The term "load" is always beeing used in threads refering to a number of things, but i want to let be widely know what it means for a subwoofer/box system.
DONT WORRY, it isn't something hard to understand. I will quickly explain it.
When you look at a frequency response graph, like from winisd you pay attention to the "tail" and the "front". Or, the low frequency side, and the higher frequency side, respectively.
Let's describe how a sealed box loads first, basically
This is the very basics, not crazy detail to make things very simple
Compare a small box to a bigger box with a given subwoofer, take a look at the Fc
which is simply the "new" Fs of the subwoofer once it is in the box since air in the box ADDs stiffness to the suspension, more air is less added stiffness btw.
So, what happens at the Fc?
The suwboofer is mechanically efficient actually. The Impedance, and Excursion
graph will show you that excursion peaks at Fc and impedance also peaks.
The Fsc(fc) says 39hz and the impedance peak is at 39 or atleast CLOSE.
The answer is not in the scope of this thread, but when the subwoofer has to travel far the impedance rises because the coil is always creating backward voltage as it moves in the field and this is the real reason impedance rise is around. Knowing that you can draw a number of conclusions like less excursion will yeild a lower impedance, or closer to the coils natural resistance which never goes away.
What else happens? near the Fc and under the subwoofer rolls off
which means that as you look to the left side, the lower frequencies have less output decaying more and more as you play lower. This ofcarse is without cabin gian, but remember that cabin gain dosn't work by affecting the subwoofers excursion and direct output, it is an acoustical thing from the cabin.
On to ported boxes
Now we have a port in the box, this ADDs a mass that is SELECTIVE.
If you are teeter tottering with a fat guy that is too big then you cannot resonaate with him, you cannot use your weight to bring him down, you will big sitting in the air while he is stuck on the ground.
A port does that, it will be excited and resonate nicely with another big guy of similar mass. So if a subwoofer trys to play to far above tunning, it is like a skiny girl that cannot get the port to really want to do much, she is bouncing on the teeter totter in the air trying to go back down but it dosnt work.
So what do you know? A ported box acts sealed ways above tunning!! This creates a typical looking sealed resonance on the impedance graph of a model and it lies over to the right side.
What else effects impedance/excursion?
, you have a very efficient system where the subwoofer dosnt even have to move every far, the energy thrown at the voicecoil from the source of POWER from the amp will do the work, controlling the subwoofer is good! Moving it rapidly and with force will yeild a good resonance. The subwoofer at this point dosn't radiate output at this point but the fact that it has LOW impedance and the port has high efficiency means the port will radiate alot of output ALONE, the sub dosn't do so much at this point.
As you go lower and lower the box is unloading
and the port acts like a big hole in the box and cannot catch or load the subwoofer and this is similar to free-air. This creates an impedance rise because the subwoofer moves VERY free and easily moves far efficiently, it also has a reduction in output because the waves will cancel out just like free-air.
Now what is important to know is that there is a PHASE
to think about.
Whenever the subwoofer is playing right above tunning and not on the dot, it is still close and similar to playing at tunning and acts basically like at tunning. But, as you travel farther up it acts less and less like it is at tunning untill it gets to the point that to where it acts sealed. So, what i mean by phase, if you know math, is that there is a degree between closed box action and playing at tunning action and they both combine inbetween. Above where it plays like a sealed box you have alot of exursion there like a sealed box does ofcoarse, and as you travel down to lower frequencies like a tone sweep does, the excursion reduces more and more untill you get to port loading where there is LEAST excursion. So:
35hz port load freq|---------------|closed box
The closer you are playing to the closed box side the more excurion VS port output you have, and visa versa.
Below tunning you know what happens, the box gradually dissapears as you go lower and lower.
So as you can see all resonances have there rolloff where they give out, it is a bandwidth thing.
Graph of ported and sealed impedance, gray is ported, yellow sealed:
The DIP and RIGHT hill on the gray is the port loading resonance, and the sealed Fc resonance respectively.
The yellow one is sealed and shows it's single sealed resonance.
I will add 4th order bandpass boxes if i get some interest.