below 20 Hz.  (Notice with the lowest drive voltage, the red curve is free from this effect.)  There is another reason why this is happening.  With the EQ set to position 1, the roll-off of the subsonic filter is changed to allow much more LF output to reach the speaker terminals.  It appears that one set of controls is for making great curves for sales, while the other set is a more realistic setting and cutoff to allow this product to produce more than adequate low end, (30 Hz cutoff) for the vast majority of music.  Because plugging the second port reduces the cutoff frequency while increasing the damping of the system, using the "Q" control allows you to make the system response flat by raising the filter "Q" setting to 0.7. This might also explain Dr Hsu's preference for me to use EQ2, with the higher order high pass filter in circuit, so as not to reveal a driver bottoming issue.

What does the system look like with the same 1 port plugged and EQ 1, but the Q set to 0.3 (Maximum damping)?  Lets see....

Same Settings as Above (EQ1) test except for the Q control set to Maximum Damping (0.3)

As we can see with the higher damping used, there is less stress on the system below 20 Hz, so those 16Hz pipes, are less likely to cause the woofer to go into distress, but even with the very limited steady state power available from the BASH amplifier, (350 Watts) it was more than enough to cause the woofer to overload when driven below 20 and above 10 Hz.  Essentially, although we can force this woofer to have a flatter curve to a lower frequency, we simply can't make it move more air than it is capable of.  In other words, the maximum output at very low frequencies is displacement related as well as power related.  At maximum power, this woofer, even with the one port plugged STILL ROLLS OFF AT 30-35 Hz due to output limitations.

As we previously saw from the HSU supplied graph varying Q for the VTF-15H the range of control of the “Q” switch goes all the way from the minimum usable frequencies up to 200 Hz.  My measurement (below) agrees, and shows the effect of the Q control from minimum damping (red) to maximum damping (blue).

Range of available control from the “Q” knob, 0.7 (blue) to 0.3 (Red) with 1 Port Blocked, EQ1

If we are to calculate the value of the frequency at a given “Q” the formula is simple, although not well known.  Response in dB = 20 Log Q.  So, for 0.7 at the cutoff frequency (Fc) response is -3.1 db.  For Q = 0.3 the response is -10.5 db.  One would suspect then, the resultant difference to be about 7 db.  Best as I can tell, the difference is only about 4 db.  (Total dynamic range of this curve is 20 db, and each small tick mark represents 0.8 db.)  Perhaps Hsu Research can shed some light on the choice of markings used on this amplifier.