Need some thoughts on a large dual 15", dual ported design.

[erikhtb]

Hi everyone, I am looking for some input on a sub enclosure for my theater. I like the idea of making the front stage area an actual sub enclosure. The only issue I have is that I have to keep it relatively shallow, (20") so I can't port it from the front. I have over 8 feet of length, so I started putting together a design that would have 2 - RL-p15's in a dual ported configuration, with the ports on either end of the sub, extending into the lower units in my front columns.

Here's a rough MSpaint design, just to give you an idea about what I have in mind, these are very rough dimensions that don't include the thickness of the MDF, just the external dimensions:

I am unsure if having the ports exhaust into the lower units will be ok or if it will cause problems.

I ran some WinISD models, although I must admit I have no idea if I'm doing it properly, and this is what I came up with:

Driver : RL-p15
Project by : Erik Eastman
Project for : you
--
Number of drivers : 2
Box type : Vented
Box size : 645.0 l
Tuning frequency : 12.00 Hz
Vent : 2 vent(s)
0.874 m length for each
0.140 m round


Relative
Freq Gain SPL
[Hz] [dB] [dB]
20.00 -8.09 84.35
25.00 -7.01 85.43
30.00 -6.00 86.44
35.00 -5.14 87.30
40.00 -4.43 88.01
45.00 -3.84 88.60
50.00 -3.35 89.09
55.00 -2.94 89.50
60.00 -2.59 89.85
65.00 -2.30 90.14
70.00 -2.05 90.39
75.00 -1.84 90.60
80.00 -1.66 90.78
85.00 -1.50 90.94
90.00 -1.36 91.08
95.00 -1.24 91.20
100.00 -1.13 91.31


I have no idea if this is acceptable or if I am even on the right track or not. I figured out the cubic meters inside the enclosure, converted to liters, subtracted 25% for driver, bracing, and stuffing volume, and came up with 645liters +/-. This seems to be pretty good.

Would it make more sense to tune it higher to shorten my port length, so I could fit 4 15" across the front instead?

If anyone can help me I would greatly appreciate it, I am constructing my room and I need to know what direction to go before I build my front stage.

Thanks,

Erik

[aceinc]

Erik,

The columns above the height of the sub are used for shelves, closets, or something?

Paul

[erikhtb]

They will hold the front left and right speakers. If this design is feasible, The lower units of the columns will be MDF boxes with the fronts open, and the rest of the screen/speaker wall will be built on top of them.

[SteveCallas]

You would want the port openings firing directly into the room......firing into the cabinets the way you have it drawn would probably act as an extension of the port and could screw things up pretty bad.

[aceinc]

Since the side cabinets are empty I see two other alternatives;

1) Extend the sub cabinet into it, and do a front firing port, or,

2) Make the port external (partially, or fully) to the cabinet with an elbow to make it front firing.

3) Use a front firing slot style port, inside the current box.

This brings up a question in my mind; In this type of environment, is it better to keep the port as close to, or as far away from the driver as possible? Does it matter?

<off Topic>
One thing that would be interesting would be to model one or two rear loaded horns using the entire width of the wall including the side cabinets. If you can stack the 2 - 15"s put them on one side, and run the horn the entire width of the wall with the mouth in the other corner. A 15' wall might yield a horn with a cutoff < 20hz. Can you say cowabunga?
</off Topic>

Paul

[erikhtb]

Could I make the enclosure the entire width of the wall (or divide in half and do two separate enclosures) and have the two ports elbow out at the ends?

I could make it similar to what I have drawn except the ports would turn 90 degrees at the end.

At what point is the enclosure too big for the sub to operate efficiently?

Thanks btw..

[erikhtb]

Would 4 18" in a sealed enclosure up front be better/easier?

[brucek]

If anyone can help me I would greatly appreciate it

I don't know much about it, but I do know how to use WinISD.

If I use Boxnotes and use your outside dimensions of 92" x 30" x 20", I get an internal volume of 26.49 cu.ft. (assuming a double thickness front)

When you subtract the drivers (~0.18 cu.ft x2) and the bracing and ports, it leaves a working volume of 24.57 cu.ft. (695 litres).

WinISD, using two RL-P15 D2 drivers with 750 watts per driver (@ 4 ohms) and two 6" ports for a 12Hz tune yields 38.5" each with one end flanged.

I guess you'd have to ask the experts here what the problems or results would be if you put an elbow in the port so it faced out front.

Below are the VENTED results I got from WinISD using those figures anyway. I used a 1st order electronics rolloff filter at 10Hz. At full power the air velocity is a bit high, but only when you get down real low. I don't know if this is a problem or not. The excursion seems fine.


SPL with 1500 watts.
SPL.jpg

EXCURSION with 1500 watts.
EXCURSION.jpg

VELOCITY with 1500 watts.
VELOCITY.jpg

brucek

[SteveCallas]

I'd switch to a 10" diameter port and raise the tune to 14-15hz.

[brucek]

I'd switch to two TC 2000 15" svcs, use a single 10" diameter port, and raise the tune to ~14hz.

Interesting.

I do see the advantage of switching to a 10" single port. It certainly helped with the velocity.

I don't really see the advantage of the driver change. If I switch to a dual 4 ohm coil in the 15" SoundSplinter so that we're comparing the same configuration with the TC-2000 15", there doesn't seem to be much difference between the two as far as WinISD is concerned except a bit of velocity gain. The comparison is below...

Perhaps I've made an error though. I'm trying to learn this stuff.


SPL compare
SPL compare.jpg

EXCURSION compare
EXCURSION compare.jpg

VELOCITY compare
VELOCITY compare.jpg

brucek

[SteveCallas]

Oops, nevermind - the RL-p15 will work fine

[erikhtb]

Wow, thanks for all the help guys..those graphs look pretty similar to the ones I was able to generate.

Any comments on this setup vs a 4 RLp-15" or 18" sealed config accross the front wall??

Ported has me a little spooked since it seems to be alot more involved and time consuming to do properly, especially since this is my first DIY sub. I just don't want to spend a bunch of money and have it sound like rubbish because I didn't know what the hell I was doing..

Again, I can't thank you all enough for your help. I am craving chest crushing bass and I will stop at nothing to acheive it.

[brucek]

Any comments on this setup vs a 4 RLp-15" or 18" sealed config accross the front wall??
Ported has me a little spooked since it seems to be alot more involved and time consuming to do properly, especially since this is my first DIY sub.

My understanding is that a sealed sub allows you to use a more compact box. But, when you have the room and are able to take advantage of a big box (as you do), then the low tune ported design gets you some great low frequency extension without any low end EQ boost required.

Seems that most people around here who have built these ported monsters have enjoyed pretty incredible frequency response.....

Hopefully others with more experience can comment.

The one thing that has me interested is whether flaring the port into an open-front enclosure would affect the port outout. I also wonder what are the ramifications of putting an elbow at the end of the port to direct the sound outward.

brucek

[erikhtb]

What I'm wondering is at what point is a sealed box large enough to allow the drivers to have the extension they would in a ported or IB design. I have the space, if I can get away with not having to port the box I would feel much more confident about building it.

This is 4 RL-p18" in a 900l box w/ 2000w. The rolloff difference between a 1000l box and a 10,000l box is fairly small. Much flatter than the ported design I was looking at. It shows a Qtc of .74, is this ok?

[erikhtb]

I found this btw...

http://www.subwoofer-builder.com/vent-options.htm

Just what I need, more options...sigh.

[Sonnie]

Anything you find there will be well worth your time reading...

[erikhtb]

Do you guys think it would be better to do slotted ports instead of tubes? It looks like it would certainly be easier.

I was looking at two seperate sub enclosures laying end to end, each with either 2x 15" or 2x 18" towards the middle of the room, with a slotted port on the very end of each box near the outside walls.


I am unsure of the way the slotted ports would need to be constructed, are they actually rectangular ports flush mounted into the box, or are they just rectangular cutouts in the face of the enclosure?

If I can get teh numbers to work, this looks like it would be very doable for me. How do you figure you spl numbers etc with two boxes? Do you just model it as if it were one big box when figuring what total approx output of the two will be?

I think this looks baddass and I would like to give it a try.

[SteveCallas]

erik wrote:

Ported has me a little spooked since it seems to be alot more involved and time consuming to do properly, especially since this is my first DIY sub. I just don't want to spend a bunch of money and have it sound like rubbish because I didn't know what the hell I was doing..

What I've found is that with ported you may have to do a bit more design work up front, but you'll have less "concerns" when it's all done. I still have yet to read a case of an enthusiast taking the driver from a LLT, going sealed, and preferring the latter. On the other hand, it is common for people to start sealed, be underwhelmed, convert it to a LLT, and be happy.

erik wrote:

What I'm wondering is at what point is a sealed box large enough to allow the drivers to have the extension they would in a ported or IB design

You'll never "catch up" to a ported design, as port output will always allow you to go deeper with more clean headroom. As you've noticed in your simulations, at some point the difference in volume for a sealed design doesn't make much difference.

erik wrote:

Do you guys think it would be better to do slotted ports instead of tubes? It looks like it would certainly be easier

Cylindrical ports are better. For the size enclosures you are considering, if you want to go ported and use four drivers, you'll definitely want to use the RL-p15s as opposed to the 18s.

brucek wrote:

The one thing that has me interested is whether flaring the port into an open-front enclosure would affect the port outout. I also wonder what are the ramifications of putting an elbow at the end of the port to direct the sound outward

I'd have to think the volume of air in the port would couple with the volume of air in the open faced cabinet, and things would be pretty far off from the modeled design. Elbows in ports will increase turbulence, making the onset of port noise happen sooner.

[erikhtb]

Thanks Steve.

So if cylindrical ports are better than slotted, and putting elbows at the ends of my ports to bring them out to the front of the enclosure will cause problems, as will exhausting them into the lower cabinet units, what do you suggest I do that will stay somewhat close in design to what I have in mind? I posted a link that ported subs with elbows in them, some more than one. I guess the million dollar question is if you had to do one or the other, which would have less compromise? Slotted ports or tubes with elbows?

SteveCallas wrote:

As you've noticed in your simulations, at some point the difference in volume for a sealed design doesn't make much difference.

What I found was at a certain point increasing the box size stopped having any significant effect on the response. When modeling IB designs, you are supposed to make the box volume something very large, say 10,000 liters. When I modeled a large sealed at 1000l, then did it again at 10,000l, the difference was slight, making me think that at some point multiple drivers in an very large sealed enclosure will mimic the response of a "small" room IB, like one that is installed facing out of a small attic space or closet, for example.

But of course I really have no idea what I am talking about, but to me this sounds logical.

[aceinc]

Steve:

Can you be a bit more specific, or point me to some reference material as to why slotted ports are less desirable?

Paul

[brucek]

and putting elbows at the ends of my ports to bring them out to the front of the enclosure will cause problems

I haven't found anywhere that it says it's a big problem. If you modify the ports to a single 10" port (as Steve suggested and I modelled above), the air velocity seems reasonable to me.

Why are you so hesitant to use a port? It seems simple enough. A single 10" port with an elbow and a flare should be relatively easy to create. You can use 10" PVC pipe.

brucek

[SteveCallas]

A slot port has edges, increasing turbulence - fluids flow better in cylinders. If having to choose between a cylindrical port with an elbow or a slot port, I'd go with the elbow. Another option might be mounting the port at a 45 degree angle?

[aceinc]

Steve:

I thought edges were more problematic as the freq increases, and for low freq's it wasn't so critical.

How much would a roundover on the input and output sections help?

Paul

[brucek]

How much would a roundover on the input and output sections help

From collo's site that you mentioned above......

brucek