# Speaker Box Design



## Omega3k1 (Jan 6, 2005)

Ok well i've got a final to do in my shop class at school so i was going to make a custom speaker box to fit in the trunk of my B14 Sentra... Well i was wondering what you guys think of my design. Its a Slot Ported 31in long x 14in high, by 16in bottom width, 11in top width. I'm making it out of 3/4in OSB cause my school is cheap, and i'm going to carpet it with some grey carpet. Ohh and its for 2 cheapo 12in subs i've got laying around but if i get the money i'll pop some higher quality one's in.


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## sr20dem0n (Mar 19, 2003)

You should angle the port entrance the same as the port exit. Right now the top and the bottom of the port have different lengths and that's not a very good idea. Do you have any idea what the tuning frequency is anyway? It looks like it would be tuned very high, like in the 50hz range, which would sound like crap.


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## Omega3k1 (Jan 6, 2005)

ok, is see what you mean... and umm yeah i have no idea what the frequency is of it... i'm not even sure how to determine it. I just built it what i thought would do alright... i mean its only for 2 cheapo 12in subs iv'e got(they're 100w max apeice). I just was going to build it so i could have a box to put these subs in. And as in angle the port entrance, do you mean like angle the baffles like \ so that they're the same length all the way through. Yeah i'm kinda a noob to all this and i was just going w/ what looked about right. So any help is apreciated thats why i put this up on here.


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## captain_shrapnel (Sep 10, 2004)

The box makes the subs, especially in a ported box. Check the speaker manufacturers website for recommended box and port sizes, without it the box is useless (unless you have some sophisticated program like LEAP).


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## sr20dem0n (Mar 19, 2003)

You can figure out the tuning frequency using a program like WinISD

This would be the size port necessary to give that box a reasonable tuning frequency:
http://edesignaudio.com/Products/13Ov2Subwoofer/BoxDesigns/twoidealvented.gif

See how long it is? If you make the port too short it will be tuned way too high. Say it was tuned to 50hz, that means that at 50hz the subs would be moving the least and you would have the most output (the port is doing all the work, the subs' cones and the air mass in the port are in resonance). Above this the output would slowly decrease until you got to about 100hz where the output would be similar to those subs in a sealed box.

As you passed below 50hz the output would slowly drop off until you reached about 35hz and then the output would drop like a rock and the subs' cones would go psychotic. This is called unloading, the subwoofer cones are moving so slow in comparison to the port's resistance that the port, instead of helping to tame the subs' excursion and increase output, it actually functions as a huge hole in the side of the box and there's practically no pressure inside the box keeping the subs excursion in check. As a result they start moving a LOT farther on a given amount of power.

Mix this response with the transfer function of the car, and what you get is an ungodly huge output spike at 50-55hz, not much output above that, and virtually no output below that. On top of that, if you feed your subs frequencies around 35hz or lower (which many songs have) at anywhere close to med-high output, they'll bottom out and possibly damage themselves.


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## pete? (Jul 6, 2004)

i know i know nothing of what sr20 demon does. but i have found my sub sounds best when its turned around. so you may want to flip your design around. :cheers: 

(only listen to this if that angle if made to sit up against the back seats.)


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## Omega3k1 (Jan 6, 2005)

i cant flip em around cause i'm mounting it flush against he back of the trunk so its facing right into the cabin. Thats why its angled. I think i'll just go w/ a sealed box cause it'll be so much easier for me 

My design is ported nearly to those specs other than the baffles not being there and such... Other than it being at an angle which i'll fix some how... but the port is 12.5 tall by 2in length x 13 on bottom and 8 on top... So yeah i'll try making a design in that WinISD. Thanks for the help though

I think i might just go w/ a sealed box.


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## sr20dem0n (Mar 19, 2003)

Omega3k1 said:


> i cant flip em around cause i'm mounting it flush against he back of the trunk so its facing right into the cabin. Thats why its angled. I think i'll just go w/ a sealed box cause it'll be so much easier for me
> 
> My design is ported nearly to those specs other than the baffles not being there and such... Other than it being at an angle which i'll fix some how... but the port is 12.5 tall by 2in length x 13 on bottom and 8 on top... So yeah i'll try making a design in that WinISD. Thanks for the help though
> 
> I think i might just go w/ a sealed box.


The port in that link is 41.25" long, the port in your design is 8" long at the top and 13" long at the bottom....that's not very similar. That "baffle" in the back is the port wrapping around, to get a respectable tuning frequency with enough port area, you need to have a port that's at least 30-40 inches long. For example, the port in this picture is almost 50" long.

I think that sticking with sealed is a good idea, just check the specs of the sub to see what size would be best. For a pair of 12's, an internal volume of around 2.4cf after sub displacement should work for most everything out there.


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## pete? (Jul 6, 2004)

Omega3k1 said:


> i cant flip em around cause i'm mounting it flush against he back of the trunk so its facing right into the cabin. Thats why its angled. I think i'll just go w/ a sealed box cause it'll be so much easier for me


i mean mount the box on the flat side instead of the angled side. but its up to you.


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## Omega3k1 (Jan 6, 2005)

i see about the slot thing. I think i'm just going to go with a sealed box now. It'll probably sound better anyway. Currently w/ my box specs its 2.6ft^3 so its about right, i'll live w/ it anyway. I mean i get to make the thing for free and keep it(dont even have to pay for supplies :-D), so if it sounds like crap it sounds like crap i still get a grade for it. 

Also for those of you who want to know i made that 3D image using 3d Studio Max... Its not a CAD program though. Its 3d Modeling software for like games and movies and the like. I've been messing with it for about 3 years now and i can do some pretty cool stuff with it (i started modeling a Supra but lost interest).


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## captain_shrapnel (Sep 10, 2004)

Its pretty hard to dick up a sealed box, lotsa fudge factor there so have at it. A vented box, in my experience, can eat a sealed box alive but requires precise tuning (to get an idea, read the Loudspeaker Design Cookbook by Vance Dickason) so it is usually better to put off that project until you have built at least one other enclosure. Yeah, make the sealed box. Shop teacher may not give you an A for complex design, but it will probably sound much better for you.


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