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Old 05-23-2011, 11:00 PM
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Default How boxes "LOAD"

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.

IMPEDANCE


The Fsc(fc) says 39hz and the impedance peak is at 39 or atleast CLOSE.

Why?
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.

Anyway!....

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?
AT tunning, 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.

Below
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.

Last edited by Novicaine; 05-23-2011 at 11:05 PM.
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Old 05-23-2011, 11:03 PM
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good writeup bro
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Old 05-24-2011, 05:13 AM
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impedence curve unloaded

impedence curve loaded (this should explain alot)

test enclosure for loading unloading test
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Old 05-24-2011, 11:42 AM
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Hurricane, you're getting better at explaining stuff, but I think it's still a tad convoluted. I think what people are talking about with "loading" is summed up concisely here:

Quote:
Originally Posted by Prometheus View Post
Loading is an industry term. Normally in reference to the placement of a sub in an enclosure or an enclosure in the vehicle. When applied to speaker placement it is reference to the amount of back pressure or restoring force created by the enclosure. I.e. if you were to place a subwoofer too close to an oversized vent in a large vent you run the risk of the subwoofer not getting enough backside reinforcement and "unloading" or running somewhat free-air. Another example is if you were to fire the driver into a hard reflective surface you can "load the cone" giving it additional support from the front of the cone as well. The danger is too much loading will cause excessive heat build up in the gap, which could in turn lead to premature woofer failure.

When speaking of placement in a vehicle the term is similair to "corner loading" a term normally associated with home theater. You want to place the enclosure where you will recieve the largest amount of acoustical reinforcement for maximum output, the drawback is the same as in home; too much loading/reinforment and the bass will become muddy or artificial sounding.

subwoofer loading - Realm of Excursion

p.s. thank you for taking the initiative, it's appreciated.
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Last edited by Wheres The Butta; 05-24-2011 at 11:55 AM.
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Old 05-24-2011, 05:56 PM
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Quote:
Originally Posted by Wheres The Butta View Post
Hurricane, you're getting better at explaining stuff, but I think it's still a tad convoluted. I think what people are talking about with "loading" is summed up concisely here:




subwoofer loading - Realm of Excursion

p.s. thank you for taking the initiative, it's appreciated.
I wasn't referring to what people are usually meaning, i know people are often referring to subwoofer placement mostly, either of the subwoofer inside the box or the subwoofer/enclosure in a vehicle. In a vehicle it has to do with cabin gain called "spatial loading".

This thread was about how enclosures load with the subwoofers and the way the response is created. I wrote it for those who are wanting to know more about how to design ported, selaed, and bandpass boxes. But it's only the fundamentals.

I did get a little into it, but i wanted people to understand alot more then a simple few lines about it.
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Old 05-24-2011, 08:33 PM
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Quote:
Originally Posted by HACKMUNCH View Post
impedence curve unloaded

impedence curve loaded (this should explain alot)

test enclosure for loading unloading test
How dare you wear a Pm sweatshirt...
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Old 05-24-2011, 08:56 PM
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Quote:
Originally Posted by cgriffin224 View Post
How dare you wear a Pm sweatshirt...
hell is that? lol....
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Old 05-24-2011, 11:46 PM
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Quote:
Originally Posted by Huricain3 View Post
I wasn't referring to what people are usually meaning, i know people are often referring to subwoofer placement mostly, either of the subwoofer inside the box or the subwoofer/enclosure in a vehicle. In a vehicle it has to do with cabin gain called "spatial loading".

This thread was about how enclosures load with the subwoofers and the way the response is created. I wrote it for those who are wanting to know more about how to design ported, selaed, and bandpass boxes. But it's only the fundamentals.

I did get a little into it, but i wanted people to understand alot more then a simple few lines about it.
loading isn't just the enclosure though - it's also to do with the placement. it can refer to either... and from what I understand, it really doesn't have anthing to do with cabin gain. they both affect frequency response, but they don't affect each other to my understanding. I could be wrong, so if I am please explain how they influence each other.
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Old 05-25-2011, 02:02 AM
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Quote:
Originally Posted by Wheres The Butta View Post
loading isn't just the enclosure though - it's also to do with the placement. it can refer to either... and from what I understand, it really doesn't have anthing to do with cabin gain. they both affect frequency response, but they don't affect each other to my understanding. I could be wrong, so if I am please explain how they influence each other.
Don't take this the wrong way, but you need to learn more about all of that.

You have Spatial loading, as the name implies it's a form of loading, also as the name implies, space. When you place the subwoofer/box in the back if your vehicle the dimensions of space is loaded. It contrains the soundwaves. With a flashlight's waves, without that reflective sheild behind the bulb to contrain and direct the stray waves forward, there would be no beam, it would be full space and the light would be a globe. This is a part of cabin-gain, so yea.

The other sort of loading that you are refering to is what this thread is about but it has to do with the port. It is just what i am talking about, the port "loads" the subwoofer. When people are talking about the subwoofer beeing loaded they mean how the port works with the subwoofer. I often see people talking about loading issues which is what you are referring too.

If the box is too small, too little Port area, subwoofer too close to the port, missing required flaring, port beeing too close to something either inside or outside etc....This is what you are refering too. This is a lossy affect that is common that will change a predeicted response from something like winisd if it is very bad. What also may be bad is to have a subwoofer with too high Qts, subwoofers like this can have bad loading aswell.

With a box that "has bad loading", it generally means that the resonse is wrong, it dosn't work linearly.

Last edited by Novicaine; 05-25-2011 at 02:10 AM.
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Old 05-25-2011, 05:07 AM
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Quote:
Originally Posted by cgriffin224 View Post
How dare you wear a Pm sweatshirt...
i know lol, was gonna use it to wax my car with lol.
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