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PostPosted: Fri Jan 02, 2004 2:54 pm 
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It's a fact that most people who upgrade there sound do it with a subwoofer system. They may do a deck first but the next upgrade is almost always a subwoofer. A lot of people like to get sound in their cars fast so they go to FS or BB and grab the "special of the day" sub and then grab one of there prefab boxes to load it into. After they (or an installer) get the stuff in they play some music and the bass is simply awesome. Two weeks down the road though they begin to notice that it's not exactly what they want. Most will notice that the bass craps out on low bass notes, or worse, they blow the sub out and figure that they need one that is more powerful/better quality. The fact is that that ported enclosure that the sales guy sold you is mass produced by people who don't understand the way that a vented (ported) enclosure works. Most have the port there as a selling tool for the box. It does indeed go louder than the sealed enclosure but unless you paid for a high end enclosure it likely booms real loud and doesn't sound that great doing it either. 99% of the time it's not the sub at fault. 99% of the time it's the enclosure that's wrong for the sub.
You can build your own sub enclosure that sounds better than any prefab out there for about the same (in many cases less) money. The reason I say the same amount is because you will be bulding with better materials to begin with. Little differences count. 5/8" MDF looks close to 3/4" but it's not. Lack of internal bracing in a box let's the MDF flex and your sound sufers because of it. Look in that $99 prefab and tell me if it's braced well. Terminal cups on really cheap enclosures are merely pressed in. Better ones have screws. Yours will have the screws and it will be sealed from the inside so that it doesn't leak air when your playing it.
To build a good enclosure there are a few things that you must make sure you do. Here are some problems that you run into.

1. Flex is the devil. Any flex in your enclosure will tarnish the sound. When you build your box think atomic bomb shelter. Overkill is good here. Use minimum 3/4" MDF. Unless you are doing a low powered (100W) system with a single 8" speaker never use 5/8". If you have a monster sub I would recommend making the baffle (the part that you bolt the sub to) 1" MDF. Use bracing inside the enclosure to keep it from flexing. Cross bracing is the best. Period.

2. Air leaks change the way a subwoofer functions and makes really nasty sounds too. Glue holds much stronger than screws. When I build a box from wood I use lots of wood glue and screws (1.5") every 3". You can get away with screws every 5" because the glue's hold is so superior. The screws are there to hold everything together while the glue dries. Why do I do it then? Overkill. Every hole is predrilled and countersunk. Before the screws go in the holes are filled with glue. After the glue has dried use some silicone (hot glue works well too) and fill every seam on the inside of the enclosure. I use hot glue for this and here's why. When silicone cures it gives off acetic acid. Not fun to breath and some surrounds (the foam part of the subwoofer) will literally get weakened from it. Hot glue is dry almost instantly whereas silicone takes a few hours. Terminal cups on the sub enclosure must be inthere tight. Better ones don't leak but if your unsure seal it with glue (after you've soldered your wires in place) to be sure. Ports often go unoticed. They too must be mounted solidly. A little know fact about vented enclosure is that they have more air pressure inside than a sealed enclosure. Don't ask me to explain 'cause it deals with physics that I don't understand 100%.

3. Tuning frequency (for vented applications only) is what makes the sound of the enclosure. Almost every prefab enclosure that a rep brought to my shop walked out with him. Partialy from reasons stated above (cheap construction) but mainly for the following reason. After I measured the box volume and then the port length and diameter most of them came out with a tunning frequency between 45-55Hz. Some were down around 35-40Hz but IMHO it's still too high for good sound. How does tunning frequency change the sound? I'm going to simplifiy this 'cause I could spend a page just touching on the subject.

(a) High tunning (40-60Hz) gives you a wicked spike in the output (which is why you have to be careful) with very steep slopes on either side of the tunnig (I have a picture of this added so that it's a little easier to understand) frequency. Another little fact. Below tunning the subwoofer starts unloading (doesn't see the box anymore) and becomes completely uncontrolled. That's why you were blowing subs. Do you want to compete in SPL competition? Interested in quadrupling power to your subs at one frequency? This is were you would be tunning.

(b) Middle tunning (30-40Hz) doesn't have a sharp spike like the high tune enclosure. It's more of a "fat" bump near and around the tunning frequency. Much less likely to unload but it's still a good idea to run a subsonic filter 3/4 octave down (halving of frequency ex. 30Hz tune run subsonic around 20Hz) Do you listen to rap? Do you like seeing blurry when your playing Ying Yang Twins? This is the tunning for you. Right around 30-35Hz you get a nice bump that swells the car with sound. Listen to rock/heavy metal type stuff? This works well for you too!

(c) Low tune (17-30Hz) mimics the rolloff of a sub in a sealed enclosure. Usualy does it a few DB's higher though. No major spikes (unless the enclosure is huge) in the slope either. Chances of unloading the sub are practicaly nil unless you play stuff below 10-15Hz. Do you like scarring your friends? Have you ever listened to Jurassic lunch and had your vision blurred before you could hear the sound? Like sound quality and/or large pipe organs? This is for you. **EDIT** Look at the specs of your sub and find one that says "Fs". If it's a higher frequency (30+Hz) be careful about tunning super low. You will usualy have to go with a huge enclosure. You can get away with it, but you need to be a little more careful. "Fs" is the subs resonant frequency BTW.

These are the types of questions you have to ask yourself when building your enclosure. It's very important to make it the way you want it. So how do you do all this and get it right the first time? First download WinISDPro at http://www.linearteam.dk/default.aspx?pageid=winisdpro . This is a free program but there is a spot to donate if you like. This program is worth donating a few bucks to. First thing to know about ISD is that it is only good for vented and sealed (closed) enclosures. It's way off on the 4th and 6th order enclosures. The complexity of those enclosure IMO aren't worth the hassle unless you are really trying something different. In any case they are way beyond the scope of this FAQ.
This is what you need to know before you get started:

1. What subwoofer am I using?

2. I need the specs of the subwoofer to run the program. Do I have them?

3. What do I want from the enclosure? (SPL/SQ/ a bit of both [SQL)

4. How much space in the car am I willing to sacrifice?

5. After I'm done this will I actualy be able to build a box in the first place?

Now comes the fun part. Let's get started. The following pics are screenshots of the program. I'm using two Adire Audio Brahma 15" subs for this example. The specs of quite a few subs are already loaded in the program so you may not need to enter them yourself.

1. Open ISD. It will open to an empty graph. You can configure upper and lower levels by going to "file" and then "options". If you are going to be doing a big system you can put the SPL up to 130db. Frequency response (range) can be set to 10-100Hz because you will usualy have your subs crossed over around 80Hz. Once you learn more the other settings can be set to your liking. Most of them you won't use until you get really specific with what you are building.

2. Click on "new project". A small window will open and give you a selection of drivers to choose from. If your sub isn't in there you will have to click on "editor" to enter the sub and it's parameters. It's likely you won't have all of the parameters that it wants. That's OK. The following are the ones that are most imortant:

Qes

Qms

Qts

Fs

Vas

Re (impedance in Ohms)

SPL (sensitivity)

Pe (how many watts RMS is the driver)

You'll notice that some of your specs may be in a different measure than on the program (ex. Vas in cubic feet instead of liters). Just click on the abbreviation and it will change for you. Once your done inputing your driver info click save and your ready to go.

3. Click on "new project" again and find your subwoofer. Click next. Enter the number of drivers you will be using. Placement will be normal. Click next again. ISD calculates what type of enclosure your sub will work best in. You can, however, choose to do it in a different type. Click "vented" and then click "next"

***Some drivers are specificaly made to run in a sealed enclosure!! The literature that comes with the sub will give you all sorts of warnings about using it in a ported enclosure. If you have one of these subs don't run them ported unless you really understand the driver's capabilities. The original Kicker solobarics were like this.***

4. Here is your first screen. ISD defaults to an enclosure that has the flattest frequency response. Problem is, on a lot of subs this will give you a ridiculously large enclosure. This is where we start to fine tune the enclosure to our liking. Frequency response is the green line. If you want to change the colour click on the green bar in the grey box and pick your new colour. This is handy when you work on more than one enclosure on the screen at a time.
Image

5. First thing we are going to do is input the size of enclosure we want to work with. Two things to remember here.

a. ISD sees box volume as internal so if you have 2 cubes max you better figure it out internaly for it.

b. When you port an enclosure and the port is inside the box (as opposed to seeing the port tube on the outside) you have to add the port volume to the overall volume of the box. Same goes with bracing and such. ex. You have 2.0 cubes of airspace inside your enclosure. After you measure the volume of your port you see that it take up 0.15 cubic feet of air space. The bracing that you put in also takes up another 0.15 cubic feet. Add them together and you get 0.30. Your sub will also displace a bit, on average 0.10 cubes, which must be added to your figure of 0.30. (bear with me on the basic math...I know). This get's subtracted from your total volume. So now you have 1.6 cubes total. If you want to end up with 2.0 cubes you will have to build for 2.4 cubes internal. The easiest way to figure the enclosure out is to input a little smaller than what you have so that you can compensate for it later.
Go to your grey window and click on "box". It will give you a volume and tunning frequency (pic a). Enter the volume you have and the tunning your after. Notice the slope change? Hard to get a reference point huh? What I do is usualy open a second project with the same sub but do it as a sealed enclosure with the same internal volume. This gives you a reference point to compare to your other graph. You can see how this work in the second pic.

a.
Image
b.
Image

Pic b. also shows you how much more gain you can get from a vented enclosure over a sealed one. This tunning is at 25Hz and the slope closely matches the slope of the sealed enclosure. Basicaly you are getting free power here.

6. Remember when I said earlier that different tunning frequencies give different slopes? This next pic shows just how radical you can get.
Blue line is 55Hz. Notice how steep the slope is? With this enclosure you get awesome output right around tunning but lose output everywhere else. Only good for SPL comps.
Pink line shows a tunning frequency around 35Hz. Peak is gone but you get a nice bump over a wider area. Like I said, this is the most common tunning for most people. Strong output over a wide range.
Green slope is same enclosure at 25Hz tune. Bump is almost all gone and slop is even shallower. Notice how much more output you have at the lowest frequencies though? SQ guys usualy go for this. If you are fortunate enough to have huge subs, 500+ watts, and don't care about space this is awesome sounding in a little car BTW. It's the tune I use in mine and the output is sickening (literaly).

Image

7. Want to learn why your subs blow so easy in a high tuned enclosure? This next pic shows what happens to a sub when it unloads from the enclosure. ISD can draw this up for you if you have all the specs for the sub. This show the subs with 2000W and the differences in enclosures.
Yellow is the sealed enclosure. Not much happening here. This is what happens normaly to a sub. Power is a little high here to show the differences better. Sealed enclosures actualy take less power than vented to get the subwoofer to max excursion.
Vented works very differently. Didn't believe me about the higher pressures in vented enclosures? Look at the dips in the slopes. They are lowest at tunning. This is where you have the highest pressure in the enclosure. You can also see how quickly excursion goes up below tunning. Green is at 55Hz and blue is 25Hz. Which sub is gonna crap out when the bass in "Get Low" drops to 30Hz? You can now see why we get away with more power in the vented enclosure. At tunning the sub is barely moving. That means that we can cram more power into it without testing the limits of the suspension (the subwoofer's not the car's;)). I could easily triple the power at 55Hz for a short time ( like during an SPL burp) without bottoming the sub out. Of course if I ran that power anywhere below or above tunning I would destroy it. I'll warn you now. If you try this and blow up your subs don't bitch at me. This is the kind of stuff you do at your own will. It is still not a safe practice to do. It's is generaly only done for SPL comps. You will void your warranty. Your friends will laugh at (or with) you. Don't forget, even if the sub isn't moving as much it's still getting 2000W of heat in the coil. Less movement=less voice coil cooling=burnt subs that smell funny and smoke...seriously

Image

8. Last thing to do. You've crunched the numbers and you want to make a box tuned to X frequency. What size of port do you need? Go to your grey window and click on "vents". It will show you number of vents (same as port) as well as size. You can also choose the shape of the port as well. What you need to do is choose the size you want to use and then ISD will show how long it needs to be. You'll notice that the bigger the diameter of the port the longer it will need to be. The same is true for the number of ports you wish to use. More ports = longer ones. I generaly use a single 4" port for enclosures with a decent 15" sub or smaller. 3" port is usualy feaseable with "normal" 8" subs. 1.5"-2" I use on 6" and under. When you get into the "super" subs like the Brahma you should bump it up to at least two 4" ports per sub. In some cases even that's not enough. Ports can make noise when you get a lot of air going through them. More ports let the air move slower through them thereby lessening the chance of port noise. Again, ISD can show you how much velocity the port will get from a given amount of wattage as well as throughout the frequency range. Don't spend too much time worrying about this. If you follow the guidlines I've given you, you will be OK. Most prefabs have 3" ports anyways so it will be a huge improvement regardless. I'm only showing you this to give you an idea of what the program can do. Again, for this function to work you will need all the parameters regarding cone movement and displacement. I've used the 25 (blue) and 55Hz (green) tunes again for this. The magic number here is 17m/s. Anything above that is most likely heard. Couple of exceptions. If you are getting 20m/s at full volume on your system that has 4 15's and 4000W you likely won't hear it. If you are seeing more than 17m/s of velocity at frequencies that you are not going to be playing you will not likely hear them. If your subs are in a trunk you probably wouldn't hear 30m/s. Again, this is a gauge to try and follow but it's not written in stone. At normal listening volumes you will be fine. Try entering your wattage as 100W (you would be suprised at how loud that is) I bet you dont get over 15m/s You'll notice that with some subs you would need 4 5" ports to keep it under 17m/s at high power.
Image[/i]

9. Finaly!! Once you've done all this you are pretty much set to go. You will need to measure out the space and fit your dream box in your car! This program does a lot more but I think this is enough for now. It has a great contents section in the help menu too. Parameters are explained as well as all the functions of the program itself.

10. Here's some quick little tips to help better the sound.

-Smooth your ports. You don't wan't a hard edge on them.

-Screws get loose but if you use T-nuts and bolts they will stay tight and you won't strip the MDF. A loose sub will leak air and sound like ass.

-Make sure the T-nuts are in tight. All three "feet" must be all the way in the MDF. After they are in, cover them in wood glue to make sure they don't back out. If one of them strips because it wasn't in all the way it will just spin and you won't be getting the bolt out easily (if at all).

-Weatherstripping works great for making a sub gasket. The sub has to be sealed to the box.

-MDF is the cheapest material but weighs a ton. A 4x8 sheet is close to 100lbs. You can get low void (absolutely has to be low void) plywood that works just as well but weighs less than MDF...its also double the price.

-Plexiglass should be a minimum 1/2" thick for a 12"x12" window.

-Fiberglass weighs a lot less than MDF or Plywood. Should be anywhere between 1/4"-1/2" thick. Thinner where it curves ('cause the curve makes it stronger) and thicker on flat parts. Don't be afraid to brace it either. It is obviously more difficult work with than wood for a first time project.

-Got a weird shaped enclosure in mind? Can't figure out the internal volume? Fill your box with packing pop-corn, sand, or water (not recommended for wood enclosures). If you use sand vacuum the enclosure out afterwards.

-SPL ports should have lot's of area. Ports that you can crawl into are not uncommon in SPL cars. Bigger ports give higher SPL. Look at fluid dynamics and check out velocity vs pressure. Again, I won't explain it here because if you are at that point your well beyond this tutorial anyways and you just reading it to make sure I didn't give any false info ;)

-Threaded steel rod works exeedingly well for bracing SPL enclosures.

-Make sure your port isn't too close to a reflecting surface in your enclosure. A 4" port should have 4-5" of clearance around it. You want the air to have an unobstructed path to the port. General rule of thumb. You want clearance equal to the width of your port.

-ABS tubes work great for ports. They are stiff and won't collapse from pressure.

-Many people are starting to line their enclosure with foam rubber. The kind that has all those little bumps in it. The reasoning is that it helps to further "deaden" the enclosure. IMO a ported enclosure should have nothing in it to obstruct the air. Brace carefully not to disrupt the airflow too. If you do this make sure to account for the volume it will take up.

Can't think of any more. If I do I'll post them later. I've been doing this FAQ straight for last hour or so so some of it may come off as a little confusing. If I've explained something poorly post or PM me and I'll re-explain it better. I'll come back in a few hours to read it over again and clarify anything that looks weird.

Hope this helps a bit.

Andre

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Last edited by m on Mon Apr 09, 2007 4:04 pm, edited 4 times in total.

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PostPosted: Sat Jan 03, 2004 5:20 am 
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MuE Patient Zero/The Link Man
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:shock:

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n1tr0 wrote:
not the quickest suzuki on the block eh ?


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PostPosted: Sat Jan 03, 2004 5:36 am 
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You coudnt have said it any better! :D
2 more words for sealing an enclosure,,,,, Kitty-hair!
Gotta love bracing!
I miss car audio,,wait,i dont miss the customers :roll:
Dan

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PostPosted: Sat Jan 03, 2004 11:25 am 
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Yeah I was going to mention coating the inside of the enclosure with resin but if you use lots of glue and then run a bead of silicone/hot glue you should be 100% sealed.

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PostPosted: Thu Jan 22, 2004 8:35 am 
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Thats a good write up on subwoofer construction.The computer programs,or even a good book are a great start for beginners,or for forming the basis of an enclosure.

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PostPosted: Thu Jan 22, 2004 1:13 pm 
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You think I would have noticed this before playing around with the program the other day... Definetely a big :shock: :shock: on the nice write up.

Answered some of my questions on that sonosub idea. Still have a few more questions specific to that I guess I'll ask in that other thread.


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