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Question: Enclosure & Port size (long & technical)

3K views 7 replies 2 participants last post by  _D_M_P_ 
#1 ·
Ok.. this outta be fun!
I am contemplating cutting a hole in the cab (under jump seats) to install 2 amps and one ported sub enclosure. Then having the metal "cage" and carpet cleaned up professionally for a nice amp rack/sub display.

Catch my drift?

Anyhow, To this end I am trying to determine the exact size of the box (H x W x D) I will build into the floor as well as the port length & size. I was looking at the specs/instructions directly from the manual my sub came with, and thats where I started to encounter some problems. The size of the box and the port length seem to conflict with each other. So I began to do more reasearch. I found a few port and enclosure calculators, and snagged a few fomulas & calculated stuff on my own; and things just don't quite add up. I can tweak the formulas to get it close to what the manual says, But not without some minor discrepancies.

I know it's probably not THAT big of a deal to go into all this detail to get it "perfect", but now that I'm in this deep, I can't let it go. I gotta find the "right" answer even if I can't build the box just "perfect". This has become more of a PUZZLE to me than just needing an answer, and I got sucked in. Know what I mean?


I have a DLS MW10 I am putting in a ported/vented enclosure.

First, here are my sub specs directly from the manual:
Code:
TECHNICAL SPECIFICATIONS FOR DLS REFERENCE SUBWOOFERS 
Type/Model                 MW10
Size                       "25 cm (10"")"
Impedance                  4 ohm
Nom. power (RMS)           400 W (max 600)
Freq. range                25 Hz- 2 kHz
Sensitivity                89 dB
Voice coil diameter        75 mm (3”)
Voice coil height          24 mm (0.94”)
Re                         3.23 ohm
Voice coil inductance      1.26 mH
BL product                 14.2
X-max                      +-9 mm (0.35”)
Suspension compliance CMS  202
SD- Effective piston area  314.6 cm2
Resonant freq. (Fs)        35.1 Hz 
Vas (liters)               28
Vas (ft3)                  1.51
Qms                        2.32
Qes                        0.36
Qts                        0.31
Cone material              Magnesium
Magnet weight              100 oz (2.83 kg)
Magnet diameter            "6.14"" (156 mm)"
Installation depth         "5.11"" (130 mm)"
Mounting hole              "9.21"" (234 mm) "
Outer diameter             "10.31"" (262 mm)"
Weight                    14.77 lb (6.7 kg)
This is the recommended enclosure size directly from the same manual.
Code:
[B]Subwoofer  Volume (liters/ft3)  Port            Damping      F3       Fb[/B]
MW10       vol: 24/0.85         3”x26cm/10.2”   Line inside  46.3 Hz  40.0 Hz
 
[B][FONT=HelveticaBold][SIZE=2][COLOR=#231f20]F3 [/COLOR][/SIZE][/FONT][/B][FONT=Helvetica][SIZE=2][COLOR=#231f20]= approximative lower frequency forvented boxes in Hz. [/COLOR][/SIZE][/FONT]
[FONT=Helvetica][SIZE=2][COLOR=#231f20]Often called F-3 dB point = the point where the power is 50% lower.[/COLOR][/SIZE][/FONT]
[LEFT][B][FONT=HelveticaBold][SIZE=2][COLOR=#231f20]Fb [/COLOR][/SIZE][/FONT][/B][FONT=Helvetica][SIZE=2][COLOR=#231f20]= Box resonant frequency[/COLOR][/SIZE][/FONT][/LEFT]
Some of you audio nuts probably already know this, but for those reading that don't here are some Additional details provided by the manual:
Code:
- All volumes are inside measures.
[LEFT][B]- Volumes occupied by speaker and ports have already been added to the [/B]
[B]given enclosure volumes.[/B]
- Use conical bass ports for best result. (DLS BP-75 or BP-110). If the ports 
are too long for the box you can use a bend. Either cut the tube and glue it 
together in angle, or use factory made tube bends. It´s easier to use the 
factory made ones. The total length must be the same as for a straight tube. 
Make the measure in the center of the tube.[B] The port opening inside the [/B]
[B]enclosure must not be closer to an interior wall than 3” (75 mm), otherwise it [/B]
[B]will have negative effects on the airflow.[/B]
- The enclosure must be very steady and completely airtight. Use 22 mm 
particle board or 19 mm MDF-board. The particle board has a self resonant 
frequency of 14 Hz while the MDF has a resonant frequency of approx. 400
Hz. [B]It´s important to do some type of bracing inside the enclosure to avoid [/B]
[B]vibrations. Volume taken up by bracing should be added to the enclosure [/B]
[B]volumes.[/B][/LEFT]
 
 
[LEFT]ENCLOSURE DAMPING
[B]Most enclosures should be damped inside with syntetic (acoustic) wool or[/B]
[B]damping mat (line). Attach the damping material on the wall opposite from [/B]
[B]the speaker and port.[/B] A sealed enclosure should be filled up to 70-100% 
with acoustic wool. In a vented enclosure the speaker and port should be on 
the same side, otherwise a fade-out of some requencies can occour.[/LEFT]
I was having trouble coming up with the measuements for a box small enough, yet still have a 3 inch port 10.2 inches long. So, I Then found a few formulas to calculate the optimal size & length of the port and I come up with some completely different values.

Minimum vent (port) diameter:

dv - is the required diameter of the port in inches.
Fb - is the tuning frequency of your enclosure in Hertz.
Vd - is the volume displaced by the driver (in cubic meters) traveling through it's full excursion (peak-to-peak). To figure out Vd for a speaker, find the Sd value for your driver in the table below, and multiply that number times the Xmax (in meters) of your speaker.

Vent Length:

Fb - is the tuning frequency of your enclosure in Hertz.
Lv - is the length of your port in inches.
R - is the inside radius of your vent tube.
Vb - is the internal volume of your enclosure in cubic inches. To convert cubic feet to cubic inches, multiply by 1728.

Oh.. and just for giggles... for someone that might want to use a square or retangular port instead, here's the formula to figure that out! Just put this formula in place of "R" and solve, where "a" is the area (L x W) of your port.



NOW THE FUN BEGINS!!!

According to the manual, the size of the sub and port are already figured into the volume requirement; SO, I need to build a box to 0.85 cu feet. A port 3 inches x 10.2 inches long. Cannot come within 3 inches of the rear wall. (If it was a 4 inch port, it would be 4 inches from wall). At a tuning frequency (Fb) of 40 Hz. Well that doesn't leave me with many options, does it? The closest I can come with a single port is a wedge shaped box of 10.5"H x 13.75"W x 6.5"D1 x 14"D2. (D1 is top, D2 is bottom). That's a pretty tight fit for everything, but that gives me a box of 0.856 cu ft. Close enough!

But I'm thinking the best route would probably be a curved shape port of the same length. That would cut down the depth of the port allowing me more flexibility on the shape & size of the box. However, I punch these numbers into one of the calculators online and get an error. My numbers just don't match. I can't get that frequency with the size of the port, box, etc. So i tried using the formulas...

Using the formula for Minimum Vent (port) diameter, it says I need a port 4.19 inches wide (ok, 4 inches is cool). Immediate red flag! Is it supposed to be 3 or 4 inches?!? :shrug:

Then using the vent length formula using the value I just got, I get a required length of 24.26 inches. Houston, we have a problem!! :silly:

Sorry, 24.26 inches just ain't going to work. However, if I just arbitrarily plug in 3 inches on the second formula is spilts out a length of 11.81 inches. That's at least close to the manual...



So what is the real answer?
If I buy a ported box directly from the manufacturer, it is actually 2 liters larger than the recommended size for my sub and 2.8 liters smaller than theer other 10" reference sub. That an error of 15% or more. I'm assuming that's ok, but not optimal, right?

So how close do I have to be to the manuals spec to be "good enough"?
Or is it the manual's spec that is just "good enough" and I can do better by calculating the correct formulas?
Given the info I have provided, what do you think is the optimal size for my box & port?

I know 0.85 cu ft sounds small for a ported enclosure, but that is in fact what the manual is saying. How much performance would I use going a little bigger. I currently have a basic ported for a 10 inch sub. It's not custom tailored to mine, but it sound ok. It just won't fit where I want to install my custom box. (I'll get the dimentions later). Will I notice a difference if I try to make it "perfect?

In the end, I'll probably just get as close as I can, and call it good.

what do you guys think?
 
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#2 ·
to me it sounds like they are trying to make the box as small as they can, 40hz is kinda high, is this supposed to be a high output box or something all the freqz below 40 will be gone when a ported box plays below its tuning freq the woofer falls on its face i would shoot for something lower like maybe 36 or 34 and make it a 4 inch port remember when calculating box vol you need to calc in the vol of the port so if you need 1 cu and the port is .5cu the box needs to be 1.5cu. unless the woofer is massive and has a lot of displacement don't even worry about it. just by the sound of what they recommend i would shoot for a box around 1.2 cu total port 4"x19" (1.13 cu box with this port will tune at 36hz, the 1.2 gives you some room for the woofer. and any extra will just tune it a little lower at most 35/34hz) fire the port to the side off the cab wall give it half the length of the port between the end and the wall. should be very pleased with the result
 
#3 · (Edited)
Yeah, I agree... 40Hz is a bit to high for a subwoofer. I don't know what they're trying to accomplish with this setup (just thought I'd follow the manual recommendations). But driver unloading everytime it goes below 40Hz (probably often) is gonna suck!! Somthing about the manual recommended design isn't right here...

I think the design specs is to give it the greatest "efficiency" (only at or over 40Hz though) but at the expense of some frequency responce. I think I'd be willing to sacrifice some of that efficiency for some better frequency responce though. My driver is probably gonna tear itself apart if I only tune it to 40Hz!!

I was looking at the T/S parameters given in the manual for my sub, and found a problem. It lists the Vas at 28 liters and also 1.51 cu feet! One of those calculations is wrong!!!

28 liters is .99 cu feet
1.51 cu feet is 42.76 liters

So which one is it?
It's no wonder all these freakin calculators I'm using don't make sense.
The parameters I'm using aren't even right. Makes me wonder what else in the manual is wrong... :silly:
 
#4 · (Edited)
another thing about building the box larger and starting and a lower freq. is that if it doesn't sound quite right the box is easy to tune, one the port can be made a little shorter and if need be you can fine tune volume with closed cell foam. each calculator that is online may be a little different. Not that they don't work but some may include the vol of the port some may not. this will throw your numbers off and seem like someone is wrong. just because the manufacture states it's .85 do the math and see if that is after the port or before. it may be closer to 1cu if you add the .13 cu for the port. and they say to use acoustic wool this makes the box seem bigger so a bigger box wont need it.
 
#5 ·
Yeah the 28 lites/1.51 cu ft is a typo.
I compared the mfg specs and did a bit of calculating and the vas should be 42.8 liters. 1.51 cu ft is correct. (There is another woofer in thier line with the same spec)

I also tracked down a bunch of formulas, and also used a couple online calculators and while there is a bit of variation between the the results, I have come to the conclusion that the factory reccommendations are pretty damn close. .85 Cu ft is the volume with the woofer and port added (the actual calculated values without port & woofer are much smaller).

The only debateable issues might be the tuning frequency and port size (and hence length as well). Using a 4 inch port in the size box they recommend would require it's length to nearly double! (and therefore the size of box to increase). The 3 inch port they recommend is a compromise of size constraints and efficiency. If my calculations are correct, this single 10" sub should be capable of about 125+db at 43-44hz. Right in the "seet spot"! Not too shabby if you ask me. Using a flared port should hopefully reduce some of the port noise I might encounter. We'll just have to see how it sounds after I build it...

Also, after some research, I found that a tuning frequency of 40Hz is probably acceptable for the majority of music one might listen to. The stage monitors of a typical band are tuned to 40Hz... The bass boost provided by some head units and sub amps is at 40Hz or higher. (The bass punch knob provided with Rockford Fosgate amps is a boost at 45Hz). Many large enclosures for home theater sytems are often tuned at 40Hz or more.

While some electronic and modified bass instruments and maybe movie sound effects may play still below 40Hz, I think their occurance is small enough to be of negligable importance. The majority of the Bass I will want/need will be produced in the 40-50Hz range. I'll get a pre-amp filter to block the rest...

I have some extra MDF to play with. I'll make a box to their spec and see how it sounds. The addition of the acoustic wool should make the box seem bigger, lowering the frequency just a tad too. I think I'll make another box slightly larger and use some material to take up the space and play with the frequency just a tad. Using up more space to lower the frequency just a few Hz migh not even make any difference with what I play. However it will make a difference how and where I install it. We all gotta compromise somewhere huh?

But at least all this has been a learning process and although not an "expert," I know a whole lot more about T/S parameters, subs and enclosures than I ever did before. :headbang:


I'm not really going for SPL here... DLS subs are more geared towards SQ. Even thier ported setups are quite a bit more precise and have better SQ than many sealed setups I've heard. I think this particular ported setup will be a good balance of both. DLS is a good company, and I think they knew what they're doing when they designed it. After all the research, I'll give 'em the benefit of a doubt and build a box to spec. I have plenty of MDF, I can always build a second one later if I want... ;)
 
#6 ·
After i last posed on this thread i was talking to another installer about this, and we went through some calculations of our own. Yes their math is right, but a box that small tuned that high isn't going to sound as good or be as loud as a bigger box thats tuned lower. the bigger box will have a smoother sound and the larger port size will increase the volume (loudness) of the sub. but i suggest you do build a few boxes because thats the only way you'll know for yourself. i'm going to look for some tech info i got some where to post in this thread it might help you out.
 
#7 ·
Post away!! :waytogo:

Oh and thanks for the help.

So far I have a slot ported box that got from the shop about 1 cu ft +/-
I'll try building one to spec and then maybe one more a tad bigger.
Compare the 3 and see which I like better...

Another interesting comparison, the guy at the shop had the same sub in a custom build box. It was larger than spec (a little over 1 cu ft I think) with a 4 inch port, and he commented that my smaller slot-porter box sounded better (SQ & SPL). Just one reason why I'm leaning toward the specs... but I'll give them all a try.
 
#8 ·
it's not just size so much as it is tuning and the bigger the port diameter the less whistle you'll get (and is a lot easier to get a larger diameter port in a big box). and if you load the port into the side of the truck it will help the bass develop (kinda like when you put a home theater sub in the corner of the room).


and with the slot ports, thats my favorite. but make sure you do the math right when converting. Your 4inch port is 12.56 sq inches you can achieve that in any way buy simple math. Just keep the area (12.56) the same the length will be the same as your 4 inch round port length. it can be any shape as long as the area stays the same, not just a slot.. helpful for the "wedge" box.
as long as the gray area is 12.56 sq in. you got yourself a 4 inch port:waytogo: nice easy way for a ported box behind a seat
 
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