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Discussion Starter · #1 ·
I'm going to attempt building a subwoofer box here pretty soon but I don't know if I should go with a slot port or a flared tube port. A tube seems a little easier but does it sound any different than a slot port tuned to the same frequency?

Also, how do you change the tuning of a tube port? Change the length?

I'm building this box for an Alpine Type-R (went with Type-R over W3 since I was able to get it for $100). I guess Alpine recommends for a ported box a volume of 1.75 cubic feet (3024 cubic inches I believe) with port tube of 3" diameter and 9.9" length (or slot port dimensions W = 12" H = 1.25" L = 21"). Anybody have recommendations on dimensions for the enclosure? Or free programs that I could use to design it?

Thanks in advance, I appreciate the help. :wigglesmiley:
 

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to change the tuning, get either a different size port(ive seen 3,4, and 6 in. diameters for the flared ones), or cut the tube of the port

some people think flared ports sound better, some like slots. ive also heard that flared ports will get you louder than slots, but by how much, or whether it is true or not, i dont know.
 

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if anything id go for the slot port. tuning it depends on port width, depth, and size of the box. but making a ported box only gives the sub a select amount of good frequencies.

id say either build a sub box to spec from what alpine says or u should look into a snail shell box. ive built plenty of those
 

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Six eyes
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- tuned correctly, a tube port is no different from a slot port, however there are some assumptions to be made when using a slot port, especially if one side of the slot is also a wall of the cabinet
- you cannot change the tuning simply by changing the port since tuning is a function of both volume and port dimensions

making a ported box only gives the sub a select amount of good frequencies.
- all things being equal, a ported box actually has a greater range than a sealed box, since it has greater low-frequency extension. A bandpass box has the narrowest range, which is what makes them tend to sound "monotone".
 

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Discussion Starter · #5 ·
Crutchfield says the recommended dimensions in the subwoofer manual for a ported enclosure are wrong and looking online I can only find what I already wrote in the first post; volume and slot dimensions.

Do the dimensions/shape of the box have major effects on the sound? If the sub fits with enough room for movement, the port is built the right size, and the internal volume is right, it would be fine hm? If I go with a tube port, would changing the distance from the inside end of the tube to the wall of the box change the sound?
 

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As stated, if you build a slot port using a wall of the enclosue as one wall of the port, it tunes as if it were half of the port width longer (without accounting for end correction). Acoustically, you won't notice much difference between the two, provided you do not exceed around a 7 -1 ratio of for the shape of the port opening. The big difference is going to come from the amount of port area you are using and the tuning frequency, relative to the net enclosure volume, as well as the size/number of drivers you are using. As you add port area and raise tuning frequency for a given enclosure volume, you gain output, but lose usable bandwidth, and the enclosure gets really peaky. Less port area and lower tuning offer more usable frequency range, but with less overall output. The smaller the enclosure volume, the more small changes in port length/area effect tuning. The small enclosure however, provides more mechanical support to the driver, and allows for more power to be put into it at higher frequencies without bottoming out. As the enclosure grows, it gets more efficient, and plays lower with less input, but you lose the ability to overdrive the woofers. Enclosure size also effects impedance rise, with larger enclosures offering less reactive box rise, and smaller enclosures offering more. You also have to consider port noise. Depending on the size/number of woofers you are using, and the port tuning you choose, there will be a minimal vent area necessary to avoid port chuffing and compression. Higher tuning frequencies produce more port velocity and require more port area to avoid noise. Driver Fs is also a concern. In general, for high output systems, you don't want to tune the port of the enclosure below the Fs of the driver. Doing so will severely limit input power.

I know thats a lot to digest... To "dumb it down" a little,

As a rule, for daily use, start with the manufacturers airspace recommendations, and use 10-15 square inches of port for every cubic ft. of NET box volume. Remember to factor in the displacement of the port, and of the driver, and subtract it from the gross box volume. For daily, most people shoot for tuning in the 30-35 hz range, with 30-32hz being the most popular. Some of this is driver specific, because as stated, you generally don't want to tune below the driver Fs for high output systems. Due to the small confines of the automobile, transfer function, and port loading... you may very well find it beneficial to port the enclosure at a higher frequency, depending on the application - but the above is a good starting point (10-15 in2 of port area per ft3 of net enclosure volume, tuned at 30-35 hz).

If you plan on building your own enclosure, I strongly recommend mocking things up in WinISDpro. It can be downloaded free, and is relatively easy to use if you understand basic acoustics and driver parameters. It will allow you to visualize how changes in your enclosure design will effect performance, but its calculations are based off of an infinite room size. It does not account for the physical effects the car will have on the results - of which there are many. It is still very useful as a point of reference, however.

Here is the link:

LinearTeam

Enslosure design is a complicated undertaking. Especially since you are new to it, you will most likely find that you have to build quite a few enclosures, and toy with things like placement in the vehicle, tuning frequency, port area, woofer/port location, etc. reapeatedly, in order to get the desired results. Good luck, if you have any questions, feel free to ask.


Edit: I see you just asked about how box shape effects sound. The shape isn't a huge issue. Ideally you want to avoid squares/rectangles, symetrical shapes, and square corners to reduce standing waves. In reality, this isn't normally all that big of a deal, and I wouldn't stress it too much. You do want to make sure, however, that the opening to your port is at least it's own diameter away from any obstructions, such as the speaker, box bracing, or a box wall.

Hans
 

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Discussion Starter · #7 ·
Cool, thanks for that program.

I went ahead and put in for the box;
-Volume: 1.8 ft^3
-Tuning Freq: 32Hz
When I click the "Vents" tab, it gives me a circle port with a diameter of 3.23" and length 9.19". I'm guessing this is the size tube I need to get.

I've looked at a few guides on how to use this program and they say something about the air velocity through the port. For what I've entered, it peaks at almost 124ft/s rms at 24Hz. Should I worry about that? It's at ~115ft/s at 30Hz. What can I do to lower this?
 

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Alright,

I am assuming you are going off of a specifiic alignment, right? You don't necessarily have to do so. The alignments they list are functions of certain calculations, that yield the most linear, musical output curve for a given speaker in a certain situation. You don't need to adhere to them, however, bear in mind that the farther you get away from a specific alignment, the more pronounced certain musical "imperfections" become. How much that bothers you is obviously subjective. Musically, you can overcome a lot with an EQ, but if you are building for SQ as opposed to for volume, it is best to start with a properly aligned enclosure.

That said...

Program wise,

You can punch in any box volume you desire (make sure you use NET volume, not GROSS) into the "Volume" category, and any tuning frequency into the "Tuning Freq." category, on the "Box" tab.

Then if you click on the "Vents" tab, You can enter the number of ports, whether they are circle or rectangle (click on the circle itself and it changes to a slot port, or vice versa). Then you can enter the desired diameter of the port, or the dimensions (L X W) of the slot. You find port area by doing the math - (Length X Width for a rectangle, Pie X Radius Squared for a circle), and multiply it by the number of ports to figure your port area. For this application, leave the end correction on 0.732. (It will be off by a bit if you flare the port ends, but its close enough without making things overly complicated).

You can then go back and forth between the tabs - If you adjust box volume or tuning frequency on the "Box" tab, it will make the changes automatically in the "Vents" tab, telling you how long the given port(s) must be to produce that tuning. Or, you can change how long you want the vent to be, or the dimensions in the "Vents" tab, and it will show your new tuning frequency in the "Box" tab. (Everything in the program is connected, its performing mathematical formulae to adjust the numbers depending on what you key in). Just make sure you add or remove box volume as you change the length of the port if it goes inside the box... because the volume of air inside the port doesn't count as box volume and must be subtracted, unless the port is outside of the box. Remember that for a given enclosure volume, as you add port area or lower tuning frequency, the port gets longer. As you reduce port area, or raise the tuning frequency, the port gets shorter. Obviously you need to be reasonable with the amount of port area - you can't have a huge port that needs to be 50 ft. long to get the tuning you want (not to mention the woofer wouldn't be able to support that large of a port...) , but at the same time, you can't use a single 1" port with an 18" speaker, otherwise you have a sealed enclosure with an air leak, not a ported one. Thats why I gave the earlier recommendation of 10-15 square inches or port area per cubic foot of net box volume, to use as a ballpark. If its a two cube box, start with 20-30 square inches of port, three cube box, 30-45 square inches, and so on. As you add port, it tends to increase overall output, but the box gets peakier and less musical. Obviously you have to be reasonable and use a port that will fit in the space you need it to, and give a tuning close to your desired frequency.

For the record, you can also change the units of any item (like from inches, to cm, etc...) by clicking on the word itself. (Not sure if you figured that out... but it helps, so you don't have to keep converting figures to punch them in).

As for the port velocity... if you tune the enclosure lower, port velocity at resonance is lower. As you tune higher, port velocity increases. If you are concerned with lowering port velocity, tune the port to a lower frequency. But high port velocity isn't necessarily a bad thing, provided you have adequate port area to avoid noise. The higher you tune, frequency wise, the more port velocity you will have and the larger the port area will need to be. For music, I wouldn't port the enclosure much above 40hz, unless you have something very particular in mind. Barring certain special instances, you will lose the ability to play the really low frequency notes well. Also, depending on a bunch of factors, you may need to use a subsonic filter to avoid damage to the woofer(s). Again, I won't go into details on all this unless you are curious).


Everything's a compromise.


Hope that helps,

Hans
 

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Discussion Starter · #9 · (Edited)
I was playing around with it and I think I found what I need. I set the box volume to 1.7 cubic ft and tuning frequency to 32Hz and for a single tube vent it gives me the 3.23" diameter and 9.9" length, like what Alpine recommended. The end correction for me says 0.850 automatically though instead of 0.732. Is this important or can I just leave it as is?
This also lowers the air velocity a bit. If it's not too much trouble, could you (or anybody who knows) give me a rundown on subsonic filters and under what circumstances I may need one?

And about the net volume, the 1.7 cubic feet I put in doesn't count the port or the subwoofer displacements? I would just add the sub displacement (0.071 ft^3) and the port displacement (0.187 ft^3) to that 1.7 when I'm coming up with dimensions for the box, right? So I would build a box with internal volume of 1.958 cubic feet and then put the tube and sub in and have 1.7 cubic feet of air? (haha, sorry if that sounds redundant, just want to make sure I've got it right)

Oh, what about adding a window to see inside? I know it wouldn't benefit the sound or anything, but I think it would look cool. What kind of material is best for this? Would it have adverse effects on the sound?
 

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It's not quite that simple, because as you add volume to the box, it changes the necessary port length to retain the same tuning. The best thing to do, is to first remove the speaker displacement (as long as the speaker is mounted normally, with the magnet inside the box), right off of the bat. Then figure out what a 1" cross section of the OD of the port displaces, volume wise, and multiply it by the desired port length to get your new volume, and plug it in. You'll go back and forth, changing the volume and the port length to retain the tuning you want. Eventually you wind up chasing your tail, but you can get close... it will make more sense as you plug the numbers in yourself. This only applies for the section of the port that is INSIDE the enclosure. Obviously, any part of the port which is on the outside of the box, doesn't need to be removed from the internal dimensions.

From what you are talking about, for 1.7 cubes, a single 3" port isn't much. Thats roughly 7 square inches of port. If you are interested in getting any kind of volume out of the thing, I would say you want to be around 20-25 square inches of port, for that enclosure size. You could either use three or four 3" round ports , two 4" round ports, a single 6" round port (For any of these, you'd probably have to use PVC, the aeroports wouldn't be long enough to get the tuning you want), or build a slot port. But a single 3" port isn't much, for that size enclosure.

As for the subsonic filter.... it depends on a lot of variables. Its hard to say, "Just set it here and you're good..." it doesn't quite work like that. What happens is that below the port tuning frequency, the driver starts to unload, meaning the enclosure stops offering support to the driver's backwave, and it acts like it is playing free-air and starts excursing like crazy. How fast this occurs depends on the driver design, the input power, and the load on the front of the port & speaker, among other things. The purspose of the subsonic filter is to attenuate the low notes, so the speaker doesn't over-excurse trying to play them. You'll want to start with it around the tuning frequency of the port, and slowly back it down towards flat, playing 0db tones at full output, until you reach a point where the woofers are starting to over-excurse. From there, you bump it up a hair and call it good.


Material wise, there are really only two basic options (provided you are using wood - some folks make enclosures out of fiberglass, plexiglass, or other materials, but wood is by far the most common). Either MDF, or Gapless (sometimes called Void-Free) Hardwood Birch Plywood. The MDF is cheap, and readily available in 3/4" thickness in 4X8 sheets. You can also get 1" MDF, at a good lumber yard. The trouble with MDF is it is really heavy, and caustic to work with (it contains formaldehyde). The other option is the Hardwood Birch Ply (There are various types that are suitable - Baltic Birch, Cabinet Grade, or Marine Grade Plywood, etc.), all of which are only available at a good lumber yard. They are imported, and come in 5X5 squares, which cost at least three times as much, for less material. Being imported, they are measured in mm, as opposed to inches. However, for a given thickness, the birch is considerably more rigid, as well as much lighter, and non-toxic. There are other crazy woods out there that some people use, but honestly, the above two will work just fine.

If you are building the enclosure for SPL, you want to make it like a tank, double walls of 3/4" or 1" material, huge internal bracing, and coated with fiberglass resin (sometimes mesh as well) on the interior & exterior. Anything you can do to reduce flex and increase hardness and smoothness of the surface will increase output. Obviously, this makes your box heavy as sin, and not very musical... but if you are trying to get loud, this is the way to go.

For more "Normal" applications, a single layer of 3/4" with moderate internal bracing should be sufficient.

As for your window, if its for SPL, it would need to be structural, meaning thick plexiglass that won't flex. If its for show, and you aren't as concerned with output, then this is less of an issue.


Hans
 

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- tuned correctly, a tube port is no different from a slot port, however there are some assumptions to be made when using a slot port, especially if one side of the slot is also a wall of the cabinet
- you cannot change the tuning simply by changing the port since tuning is a function of both volume and port dimensions



- all things being equal, a ported box actually has a greater range than a sealed box, since it has greater low-frequency extension. A bandpass box has the narrowest range, which is what makes them tend to sound "monotone".
only if the ported box is really well made. most ported boxes are built and tuned to a frequency.

the snail shell box is what bose uses for every one of their speakers. basically uses the reverb of the back of the speaker for a "two speaker in one" sound. and it does it better than a ported box and covers more frequencies better than a regular ported box
 

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Discussion Starter · #12 ·
civicsidoug; can you give me more info on snail shell boxes? I've searched around but I can't find anything substantial. It sounds interesting but your reference to Bose worries me a little.

back to ported enclosures...
Is it important to flare the ends of the tube? I mean, PVC is easy to find but I haven't seen any flared PVC pipes or flares that can be put on the ends (and they're all like 10 feet long so they need to be cut a lot). I hear flared ends make it easier for the air to move so there is less port noise but if the peak port velocity is only around 40ft/s, would it be fine to just use the straight PVC pipes found at home depot or wherever?

The Cross Area given by WinISD, can I use that and then multiply by the length needed to find the volume that the ports will take up? It makes sense to me that that would be the volume of the vents.
And the Vent Length given, is that the total vent length? IE if it says 33in for three 3in tubes, then would that be 11in per tube?

If it comes down to it, I guess I could build a slot port but I much prefer the style of tube ports.

I appreciate you all putting up with my questions, especially Hans.
 

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Some people refer to horn enclosures as "Snail Shells" because they operate like a snail's shell, starting small and consistently getting larger as you move towards the opening. They work just like a bullhorn, converting velocity to pressure & vice versa. If you've ever been to a concert or nightclub, and seen a bass bin where you can't see the woofer, that's an eaxmple of one type of folded horn, or "Snail Shell" enclosure.

I'll let civicsidoug explain the rest, since he brought it up.

Some pics:










Back to your port questions. The area they list doesn't include the outside material of the port, so no, you can't just use those figures. You have to figure the area of the outside diameter of the tube, and use that.

As you increase the number of ports, each one has to be longer to get the same tuning, plus they take up more wasted space. If you have it entered as say, four 3" ports, and it says 33", that means 33" for EACH port. For this reason its generally better to use a single large port, as opposed to multiple smaller ports. Also, if you can mount all or some of the port externally, that will help as well. If you can't realistically get the tuning you want, you'll have to use less port area (a smaller port) - but output will be reduced. Your other option would be to raise the tuning frequency some, which will shorten the necessary length of the port. Fiddle with numbers and see if you can find a happy medium.

As for the flaring, its done to prevent port noise at high volumes, especially when you are cheating and using less port are than you really should be. Yes it helps, but its not necessary, especially if you have enough port area to begin with. You can actually flare PVC tubing yourself, with heat and a die, if you care that much. But in reality, putting a tiny roundover on the edge of the tube and sanding it smooth is usually more than adequate. (A router with a small roundover bit cuts schedule 40 PCV quite nicely, if you have one). The main idea is to not have a sharp edge on the port, or it will whistle and make all kinds of funny noises... but it doesn't need to be a huge flare, just enough so it's a smooth edge. Keep in mind that flaring the end of a port makes the tube act shorter - which raises your tuning frequency. If you are tight on space, I wouldn't put a big flare on the ports.

Here are a few examples of home flared PVC ports:







Again, hope that helps...
Hans
 

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Discussion Starter · #14 ·
What about putting an elbow in the tube? How would that change things? I could certainly run the tubes along the outside if I curve it, leaving some inside and the rest outside curved around the enclosure. But if something bad happens when you put an elbow in the tube then I'll have to figure something else out.
 

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Elbows are fine... not a problem at all. You just have to figure out how much length the elbow gives you. The length measurement is based on the dead center of the tube. You can either do it mathematically, or if you prefer, with string. Lay the elbow flat on a table, and take the string and stretch it around the outside radius of the elbow (to the points where the PVC will butt against the inside of the tube,usually about an inch and half in from each end), and then straighten the string and measure it. Do the same for the inside radius. Now add the two numbers and divide by 2. That should give you the length of the imaginary line through the dead center of the elbow, which is what you need for your calculations to figure out the port length.

Sounds like you're on the right track. I know it's a bunch of work, but the results will be much better than anything you can buy off a shelf - and it gets easier the more enclosures you design. (Which all your friends will be bugging you to do, once they hear yours) :biggrin:

One thing to consider is which direction the port will be firing relative to the woofer, and how close its opening will be to a solid object. This will effect certain things. If you tell me which direction you plan to have the woofer and port firing (up, back, sideways, forward), where the box will be located in the trunk, how close the opening of the port will be to a solid surface in the car, and what surface that would be... I can tell you a little bit about what to expect soundwise.

Hans
 

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Discussion Starter · #16 ·
Cool cool... I hope it comes out as I plan it, haha. Thanks for the encouragement.

I'm going to put it on the left side of the trunk (civic sedan), a few inches away from the back of the back seats. I think it'll be a little more than a foot from the trunk door. the woofer and the ports will be firing back, so when the trunk is open it'll shoot out and away from the car. I think this will leave me with the most space in the trunk, but I don't know if there's a different placement that would improve sound.
 

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Sounds good man.

Most of the time in a trunk car (short of corner loading a pair of drivers, one in each corner in a separate enclosure), you'll get the most output from either woofer back, port back... or woofer up, port back. Every car is different, and it depends on a lot of factors, but in general, thats often the case. If you were to remove the rear speakers, or cut holes in the rear deck, sometimes you can benefit from firing the ports up, through the rear deck into the cabin. Personally, I wouldn't mess with all that, and just keep them pointed back.

One thing that would would help increase output would be moving the enclosure backwards towards the trunk, so that the port and woofer are firing directly into the trunklid (just a few inches away). Having the port & woofer cone firing directly into a solid object (in this case the trunklid) would load the port & driver face more, which would allow you to cheat and run a higher tuning with more port area, and still be able to get decent low end extension (It would make the box act as if it's tuned lower, while still keeping the midbass hump of the higher tuning, which helps output volume). It also creates a bit of a natural bandpass effect. This setup is a lot less practical though, since all the weight is in the absolute rear of the car, and you have no real usable trunk space at all.

Whatever you do, don't point the woofer or the port at the back seat, unless you plan on sealing off the space around it completely, and always driving around with the rear seat folded down.

What you got planned sounds like a good compromise for daily. You still plan on porting it at 32 Hz?

Post pics when you get to building.... it should sound killer.

Hans
 

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Discussion Starter · #18 · (Edited)
it came out to barely less than 33Hz after I adjusted port length, which is good enough for me.

I can't wait to get started. I just got the materials so I'm going to draw up my final plans in a little bit.

edit: another question, what do you recommend for bonding the MDF together? Can I use Liquid Nails to hold the boards together and then seal up with silicone caulk at the seams? Or should I screw it together? Or both Liquid Nails and screws?
 

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it came out to barely less than 33Hz after I adjusted port length, which is good enough for me.

I can't wait to get started. I just got the materials so I'm going to draw up my final plans in a little bit.

edit: another question, what do you recommend for bonding the MDF together? Can I use Liquid Nails to hold the boards together and then seal up with silicone caulk at the seams? Or should I screw it together? Or both Liquid Nails and screws?
use wood glue. liquid nails is not really a bonding agent, whereas wood glue is. titebond is probably the most common glue used by audio enthusiasts. use that with screws spaced about every 6". the screws just hold the 2 pieces together while the glue dries.

if you do want to seal it though, either use a bead of glue when it dries, or you can use a silicone based caulk. just make sure the silicone is completely cured before loading in your sub. depending on the type of glue used on the sub, the fumes from certain silicone caulks actually eat the glue.
 

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Yeah, when bonded properly, wood glue is stronger than the wood itself.
For sealing, I use bathtub caulking because it's not silicone-based. Works great and no stink :)
 
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