Internal bracing vs SPL

[firepinch]

Hi everyone! Im in the process to build a ported enclosure. But when I was talking about braces with a car audio freak friend, he said that braces design is very important in order to not compromise SPL. What do you have to say about this? Do you think that bracing can compromise SPL in a certain way? I must add that I know that you have to put the end of a port or the back of the woofer enough far away from any boundaries (such as enclosure walls or justly, braces). My question is really just about how bracing an enclosure can affect the final performance.

Thank you!

[lndm]

For one thing, the nature of resonances with a room/box/driver can change with SPL. A hum can turn into a buzz or a rattle, for example. Try it and see. Allow bracing to be added to a design later, perhaps with a panel that temporarily remains removable.

[Rademakers]

Say you have a closed box. As the speaker is moving into the box it creates a pressure on the walls. These walls in turn want to move away from the box to counteract this force. If the speaker is moving outwards the cabinet, the walls try to move inwards.

The movement of the walls requires energy which is directly taken away from the speaker trying to convert it's movement into sound.
Bracing is an effective way of making the panels stiffer and stiffer walls do intend to move less, taking away less energy.
Bracing can be even more efficient than making the cabinet walls heavier/ thicker.

In PA-speakers and caraudio drags the SPL is high enough to measure a significant difference, in Hifi the fidelity is in line (coloration of sound). So that's why everybody seems to be bracing.

Wkr Johan

[firepinch]

Originally posted by Rademakers
Say you have a closed box. As the speaker is moving into the box it creates a pressure on the walls. These walls in turn want to move away from the box to counteract this force. If the speaker is moving outwards the cabinet, the walls try to move inwards.

The movement of the walls requires energy which is directly taken away from the speaker trying to convert it's movement into sound.
Bracing is an effective way of making the panels stiffer and stiffer walls do intend to move less, taking away less energy.
Bracing can be even more efficient than making the cabinet walls heavier/ thicker.

In PA-speakers and caraudio drags the SPL is high enough to measure a significant difference, in Hifi the fidelity is in line (coloration of sound). So that's why everybody seems to be bracing.

Wkr Johan

I am not sure to understand fully what you explained... "The movement of the walls requires energy which is directly taken away from the speaker trying to convert it's movement into sound. " Do you mean that if the walls move less, there is less lost SPL? So a fully braced enclosure can have a bit more SPL than a not braced one? Or you mean that when the walls move more, there is more SPL? However if that is the case (second supposition), wall movements = more distortion!

[lndm]

Energy is transferred to the walls. It is stored and released. Some of this energy re-moves the speaker, some is converted to sound in the panels, and some is converted to heat.

The energy converted to heat is lost, but this not that significant with panels. More so with stuffing, as that is its job. Try for yourself though, lots of stuffing doesn't make a massive SPL difference, and some is necessary anyway.

There was once (maybe still is) a notion amongst some in the car audio field that a resonant box helps you get loud. For the purpose of HiFi though, I disagree. As you said, it means distortion. Both in the frequency and time domains.

At the end of the day, it depends what you are trying to achieve. Bearing in mind that there are some that like to make their cabinets a little like musical instruments with exotic timber that resonates musically. This aside, I believe it does not make much sense to build a cabinet whose panels resonate.

[Cal Weldon]

If you are building a ported enclosure, the amount of bracing required is reduced as there is no or little pressure build up in the box. It is still a good idea to brace though and remember to offset the braces so they are not dead centre of any panel.

[lndm]

I beg to differ. At 0Hz there can be no pressure build up. At critical frequencies though, the box resonance and driver wavefronts meet to create considerable pressure.

Regardless, panel resonance related to the dimensions is not about pressure build up.

[Volenti]

Originally posted by lndm
I beg to differ. At 0Hz there can be no pressure build up. At critical frequencies though, the box resonance and driver wavefronts meet to create considerable pressure.

Regardless, panel resonance related to the dimensions is not about pressure build up.

I agree, the forces acting on the enclosure walls at Fb is greater in a ported box than sealed at the same frequency, you can't have higher spl outside the box without having higher spl inside it.

[BassAwdyO]

I'm sure this has been covered numberous times in this forum before....

But to sum it all up
1.Bracing is important in subwoofers to increase panel stiffness so that the panels move less and hence offset the sound pressure created by the driver less.
2.Bracing in subwoofers need not be offset to create multiple panel resonances(we are out of the resonant frequency range of the panels) so a brace in the center will be the best
3. Ported boxes will have higher pressure inside of the enclosure near fb(which is why the driver excursion goes way down)


However, Unless you are competing or demand overkill to satisfy your diy cravings 3/4" MDF really doesnt need a brace until the length exceeds 12-15"

[firepinch]

Interesting point here that I got with this famous friend tonight. Im gonna write this post too at the DIY audio forum.

When both of us talked about bracing and the effect on SPL, Ive got really great answers from you all. And really a big thanx for this by the way! I can now understand the relationship between the two. However, lastly, he talked about air turbulence and bracing. More the frequency is low, more it is hard for the waves to travel in the enclosure and especialy by the ports. So if you put internal braces, you can restrict more the air flow created by the woofer and the port (or making waves a harder time to travel in the enclosure) in the enclosure at lower freqencies, thus, reducing SPL. This is why this is better to use external braces in order to avoid this problem. Using external braces makes the enclosure stiffer, and you dont have anymore the problems of possible air restriction in the enclosure, more especially with lower frequencies.

I just talked at his place. I mean, everything I said was coming from his mouth. It seems like logical and beleivable. What do you think about all of this now?