Box size in relation to sound output and power handling.

[KingPen]

So this is something I’ve been thinking about that some of you experts may be able to shine some light on for me and possibly others. Please try to read the full post before replying. I’ve been giving some thought to how box size, sound output, and Power input all relate to each other and I’ve come up with some grey ideas of how it all connects but I’d like to get a clearer picture. Here are some of the things I’ve seen reference which may or may not be true.

Smaller box = higher power handling, or as some call it less efficient. In either case the why isn’t important only that it takes more power to reach full excursion of the sub.
Larger box = full excursion on less power
If the box is to big the sub gets sloppy and slow to return.
If the box is to small the internal pressure forces the sub back out too quickly.

Given those to be true (if they are), I was thinking about the manufactures recommended box sizes and how they come to those number. Obviously they use a mathematical equation that I’ve seen and don’t completely understand as well as real world testing but I’m wondering if those can be manipulated a little to work in our favor. Here’s what I’m thinking.

In this scenario let’s say we have a 12” Speaker that is rated at 500w in a 2.5cuft box. Ideally you put 500w to it in this box and you reach its RMS potential. Now let’s say we up the box size to 3.0 or 3.5 etc, and its power handling goes down. Based on my understanding of how a subwoofer works you get sound output by moving the air. So in this bigger box let’s say we reach the full excursion at 400w due to the oversized box. It should be moving the same about of air right? Would this mean the sound output would be the same in terms of SPL on less power? If not why not? Sound quality aside on this by the way, I know the wrong box size can mess with how it sounds.

[SINTORMAN]

Yes increasing box size can make a driver be more efficient to get louder on less Power but after a while it stops helping. (and becomes almost free air)

A box bigger than spec doesn't always result in a "sloppy" response. That can happen in any size of box and that occurs when the cone is traveling too far out of the gap and you lose cone control.

And I don't think that last one has any truth behind it. a sub at rest in a box has equal pressure one either side. As soon as it moves one way it should be the same amount of work done to either compress or expand the air inside the box.

They arrive at spec numbers by using the t/s parameters to find a nice balance between cone control and output on the suggested power.

[KingPen]

Originally Posted by SINTORMAN

And I don't think that last one has any truth behind it. a sub at rest in a box has equal pressure one either side. As soon as it moves one way it should be the same amount of work done to either compress or expand the air inside the box.

I may need to reword this part but that's just simple physics and does apply, especially in a sealed enclosure. For example. If you have a 1 liter tube that's 10 inches long and you press it in say 2 inches, the remaining 8 inches of tube will be under a measurable amount of pressure. Now lets say the tube was 20 inches and 2 liters, pressing it in the same 2 inches the remaining 18 would be under a fraction of the pressure of the other tube. Reason is your forcing that 2 inches of substance into a space that's smaller based on ratio. The same thing with the subwoofer. the cone displaces the same amount of air. when forced into a box the resulting pressure is determined by the ratio of displacement to total internal volume.

[SINTORMAN]

Originally Posted by jcarver1112

I may need to reword this part but that's just simple physics and does apply, especially in a sealed enclosure. For example. If you have a 1 liter tube that's 10 inches long and you press it in say 2 inches, the remaining 8 inches of tube will be under a measurable amount of pressure. Now lets say the tube was 20 inches and 2 liters, pressing it in the same 2 inches the remaining 18 would be under a fraction of the pressure of the other tube. Reason is your forcing that 2 inches of substance into a space that's smaller based on ratio. The same thing with the subwoofer. the cone displaces the same amount of air. when forced into a box the resulting pressure is determined by the ratio of displacement to total internal volume.

you are forgetting the equal and opposite force of when the cone travels in the other direction.

pushing in a 10" tube 2" should = pulling out a 10" tube 2 inches. Therefore it will always be equal no matter the size of the tube as long as cone excursion is linear.

[KingPen]

Originally Posted by SINTORMAN

you are forgetting the equal and opposite force of when the cone travels in the other direction.

pushing in a 10" tube 2" should = pulling out a 10" tube 2 inches. Therefore it will always be equal no matter the size of the tube as long as cone excursion is linear.

Equal in both directions in the same volume yes but not equal to the same displacement in a larger volume, as in a subwoofer box to subwoofer comparison. If you dont believe me try the tube concept. Make one 5" long 2" in diameter and try to compress 2 inches of air further into the 3" remaining. You will see how much work this takes and how hard you have to press. Do the same thing with a 5 foot tube and you will see its no problem to get 2" inches in or even 4" which is nearly impossible without a high pressure walls and a mechanical driving force for the smaller tube.

[SINTORMAN]

Originally Posted by jcarver1112

Equal in both directions in the same volume yes but not equal to the same displacement in a larger volume, as in a subwoofer box to subwoofer comparison. If you dont believe me try the tube concept. Make one 5" long 2" in diameter and try to compress 2 inches of air further into the 3" remaining. You will see how much work this takes and how hard you have to press. Do the same thing with a 5 foot tube and you will see its no problem to get 2" inches in or even 4" which is nearly impossible without a high pressure walls and a mechanical driving force for the smaller tube.

what I am saying is it will not push it out too quickly and it doesn't matter. The cone will only travel as far as it can based on the internal volume (air spring), spiders and power allow. Putting a sub in to small of an enclosure will only limit its output but it will not affect linearity unless it is extremely small.

when you said pushing out I thought you meant only being affected one way. So I guess yes and no. but overall no it will not "force the sub back out too quickly"

[Fallen]

TL;DR

Sloppyness also comes from high group delay. Which is a product of large vented boxes.

[KingPen]

I understand what your saying, all good information.

[KingPen]

Originally Posted by Fallen

Sloppyness also comes from high group delay. Which is a product of large vented boxes.

Given to large it will do that but I would think there has to be some kind of tipping point? Any thoughts?

[Brian10962001]

Not trying to be that guy, but TOO large and TOO small.

[SINTORMAN]

Originally Posted by jcarver1112

Given to large it will do that but I would think there has to be some kind of tipping point? Any thoughts?

It isn't a tip the balance kind of deal. It happens gradually.

[Fallen]

there is a threshold for where the group delay becomes perceivable . its frequency dependant . anyway Google group delay for more info

[tjamz]

Originally Posted by jcarver1112

So this is something I’ve been thinking about that some of you experts may be able to shine some light on for me and possibly others. Please try to read the full post before replying. I’ve been giving some thought to how box size, sound output, and power input all relate to each other and I’ve come up with some grey ideas of how it all connects but I’d like to get a clearer picture. Here are some of the things I’ve seen reference which may or may not be true.

Smaller box = higher power handling, or as some call it less efficient. In either case the why isn’t important only that it takes more power to reach full excursion of the sub.
Larger box = full excursion on less power
If the box is to big the sub gets sloppy and slow to return.
If the box is to small the internal pressure forces the sub back out too quickly.

For the sake of this thread I'm going to reply as if we were talking solely about sealed enclosures. Below are some rules of thumb to go by.

Smaller box= "tighter" bass at the cost of low frequency reproduction and power handling (electrical.... not necessarily thermal). Boxes designed with a QTC of around .707 are considered to be the most musically accurate and will likely be pretty efficient. Using a QTC of .9 is considered a pretty good "compromise" enclosure. It will still play fairly deep and be rich sounding, but will also accentuate the "snappiness" that some people love about their subwoofers. Moving up to a QTC of >1 will usually result in a "tighter" sound but will sacrifice bottom octave output. (lower number QTC is indicative of a smaller box, higher QTC's result in smaller boxes).

Given those to be true (if they are), I was thinking about the manufactures recommended box sizes and how they come to those number. Obviously they use a mathematical equation that I’ve seen and don’t completely understand as well as real world testing but I’m wondering if those can be manipulated a little to work in our favor. Here’s what I’m thinking.

Relatively easy to come up with those numbers. They hook their speakers up to a tool that measures the various electrical and mechanical parameters (commonly referred to as Thiele-Small Parameters or TS Parameters). From there they are able to plug them into various equations which can simulate/predict how low a speaker can play, etc... Or, (more likely) they simulate them using software from LinearX (L.E.A.P.) or something similar