Thank you for your time to put this together for me. I'm curious why to only cover half of the inside with polyfill? I have always covered the whole thing. This is probably a stupid question, but shouldn't a larger speaker work better? I know bigger isn't always better. I'm also thinking my amp may be over kill. I also have a Behringer ep 1500. Would that work better for this sub?
You asking these questions as you don't understand the fundamental physics of audio reproduction. So I will have to start from zero pretty much.
The first thing to understand is that speakers are resonant systems. Further they are compound resonant systems.
So lets consider the simple pendulum. If you either, lengthen the pendulum or both the pendulum will swing slower. So the frequency is lower. That swing is the resonant frequency.
Obviously if we do anything in the opposite to the above, the frequency will be higher. You alter the time keeping of a clock by raising or lowering the weight on the pendulum.
Now lets look at the escapement spring and the weight to our clock. Obviously the spring gives it a kick, and is the motor of our speakers, which also affects the resonance. The weight keeping the system going is our amplifier. Obviously our escapement spring has an influence as well.
So our speaker driver has weight, we have a motor system and a suspension which can be another tight or loose spring. Then we have the air in the enclosure, which has compliance, in other words that is another spring to add to the equation.
So the parameters of a speaker, which are the components of these springs we are talking about are defined by parameters called the Thiele/Small parameters. T/S parameters for short. In addition those parameters also define the electrical parameters of the driver. As in a speaker the electrical behavior also has a significant influence on our complex resonant system. Many electronic components also have electrical resonance, and since a speaker has a coil, which is an inductor it also has resonance.
So the job of the speakers designer is to optimize this complex resonant system to provide the optimal result. Which in terms of bass alignment is called, the optimal box.
So I have designed you the optimal box for that driver, and it will be a good performer.
So a box has to be the right box for a given driver. So a bigger box is not better for the driver I selected. You can model extended bass alignments. For that driver increasing box volume to around 5 cu.ft. gives you a 1/4 Hz of bass extension, and gives you a poorer quality bass as a result.
Now, how low a driver will play is pretty much determined by its fundamental free air resonance. Usually a driver will play to a little higher than its free air resonance in a ported box, now and again a few Hz lower. But it is a pretty hard and fast that a driver will roll off at its fundamental resonance known as Fs. The rule holds no matter how large the driver is.
So the driver you picked had an Fs of 35 Hz, so it likely will only play down to 40 Hz or so. I did not bother modelling it, but I have estimated it closely.
The driver I picked for you has an Fs of 23 Hz. So the sub will only be 3 db. down at 22.5 HZ and 6 db. down at 20 Hz. Its maximum output possible at 20 Hz is 106 db. So the design full fills the criteria you gave me, with margin.
As check on the validity of my design you can look at the impedance curve. The nadir of the impedance plot is 23 Hz, with the twin peaks either side. This is exactly how it should look.
As far as stuffing, as explained this is a resonant system So you need just enough Polyfill to kill reflections, so you put a little top and bottom, on the back behind the driver and a little either side of the driver. If you use too much you will kill the box resonance and the sub will NOT work.
I choose this design as you are obviously a novice. This should be a fairly easy build, and you can get peak vent air velocity down to 20 m/sec, without the complication of having to build a slot vent. It is also a highly cost effective design, and will hold its own against pricey subs. However you must follow the plan to the letter and not go off on unsound notions of your own. If you build this design to the letter, I can guarantee you will have an excellent sub.
It is a straightforward design, and if you follow instructions will give you excellent results with your amplifier.
