- Subwoofer Subsonic Filter & EQ -
The Phlood Philter is a fully adjustable subsonic filter and equalizer that performs two essential functions. One is to provide a 12db per octave subsonic filter, and the other is to provide an adjustable equalization to boost your subwoofer's output at it's lowest reaches.
The "subwoofer scene" is undergoing quite a renaissance in these past few years! Fantastic high-excursion drivers are becoming more and more available, at a better and better price all the time!
These drivers are pushing the low-bass envelope, and allowing sub builders to tune their enclosures at low frequencies that would never have been considered before! If you told somebody 5 years ago that you were building a 12" subwoofer in a 3 cubic foot box and tuning it to 20Hz, they'd have looked at you like you were crazy! (that is, if they knew what you were talking about)
OK, great, so we're getting incredible low-end extension from these drivers. Is there a problem with that?
Well no, but there is a lot of improvement to be made. The trouble with these low-tuning designs is that below your port tuning frequency, you completely lose control of the driver! It behaves essentially as if it had no box at all! To some of you, this may be be very familiar to you when you play your favorite scene from an action movie and have to flinch when the big explosion comes, because here comes the "THWACK!" of your driver bottoming out!
How do you fix this? It's pretty easy actually, and very effective! Besides turning it down, you simply need to apply a 12db per octave subsonic filter. This filter should have a cutoff frequency that is the same or perhaps slightly lower than the frequency you have your sub box tuned to.
To illustrate, here is a modeled excursion plot of a popular 15" driver in a 5cuFt box tuned to 20Hz.
The red line is what happens to driver excursion when 300W is applied. As you can see, it goes way beyond it's excursion limit when driven that hard below it's tuning frequency!
The yellow line is with the filter applied, and tuned for a cutoff frequency of 20Hz. Better, eh? You've got a lot less to worry about now!
But wait! Don't you lose low-frequency response that way?? Well, yes and no. Fortunate for us, it's more no than yes!
Here's why. Now that we have driver excursion under control, we can use the equalizer function of the filter to give us a boost at around 20Hz in this case, to allow us to get very flat response all the way down to 20Hz and lower! Typically, when you tune any driver to a low frequency like this, you lose anywhere from 3 to 9db of response at your tuning frequency! (20Hz in this case. Here is our example again...
This graph compares the unmodified response with that of the Phlood Philter set to a cutoff frequency of 20Hz, and a Q of 2.5. (about 7.5db of boost)
As you can see, we have not lost anything of much value! In fact, we have done nothing but gained output all the way down to 15Hz, where the two lines cross! Essentially, we have traded the frequencies that we cannot hear or even feel for more output at the frequencies that matter! This assisted subwoofer will now sound much deeper than it did before, and it will play louder to boot!
Works great, eh? Not only that, but it's proven to work! Many "plate amps" and commercially available subs incorporate a very similar circuit!
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