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View Full Version : The Subwoofer Enclosure Tuning Thread



LoneRanger
07-11-2004, 05:28 PM
I see tons of posts here every week asking how to tune subwoofer boxes, why they should be tuned to certain frequencies and what affect this has on the way the box sounds. Hopefully, in this thread I can provide a base of information for those who need help with these common questions.

How big should my box be?

Your box should be big enough to allow the subwoofer to reach full excursion without being an infinate baffle (so large that it seems like its not in a box). This number will vary between subwoofers and is directly related to the subwoofer's VAS spec. The manufacturer of your woofer will usually tell you how large to make the box either in the paperwork that came with the woofer, on their website or you can usually obtain this info by calling their tech support line. If all else fails, ask somebody on the forum.

***Edit***
If you cannot provide a sub as much air as is recommended (small trunk, single cab trucks) you probably want to go with a sealed enclosure. Sealed enclosures do not require as much air space.

Subs will play in smaller boxes than normally recommended, but it usually means a sacrifice of bottom end. The box should be accordingly tuned to a higher frequency to compensate for this. If the box is going to be larger than recommended (and is not for SPL purposes) it should be tuned lower.

What frequency should I tune my sub to?

*NOTE* This is just a gerneral speculation. Some woofers are different and won't respond well to low/high tuning. If you're unsure how well your woofer will perform at a given tuning frequency, please consult the manufacturer of the suboofer or ask someone for advice. ***

If you're going for extreme sound quality and tonal accuracy, your best bet is to tune that sucker low. Anywhere from 26-30 hz should provide a fairly flat response curve on most woofers, and will assure that you don't miss any of the low-lows. This also assures that your woofer won't die from over-excursion unless you really abuse it.

If you're an SQL type of person and enjoy a little boom with your quality, you should probably go for a slightly peakier tuning frequency between 31-40hz. This will normally yeild a mild peak at notes within a few hz of the tuning frequency. It will make the bass slightly more boomy at the sacrifice of tonal accuracy. Over-excursion due to playing notes under tuning frequency can be a problem, but as long as you're careful to set the gains right and don't play at high volumes for extended periods, you'll be just fine.

The SPL types will want a high tuning frequency. Most SPL guys who compete tune their boxes in the 50's, 60's and even 70hz range. If you are willing to sacrifice sound quality for boom but don't want to over-do it, I'd suggest going for the 40-45hz range. It will almost always make for a nasty sounding enclosure, but it will peak a lot heavier than a lower tuned box. You also have to be very careful not to blow the subs by playing at high volumes below the tuning frequency.

How do I tune my box?

The formula I use comes from the JL Audio website (click here (http://www.jlaudio.com/tutorials/ports/index.html)) and I consider it the gold standard. There are several online calculators like the one at BCAE, and calculators like WinISD, but they will more often than not produce different numbers than you would doing the formula by hand. Do the math twice and make sure that you figure for ALL displacements. Most subwoofer manufacturers tell you how much the woofer will displace, but if you can't find it on their website or in the literature that came with the driver, give them a call. The formula to figure the displacement of a slot port is ((W+T) x H x D), where W = width, H = height, D = depth and T = board thickness.

The rule of KISS (Keep It Simple, Stupid) applies to the design of woofer boxes. First, figure out how much air space the sub needs, then figure the port and add the displacement of the port and woofer to your total volume. Then and only then start figuring the outer dimensions. For instance, lets say my sub needs 3 cubic feet, the sub displaces .1 cubic feet and the port will displace .7 cubic feet. My car will fit a box that is 36" wide, 18" deep and 17" tall. I would make the enclosure 34" wide, 16" tall and 15.5" deep. That would fit inside my trunk nicely with a little room to wiggle it in and also produces a net volume of 3.8 cubic feet (assuming .75" board thickness). A great little calculator to figure the internal volume of square and wedge shaped boxes can be found here (http://www.eatel.net/~amptech/elecdisc/spboxnew2.htm). DO NOT USE THEIR TUNING CALCULATOR.

Any other general advice?

Take your time and measure every thing twice. In fact, do all the math twice too. A screw up on paper isn't as hard to fix as one on wood. Not nearly as costly either.

Make sure your enclosure is sealed tight. The port should be the only part of the box that lets air escape. The easiest way to check for gaps is to take the box into a dark room and shine a flashlight inside it. If you can see any pinholes of light between the pieces of wood, you'll have to seal it better. I suggest running a bead of caulk around the inside of the box after it's finished just to be safe.

Never assume anything. If you can't prove it in writing, with math or from a trusted source, it's probably not correct. Taking shortcuts is never okay. Remember what our pal Ben Franklin once said: "A stitch in time saves nine".

If you are unsure of your mathematical skill or just don't like doing math, you should probably leave enclosure design to those with experience. Several people including myself offer box design services for a minimal fee. If you feel like you just need a little help or aren't sure if you're doing something right, never be afraid to ask. Trust me, it doesn't make you look dumb to ask questions about complex math formulas. In fact, people will respect you that much more for making the effort.

Donn, why are you so **** cool?

I dunno man, I just am.


If you guys have any other questions you'd like me to address, please feel free to ask and I'll edit this post as needed.

Gauntlet
07-11-2004, 05:33 PM
Good job Donn. Consider this stuck.

saywhat?
07-11-2004, 05:41 PM
this may help some others as well if you would be so kind to answer....say your box is 34 inches wide and your port walls are 3/4 MDF. would u measure the air space u have with 32.5 inches wide......or do u figure airspace BEFORE you measure out port specs?

LoneRanger
07-11-2004, 05:55 PM
this may help some others as well if you would be so kind to answer....say your box is 34 inches wide and your port walls are 3/4 MDF. would u measure the air space u have with 32.5 inches wide......or do u figure airspace BEFORE you measure out port specs?

Internal volume is figured by the internal length, width and depth. If the box is 34x16x15.5 on the outside, you'd subtract 1.5 from each dimension which would yield 32.5x14.5x14. You then divide the product (6597.5) by 1728 (the number of cubic inches in a cubic foot) which gives you 3.812 cubic feet.

kickerlivinloud
07-15-2004, 05:14 PM
Quick links to some online calculators for you all...

http://www.jlaudio.com/tutorials/ports/index.html

http://www.carstereo.com/help/Articles.cfm

http://www.bcae1.com/spboxnew2.htm

www.audiobahn.com

www.rockfordfosgate.com

Hope this helps someone out, I know I use them a lot...


Jon

kickerlivinloud
07-15-2004, 05:35 PM
so Jmac, when are you going to start making designs agian?? I may have to hit you up for one to in a couple weeks or a month or so, so I can see if I did mine right, I should have them done by then.


Jon

ramos
07-21-2004, 01:49 PM
Meh, calculators **** ... Use a sheet of paper and a pen, draw out your enclosure, make sure it's right ...

Here's a step-by-step process of how I do it ...

1) Measure your trunk for max dimensions and note TWO dimensions that would be preferable. Subtract twice the thickness of the desired dimensions to get the desired internal dimensions (i.e. 3/4" MDF, 20" height = 18.5")
2) Figure out desired net volume, tuning frequency, and port area.
3) Figure out port length using the following formula :

Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = Port Length in inches
Av = Port Area in square inches
Vb = Net Internal Volume in cubic feet
Fb = Desired Tuning Frequency

4) Draw out a diagram of your enclosure on a sheet of paper and figure out the displacement of the port.
5) Add up all displacements (port, driver, brace)
6) Add displacements to desired net volume and multiply by 1728
7) Divide by the two desired interal dimensions to get the third internal dimension
8) Add twice the thickness of the wood to get the exterior dimensions

I hope you guys appreciate this because I'm literally throwing money away here :fyi:



We love you man :inlove: :D :)

callahanianhunk
07-30-2004, 10:42 AM
alright i cant do those formulas to figure out the tune call me stupid or whatever but i no my box size an sq ft my box is 15" deep 14" tall an 53" wide an its 5.029 cb ft internal usind 3/4 mdf but what i cant figure out is what size depth wide an tall my port should be to tune it to 40 if any of yall can help i would greatly appriciate it.

LoneRanger
08-01-2004, 01:44 PM
alright i cant do those formulas to figure out the tune call me stupid or whatever but i no my box size an sq ft my box is 15" deep 14" tall an 53" wide an its 5.029 cb ft internal usind 3/4 mdf but what i cant figure out is what size depth wide an tall my port should be to tune it to 40 if any of yall can help i would greatly appriciate it.

the purpose of this thread isn't for us to tell you the length of your port. its to educate you on how to do it yourself. rather than asking "what size should my port be" you should be asking questions about the parts of the equation you have problems with. its my experience that most times its not ignorance or lack of mathematical skill that keeps people from learning how to do design their own enclosures: it's flat out being lazy and not wanting to tackle that big ol scary math problem.

SO!

instead of just telling you the width, length and depth of the port, i'm going to show you how to do the math step by step :)

First off, you didn't follow my directions in the first place because you figured the outer dimensions first. GO BACK AND READ MY ENTIRE POST!!!!! I'm going to ignore the figures you submitted for now and use them as a guideline later.

Since you neglected to mention what size, type and how many subs you're using, I'll assume that the box needs about 50 square inches of port (this is suitable for a single 15" or a pair of 10's or 12's in most cases) and about 4 cubic feet after all displacements. However, you did at least let me know what thickness wood you're using so that's a halfway decent start I suppose.

If the max height is 14", the max port height is going to be 12.5" (14" - 1.5" for top and bottom thickness). To achieve 50 square inches of port, the port will have to be 4" wide. So far we have:

AV = 50in^2
VB = 4ft^3
FB = 40hz

Let's start plugging these into the equation.

AV*1.84 = 92
92x10^8 = 920000000

VB*1728 = 6912
FB/.159 = 251.57
6912*63287.46=437442923.5

9200000000/437442923.5=21.03

21.03 - (.823*sqrt50) = 15.21"

Your port should be 15.21" in length. I would round up to 15.25 just to make it easier on you....kinda hard to cut .21 on the old table saw :)

Now you have to figure the displacement of your port. The formula I listed in my original post is (W+T)*H*D. In this case W = 4", H = 12.5", D = 14.5" (the port passes thru the face of the box which gives it the extra .75" to make 15.25", but since that .75" doesn't displace anything inside the box, we don't count it here), T = .75"

W+T = 4.75
4.75*12.5=59.375
59.375*14.5=860.937
860.937/1728=.498

Your port displaces .498 cubic feet, so the enclosure will need to be 4.498 cubic feet after subwoofer displacement. Here's where I'm going to leave it to you. All you need to do from this point is add the displacement of your woofers 4.498, then use the dimensions you provided earlier to come up with a combination that equals 4.498+woofer displacement.

Now that I've gone thru all the trouble to do this "complex math problem" step by step and break it down into math problems my 12 year old niece can do, nobody has a reason to ever be lazy again :)

Happy Building

Donn Tha Wicked Gnome

****P.S.
WinISD Beta (a freeware program) came up with a port length of 14.6 (if you add the .75" to accomodate for the fact that the port passes thru the face of the enclosure, it's right on the button). WinISD doesn't require you to do any of this crazy math, nor does it require you to think a whole bunch. Just punch in the info and it spits out the answer.....

saywhat?
08-01-2004, 01:52 PM
hey don, ur sig says "theres no I in tag team" but there IS a ME!.....now...explain that.

LoneRanger
08-01-2004, 02:13 PM
hey don, ur sig says "theres no I in tag team" but there IS a ME!.....now...explain that.

If you wanna get technical, there's also a "meat" in tag team....care to explain that? :p:

knocturnaldrive
08-01-2004, 02:24 PM
Hey,
The manufacture of my sub said that i should have a port length of 8 inches and 4 inch diameter. Is it 8 inches long including the 3/4 deep hole that the port mounts in(box wall), Or is it 8 inches long on the inside, not including the 3/4 thickness of the wall of the box, i'm just asking because my sub isn't sounding that great in accuracy, and i know you compromise accuracy when you port, but it's really bad, so thanks for the clarification.
i did look through the post above to see if it had already been answered, and i didn't see it, but if it was sorry.

KnocturnalDrive

sumone
08-21-2004, 02:57 AM
Meh, calculators **** ... Use a sheet of paper and a pen, draw out your enclosure, make sure it's right ...

Here's a step-by-step process of how I do it ...

1) Measure your trunk for max dimensions and note TWO dimensions that would be preferable. Subtract twice the thickness of the desired dimensions to get the desired internal dimensions (i.e. 3/4" MDF, 20" height = 18.5")
2) Figure out desired net volume, tuning frequency, and port area.
3) Figure out port length using the following formula :

Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = Port Length in inches
Av = Port Area in square inches
Vb = Net Internal Volume in cubic feet
Fb = Desired Tuning Frequency

4) Draw out a diagram of your enclosure on a sheet of paper and figure out the displacement of the port.
5) Add up all displacements (port, driver, brace)
6) Add displacements to desired net volume and multiply by 1728
7) Divide by the two desired interal dimensions to get the third internal dimension
8) Add twice the thickness of the wood to get the exterior dimensions

and why do people buy plans again? it's that easy??? lol.

LoneRanger
08-22-2004, 02:28 PM
and why do people buy plans again? it's that easy??? lol.

for the same reason people buy Easy Mac over the regular kind that you have to cook on the stove :)

Acidburn
08-22-2004, 03:04 PM
and you know the cook over the stove stuff tastes sooo much better

sumone
08-23-2004, 01:43 PM
and why do people buy plans again? it's that easy??? lol.

lol, don't I feel stupid! I spoke too soon :)

I think I'm thinking too hard...

Zeph
08-24-2004, 08:02 PM
ah, good, now i can figure out without using a calculator!! :thumbsup:

thanks guys, man , im retarded to when it comes to math, but i figured this stuff out! good post

jp101
08-26-2004, 12:18 PM
thanks guys for the great formula, I'm calculating the port for my box. it will have 1 S12L5 in it and I only have one question, how do you get the AV? I'll be putting a slot port in it. this will be my first time to build an enclosure and I really want to get it right on papers first. thanks!

sumone
08-26-2004, 12:49 PM
Av is your port area.

from a post above,


...I'll assume that the box needs about 50 square inches of port (this is suitable for a single 15" or a pair of 10's or 12's in most cases) and about 4 cubic feet after all displacements

Now, I didn't know you could assume how much port area a sub needs. Maybe Donn or Jmac can answer that.

But if you already have a set dimensions of the port, it'd just be port_width * that_other_dimension. Like in my case, my box's max height was 16.5, therefore 15 inches internal. Then I just chose a port_width of 1.75 arbitrarily because just from analyzing the equation of Lv (as related to Av, Fb, and Fb), the more port area, the longer the port; which makes sense, cause the bigger the area of the port, the less resistance therefore the length of the port has to make up for that resistance. I'm just guessing though, again, more experienced people like Donn or Jmac would know the real answer/reason.

jp101
08-27-2004, 03:16 AM
will it be ok if I tune it 40Hz? I listen to rap and rock. I just want to make sure that I wont damage the speakers. any suggestions?

sumone
08-27-2004, 01:59 PM
It's not like it's wrong to tune to 40hz. As far as I understand it, mid-high 20s gives you the best SQ, 40 and above, SPL, and 30s a mixture of both. So it depends on whether you just wanna be "loud" or you wanna actually "enjoy" your music (not that they're mutually exclusive though)

LoneRanger
08-30-2004, 07:27 PM
Av is your port area.

from a post above,



Now, I didn't know you could assume how much port area a sub needs. Maybe Donn or Jmac can answer that.

But if you already have a set dimensions of the port, it'd just be port_width * that_other_dimension. Like in my case, my box's max height was 16.5, therefore 15 inches internal. Then I just chose a port_width of 1.75 arbitrarily because just from analyzing the equation of Lv (as related to Av, Fb, and Fb), the more port area, the longer the port; which makes sense, cause the bigger the area of the port, the less resistance therefore the length of the port has to make up for that resistance. I'm just guessing though, again, more experienced people like Donn or Jmac would know the real answer/reason.


You can't just assume how much it will need. I said "assume" because i was creating a hypothetical situation. The area of the port in relation to the driver's sd is just as realevent to output (read as SPL) as the tuning frequency. More port area = more output.

The more surface area the port has, the longer it will have to be to achieve a given tuning frequency as compared to a port with less surface area.

It's pretty much a given that a box with a larger port will allow the woofer to reach XMAX with less effort because there's not as much resistance. If you're designing an SPL system and your only goal is efficiency and output, go for a big port. If you're trying to maintain tonal accuracy, go smaller. Just remember, this port has to actually fit inside your box so if you're trying to tune the port to 25hz and you want 80 square inches of port, the port is probably going to be bigger than the woofer chamber.

#1AlpineFan
09-23-2004, 02:56 AM
So in order for me to understand how all these calculations work, I am using the recommend ported box for my 13w7 and trying to work the math and come up with the answer (because i know the answer).

If someone wil help here's the stuff. There is a pdf of the box diagram at this link (http://www.jlaudio.com/subwoofers/13W7.html)


I want to solve for the equation JMAC posted
Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = Port Length in inches
Av = Port Area in square inches
Vb = Net Internal Volume in cubic feet
Fb = Desired Tuning Frequency

As stated before AV=(W+T)*H*D. I think this is where i'm having problems.

Vb=2.375 cu ft
Fb=30 hz
Slot opening(not including the board)=2 inches
Front baffle = 1 inch
Rest of box = 3/4 inch
Slot height = 15.75 in

So with AV=(W+T)*H*D
(2+.75)*15.75*????

If i'm trying to solve the lenght of the port how can i know what D is? Do i just guess or throw a number in? I know 15.75 is the height because the box can't be any higher than 17.25in.

The answer is Slot length LV = 25.375. But i don't get that of course.

carguy83
09-23-2004, 02:49 PM
Very informative thread! I learned a few things already just by reading this. But I have a quick question since this is on the subject of subwoofer enclosure tuning. I know bass is non-directional, so does that principle apply to where a port(s) is pointing? Will there be any audible differences if the ported is say, for example, pointed to the rear of the car versus a port aiming straight up?

maniackilla
09-28-2004, 07:09 PM
jmac .... the reason they pay you ... is b/c the money is worth not dealing with the stress of that and knowing if its right :P

and ... your sig ... is interesting, atleast it counts by the pwoer of 2, making it representative of binary. choose a 1 or a 0, 1 means stay on drugs, 0 means get off them ..... thats also a binary based decision. as for statistics and probability ... the 1 choice of making the decision which has 2 outcomes give you a probable 1/2 chance of choosing one over the other. if it were me ... my probability of staying on drugs, even though i have a theoretical perfect chance of 1/2, would be is a 100% success rate of being a 1 ...... i just got of work ... excuse my post

/me puts down powder contaminated dollar bill, resinated rolling paper, nutritous 8oz bottle of max strength tussin with the only active ingredient of DXM, salvia pipe, and fungus filled home-made biodome .... those icons sure would come in handy about now ...

edit: edited to add some ... :silly: :smash: :p: :thumbsup: :spank:

Jimmyd_442
11-17-2004, 08:25 PM
Jmac will ur formula work with round ports to