Our company tries to respond quickly to customer needs. This issue of getting high output below 25 Hz is a good example. Previously, we felt that the majority of customers who buy the STF-3 do not need response below 25 Hz. Posts like yours make us reconsider that. We came out with a very simple and cost effective solution to make the STF-3 more appealing to consumers like yourself – plug up one port. The fact that our manual did not mention the option does not mean that one cannot do it or that it is not recommended. It is a different story if we said specifically in the manual that we do not recommend operating the subwoofer in that fashion.
The loss in output with 10% harmonic distortion when switching to one port is not as bad as you think. Thanks to the soft power supply of the BASH amp, with one port open, the impedance at 25 Hz is much higher than when both ports are open. With both ports open, the impedance at 25 Hz is very low. The maximum voltage swing the amplifier can deliver is much lower than when one port is open, where the low impedance is shifted down to around 18 Hz. The result is no loss of output at 25 Hz for 10% harmonic distortion. The woofer has a much higher excursion at that SPL compared to the two port open case (roughly 10 times higher), but harmonic distortion is still below 10% up to the same SPL. The only 1/3 octave spaced frequency measured that showed significant reduction in output with the 10% harmonic distortion criteria is 31.5 Hz where we measured a 2 dB reduction.
Regarding frequency response, using one port versus two ports and not adjusting the EQ is roughly equivalent to comparing a sealed box system with a system Q of 0.7 to one with a system Q of 0.5. The frequency response starts to roll off earlier. In this case, the roll off begins at around 40 Hz, being down about 2 dB by 25 Hz compared to both ports open. However, Edward’s expectation of the frequency response being down 7 dB at 20 Hz compared to the two ports open situation is erroneous. Edwards forgot that with both ports open, below 25 Hz, the output of the port gets out of phase with respect to the woofer output. This result in a 24 dB/Oct acoustic roll off below 25 Hz. This is absent in the one port open condition, down to the single port resonance of around 18 Hz. This means that the roll off rate between 25 and 18 Hz is far gentler with one port open. The result is the frequency response at 20 Hz is down less with one port open than with both ports open, even without changing the EQ.
Now, some customers do prefer an overdamped frequency response. Some feel that it gives a ‘tighter’ bass. Hence I would not dismiss this option entirely. In fact, I will make it official that customers who prefer an overdamped characteristic can set their VTF-3 or VTF-2 to two ports open EQ, and plug up one port. It posts no danger to the subwoofer (on the other hand, we do NOT recommend opening both ports and setting the EQ to one port open).
