Printer Friendly Forum Posts - Best Way to Brace a Box

im about to build a box that is about 9Ft^3 internal. the front baffle will be doubled but i dont want to double the rest of the box because i dont have the space. i have about a 1/2 cubic foot to spare for bracing and i want to make the most of it.

my question is what is the best way to brace the walls of a box without obstructing the air flow inside the box?

box dimensions are 49 wide X 22 tall X 19 deep external. the box will also have a port on one side of the box that is 4 X 20.5 X 14.2.

the subwoofer going in is an 18" SX from RE audio. im designing this box more for quality and will be turned down 90% of the time but i need this box to be strong for the other 10% of the time that i have it shaking the block.

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There are many approaches. The key is to connect the front and back baffles, the top and bottom, and the side to side with something rigid. Many people cut holes and patterns into sheets of MDF and glue and screw to connect the baffles (one piece can connect front, back, and two sides.) Another approach if you don't have enough available airspace (or time and money) to insert two or three extra sheets of MDF is to use 1" dowels (or a reasonable facsimile like 1X1 pine.) Cut the dowels to length and wedge between opposite baffles and glue and screw in place. 6 dowels (2 each for each direction) will work.

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thanks for the response man. i never thought about using wooden dowels before but i have heard of using threaded rods and bolts, which is almost the same thing. im not looking to be quite that extreme but id rather use MDF. its not a money thing thats stopping me from double layering the whole box i would just rather not have anything in the middle of the box obstructing the air flow. dowels would be ok because they are round.

my idea was to cut a few sections about 5 X 20.5 to go up against the walls and glue and screw them to the inner walls of the box. each piece would be .044 cubic feet which would allow me about 11 of these pieces to make up the 1/2Ft^3 i need to get down to about 8 cubic feet.

i have used pine 1X1"s in a few boxes before and im not sure that it is the best solution to have a good sound. in fact thats what i have in the edges and corners of the box i have my current sub in now.

if you say that using dowels will be better than lining the walls with MDF strips then ill try it because whats the worst that can happen. im willing to try anything as long as its a serious solution.

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If you want to prevent flex, doubling the walls can work. If you want to prevent losses due to vibrations, connecting opposite walls is usually the better, easier and lighter solution. Whoever told you that the shape of the construction inside the enclosure has any impact on the output is full of baloney. As long as there are pathways that do not act like ports, square edges inside the enclosure do not impact a subwoofer system's performance.

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alright thanks that helps a little bit. nobody told me that square edges hurt the sound, its more about me not having anybody telling me anything. i have been mostly left alone to figure things out by myself so there is still a lot that im just not totally sure about and im always willing to learn something new from someone who knows better than me.

you say anything is fine as long as it does not act as a port, can you give me an example of what you mean? i understand what you are saying but im having a hard time picturing it in my mind.

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Here's an example using dowels. These could also be square 1X1's

https://www.subwoofer-building.com/images/pics/my%20diy%20subwoofer%20bracing.jpg

Here's an example using MDF with opening cut into it:

posted_image

If you have a divider with a single hole or with several small openings, it can cause the enclosure to function like a tuned chanber - a quasi-fourth order bandbass, which you do not want.

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ok so in the second picture, wouldnt every hole act as a port because you are pushing air through them? even though its still just the thickness of the wood, it still forms a cylinder inside the box? probably a very stupid question but i gotta know.

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soundnsecurity wrote:

ok so in the second picture, wouldnt every hole act as a port because you are pushing air through them? even though its still just the thickness of the wood, it still forms a cylinder inside the box? probably a very stupid question but i gotta know.

No because the area of the openings is large compared to the area of the "enclosures." The relative air pressure across the openings will be equal (or nearly enough so that no Helmholtz effect occurs.) Ports work because of a difference in air pressure.

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alright cool. i guess ill leave you alone now and do some research so that i can try to understand this a little better. do you have any links to any good reading material on the subject?

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soundnsecurity wrote:

alright cool. i guess ill leave you alone now and do some research so that i can try to understand this a little better. do you have any links to any good reading material on the subject?

You don't need to leave me alone, that's why I'm here! posted_image Reading material on enclosure design? Hmm, most of the web-stuff that I've seen is focused more on different alignments rather than the esoterica of how to design bracing, but let me look and see what I can find.

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did some digging on the helmholtz effect and found mostly examples about bottles and musical instruments. i understand it a little better(conceptually, not mathematically) though, but i wanted to know how it applies to subwoofer boxes. what got me curious was how they use the bottle as an example. i figured that if you use a car as an example instead of a bottle then the cab of the car would be the same as the inside of the bottle. i dont know if i described that the right way at all but thats how i see it. ive always known that the airspace inside the car mattered in a ported box design but now im thinking that its more important than i thought. what are your thoughts on this matter?

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soundnsecurity wrote:

did some digging on the helmholtz effect and found mostly examples about bottles and musical instruments. i understand it a little better(conceptually, not mathematically) though, but i wanted to know how it applies to subwoofer boxes. what got me curious was how they use the bottle as an example. i figured that if you use a car as an example instead of a bottle then the cab of the car would be the same as the inside of the bottle. i dont know if i described that the right way at all but thats how i see it. ive always known that the airspace inside the car mattered in a ported box design but now im thinking that its more important than i thought. what are your thoughts on this matter?

You are 100% correct. The inside of a car does work as a Helmholtz chamber, and a good SPL competitor will tune their system to activate the car's interior to squeeze out every last fractional db they can. The way a port works is the speaker moves air across the top of the port tube just like those descriptions you found of blowing across a bottle. This sets up a Helmholtz resonance based on the size of the tube and the area of the enclosure, causing the system to vibrate and make noise at a specific frequency (the "tuning" frequency.) When properly designed the tuning frequency will add to the system's output and extend the low-end performance. You don't want to create little unpredictable resonators inside your enclosure, or in your car because of opening a window while driving. :)

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but what i mean is that its the subwoofer that does the "blowing" and the cabin is the bottle, so does the extra output come directly from the port or does it come from cab itself. also if it works that way then wouldnt you tune the port using the cabin volume and not the box volume? see now im probably over thinking it but now im slightly confused...

ive always pictured a ported box working more mechanically, taking the rear wave and physically aligning it in phase with the front wave so that they work together and build higher peak output. but looking at it from this new perspective its starting to seem like the subwoofer and the port are working almost independently to create sound, and then the sound from them are mixed together in the cab to make one single louder sound. i know its way more complicated than that but my whole world is turned upside down now so i dont know what to think anymore posted_image

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soundnsecurity wrote:

but what i mean is that its the subwoofer that does the "blowing" and the cabin is the bottle, so does the extra output come directly from the port or does it come from cab itself. also if it works that way then wouldnt you tune the port using the cabin volume and not the box volume? see now im probably over thinking it but now im slightly confused...

ive always pictured a ported box working more mechanically, taking the rear wave and physically aligning it in phase with the front wave so that they work together and build higher peak output. but looking at it from this new perspective its starting to seem like the subwoofer and the port are working almost independently to create sound, and then the sound from them are mixed together in the cab to make one single louder sound. i know its way more complicated than that but my whole world is turned upside down now so i dont know what to think anymore

The cabin of the car can work like a Helmholtz chamber, with the subwoofer blowing on the "port" of how the sound wave enters the chamber. It's actually a relatively unimportant effect: much more important to the overall SPL are the standing waves created inside the vehicle. The subwoofer system will work the same way in a car or out of it. The port is a separate sound generator from the woofer. The woofer moves air which "blows" across the port, causing it to resonate based on the size of the enclosure. The enclosure is the "bottle" and just like if you blow across a soda bottle with different amounts of liquid in it, different sizes of enclosures and ports will resonate at different frequencies. It has nothing to do with the rear wave of the woofer (except that is what provides the motive force) and it certainly has nothing to do with aligning the waves. Indeed, the sound created by the port will be out of phase with the sound created by the woofer depending on where the port is located in relation to the woofer, among other things. Look up an effect called "group delay" if you want to learn more about this. It'll really mess with your mind for your next install. posted_image

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that figures, just when i thought i had a good handle on how things worked it turns out that nothing is what it seemed. thanks for your help and patience because im sure ill have more questions once i find the energy to look into group delay. ive heard the term tossed around here from time to time but nobody ever went into any detail about it.

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any good resources about group delay sir? did a quick search but didnt find anything very substantial for someone who doesnt already know what it is.

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Written by genius subwoofer designer and good friend Dan Wiggins: https://www.adireaudio.com/Files/GroupDelay.pdf

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that was a good read, havent heard the Adire audio name in a while. i now know a little more about group delay but im still in the dark on how to use it to my advantage when designing a system. it would have been nice to be able to read the response charts. maybe i need to invest in a box design program when i get a little money.

if i have this right group delay is basically the relationship between acoustic phase and output at a given frequency.

i notice that in the vented box that the group delay is the highest just below the tuning frequency, would this be another way to describe how a ported box "unloads" and loses output below the tuning frequency. is this a result of the sub and port being totally out of phase with each other?

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The reason a vented system has its worst group delay at or around the tuning frequency is because this is where the port output is greater than the woofer's output. You can't really "use" group delay to your advantage, but if you understand what it does in a system (flabby, booming sound without much definition; high distortion; loss of output due to cancellation) then you can design the system and how the installation will fit a vented system into the vehicle in such a way to minimize the effects. And, as Dan's paper illustrates, never use a passive radiator system if these effects are a concern. By playing with port position and how you control it's relative acoustic phase and output level in relation to that of the woofer's output, you can design a system so these things occur at about the same time, maximizing SPL and minimizing the effects of group delay at the listening (or measurement) position. It's very math intensive... but programs like LSPCad or Praxis can be used to do the math for you.

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i guess some experimentation is in order for me now. i might have to rebuild a new box for my current 15 and see what i can do to improve how it sounds in my truck because right now it's the worst part of my system by a long shot.

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soundnsecurity wrote:

i guess some experimentation is in order for me now. i might have to rebuild a new box for my current 15 and see what i can do to improve how it sounds in my truck because right now it's the worst part of my system by a long shot.

What driver is it, maybe I can suggest something for you...

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its a 15" RE audio SXX dual 2ohm. its the newer version but it has the same specs as the old one. specs can be found here

the amp is a PPI black ice 1800

right now the box is just under 3 cubic feet which is their minimum airspace

posted_image

that is the box i made but its missing the very top piece in the picture. its standing up in my truck with the port on the bottom and facing the back of the drivers seat.

the problems i have with it now is mainly the response seems to be all over the place. it will play some notes with nice volume but it wont have any detail to it. and other notes sound good but are barely audible. if i turn it up to hear the good notes then it also amplifies the bad notes which overpower the rest of my system. so i turn the amp up but i turn the LPF way down, i usually like my low pass set around 90Hz and mine is set to about 50Hz right now.

i listen to a lot of rock and heavy metal that has very fast drum beats and a lot of bass guitar so i need my sub to be fast and detailed, SPL is optional.

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Sounds like you have a boom issue with might very well be caused by the effects we've been discussing. Let me model that driver. Be back in a sec.

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Looks to me like 3cuft (85 liters) should be vented at 31.6 Hz. In that alignment your peak output is at 79.4 Hz, which can sound quite boomy inside a vehicle. What is the tuning frequency of your current enclosure? This driver also models very nicely in a sealed enclosure of about 1.8 cuft (50.7 liters).

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i wanted it tuned to 32Hz but i dont think i got there. from what it sounds like i maybe botched the tuning a little and im thinking its closer to 36Hz. my problem was that i only wanted to take up a certain amount of space directly behind the drivers seat. my back seat is a 70/30 split so i took the 30 side out and i also didnt want to pass the window line.

i couldnt tune that low because the port would take too much volume from the overall box volume. tuning to 32Hz would have left me with about 2.5Ft^3. the port itself is 2" X 15.5" X 21". the 2" is actually 2 and 1/8 because of a lazy cut and by the time i found out about it it was too late to re cut that day so i just went with it.

i forgot to mention that the box is also stuffed with 1 cubic foot of poly in an attempt to make it more listenable until i figured out what i should do next.

i might do sealed since you say it models nicely in 1.8 even though it isnt recommended by RE. ive never been a big fan of sealed boxes but it would help me maintain my space needs because i really dont want to take the rest of my seat out in the name of sound.

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Before you do anything else, I suggest blocking the vent and see what happens. Take a towel and stuff it into the vent hole, filling as much of the opening as you can. Set your Xovers to 80Hz. Can't hurt to try, right?

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i shall try it.

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just did it. stuffed a towel into the port and played some familiar music to compare and it definitely toned down the boominess but it also dropped some low frequency response too. with the crossover up to 80Hz it was a little to much so i backed off to 70Hz and changed the slope to -6db instead of -12db and it was very listenable.

so filling the port would have made this about a 3cuft sealed box, how would this compare to the 1.8 you suggested?

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It didn't seal it exactly, but by stuffing the port you reduce the port's output (which is why the low freq extension was reduced) and you reduce the strength of the sound wave that causes group delay as well as dampen port resonance. You also remove the SPL that may be added due to "chuffing" or air coming out of the port at a high enough velocity to be heard. By experimenting with the amount and density of stuffing you could find a sweet spot that gives you a sound you can live with. A 3cuft sealed enclosure will create a system Q of around 0.6 as opposed to 0.7 in 1.8 cuft. The difference would be subtle: higher Q means higher output at system resonance, with a Qtc of 0.707 being the most efficient trade-off between peak output and low-end response.

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na i dont think ill try to fix this box. i dont want a sound i can live with, i want a sound that i cant get enough of. ill try out the sealed box first since its relatively easy to build one. i still think ill end up building a bigger ported box than what i have now just to have more bottom end but we shall see. i should be able to build that box sometime this week so ill let you know how it goes.

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hey can you do me one more favor? can you model this this box for me and tell me what you think?

same SX15, final internal volume 4.3 cubic feet, 43L x 17H x 18D. port is 4 x 15.5 x 31, tuned to 32Hz.

i had a thought the other day as i was listening to the sub, still with the towel stuffed into the port, thinking about how you said that group delay is basically a difference in output from the sub to the port and i thought what if i just build the box and have the port all the way on the other side of the truck. this is how i had my last system set up and maybe it was just an accident but it sounded pretty sweet. this way i can maybe have my cake and eat it too.

i actually like the way it sounds with the towel in the port but i cant leave it like that obviously because i will eventually have to show this to customers and i dont feel like being laughed at. the sound is a lot more detailed than it was normally so the only thing its lacking now is in the bottom end.

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You bet. In 4.3 cuft tuned to 32Hz results in an f3 at 30Hz and a response peak at 47Hz. It would be less boomy than 3cuft (which has its response peak up around 74Hz and may be why you felt the need to turn the Xover down so far.) A larger enclosure would sound better, IMO. As far as the towel goes, I can't count how many vented enclosures I've built that ended up with a stuffed port because they just sound better, but they still have more low end than a completely sealed alignment. Go to the fabric store and get some nice black velvet, use that to stuff the port and tell your clients how hard you worked to determine the exact amount of this custom "subwoofer dampning" fabric to use to critically tune the port for the best SQ in your truck. They'll be impressed. ;)

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cool. if im not happy with the sealed box then thats what ill probably have to build. thanks again

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just finished building the 4.3 cubic foot ported box and threw it in the back of my truck. it takes up the whole back seat but it sounds awesome. probably gained around 6db and its extremely clean and smooth. now i need to build a floor so i can take the seats out completely and frame it all in with a nice custom install for the customers to see.

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Excellent!

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its actually cleaner than i expected because even at full volume there is no "hiccup" between note changes. it just transitions so nicely from note to note. obviously its still not quite as loud as my 18 but that is to be expected.

i think its funny that now that i am having to use their "SXX" version of the SX, now they are releasing the "SX PRO" which is basically the older version SX with new markings. ultimately i might still end up with the SX pro but for now i have the sound that i want.

the only difference between the SXX and the SX PRO is that the magnet will now have the vent hole in the back that they originally started with. do you know what effect this can have on a sub woofer system? honestly i have noticed no other differences other than the covering up of this rear vent and now they want to sell both versions ( covered and uncovered) can you tell me if there is a reason for this and what will be the difference? because otherwise there seems to be no other difference.

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A vented pole piece will allow a smaller voice coil to handle higher power levels because it dissipates heat more efficiently than a non-vented pole. It allows air flow through the middle of the motor. In general, it has no impact on the sound of the system, but it can create higher sensitivity ratings because of the smaller VC.

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cool, i always knew it was better for power handling due to the added air flow but i thought it might allow the cone to move a bit more freely because i figure that since the center of the voice coil is hollow i thought that the air inside would provide some sort of compression/resistance because the air would have no where to go but through the small gap between the magnet and the voice coil. i figure that having the vented pole would allow air to flow and eliminate any compression inside the motor structure and let the cone move a bit more freely/loosely. thoughts?

also just realized how many views this post has gotten, almost more view than some stickied posts that have been up there for forever and a day, so i hope someone else other than me has learned something.

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soundnsecurity wrote:

cool, i always knew it was better for power handling due to the added air flow but i thought it might allow the cone to move a bit more freely because i figure that since the center of the voice coil is hollow i thought that the air inside would provide some sort of compression/resistance because the air would have no where to go but through the small gap between the magnet and the voice coil. i figure that having the vented pole would allow air to flow and eliminate any compression inside the motor structure and let the cone move a bit more freely/loosely. thoughts?

Well, I can't say that doesn't happen, but it's generally not part of the design reason for using a vented pole piece. It's about power handling (and costs.)

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