Airspace?

[mazeroth]

Building a set of line arrays with some cheap woofers and the box came out to 0.9 cubic feet with 12 4.5" drivers. Any idea how much I should add to the dimensions of the box? I'm also using a single Fountek JP-2.0 ribbon. Here's the driver I'm going to use:

http://www.partsexpress.com/pe/showd...number=299-432

Thanks!

[johninCR]

Sealed you only need 1 litre net per driver for a Q of .7
and it's down 3db at 135hz.

Vented you need 2.43 litres net per driver tuned to 82hz.

The ribbon tweeter looks like it would be great for an array, however, you're going to need a lot more than 1 per side. One will only keep up with the array at 1 meter. At 4 meters the array will be about 6db louder than the single tweeter. It might get cost prohibitive to get the correct match and then you'll have tweeter array length and dispersion issues to deal with on top of matching sensitivity.

[mazeroth]

Thanks for the reply, John.

Where did you get those figures from? I used WinISD beta 0.44 and it says to use 2.1 liters per driver tuned to 78.53hz with a 4" port length of 4.23". I am designing now with 10 drivers and not 12 due to the last few hours of researching and drawing up plans. What I'm curious to is how you can figure out approx. how much space the driver itself takes up?

On the single ribbon, I have been talking with Jim Griffin and also listened to his Needles line array that I'm mocking with cheaper drivers. He said it was fine to use a single ribbon as long as you tweak it to compensate the nearfield/farfileld transitional distances. I will be seated at all times with a distance of 8-12 feet tops.

I think I'm going to use 10 of those JBL drivers in a 3/2/tweeter/2/3 power taper, as Jim suggests using a power taper if only using a single tweeter. Speakers should be 5ft. 10in. tall if I follow my current plans, or 6ft. 3in. if I use the 12 drivers, which I don't think I'm going to.

Thanks again!

[johninCR]

I'm sure Dr Griffin is correct. Sounds like it's going to be excellent. With the power tapering and relatively close listening distance, you should be fine.

The easiest way to measure the basket displacement is to fill a bowl with a known quantity of water to the brim. Then with the driver wrapped in plastic see how much water the basket displaces by measuring how much doesn't spill out and subtracting from the original.

If you're crossing over in the 2khz range, you've got some flexibility on the driver spacing. In fact, increased height of the line helps keep you in the nearfield in the lower frequencies.

Those little JBL's look great for some corner array horns that I've been trying to figure out.

[mazeroth]

I will be crossing around 3khz but will do some tweaking to that I'm sure. Going active x-over so we'll see what sounds best.

Another question I have. I'm still a novice cabinet maker and here are the dimensions of the cabinets I'm going to build for them:

68.75 height x 6.5 width x 6.75 depth which yields 1.02 cubic feet, right where I want to be. However, I haven't taken into account poly stuffing in these line arrays. Any idea how much I should use and which sides to line with it? Also, how much will it "add" to my internal volume?

Thanks!

[johninCR]

With a ported box you really don't want too much fill because it will restrict the air flow. I'd just put something on the back wall to reduce/disperse reflections back to the driver cones, so it's not enough to affect effective volume.

How do you plan to handle the port(s)? I'm not sure how such a tall narrow cab will affect the operation of a single port. Multiple ports are warranted to give you the line array effect on their output. Keep in mind that multiple ports don't exactly sum up to a single port either. 4 or 5 is sufficient based upon the frequencies exiting the ports.

With regard to your exterior cabinet dimensions, be sure that it is really 3/4" plywood, not 18mm plywood, because it makes a difference of almost 1 Liter or 3% of volume.

Hint: solder all your driver wires and do any basket damping before putting your drivers in. It's much easier before, rather than after they are on the baffle.

[mazeroth]

John, that's a very good argument. I remember reading about WinISD and not being able to accurately calculate more than one port. Seeing how my woofer length is over 5 feet long and I had initially planned on using a single 4" port, the more I think about it, the less I like it. Any chance you or someone else reading this could plug in this driver to a program or spreadsheet you may have to give me the optimal box size with multiple ports? I'm thinking 2 would be a step up and 3 would be better. Agree? Also, could you tell me what size box your program recommends with the ports? Might want to add 2.5 liters to the box size to accomodate for the woofers and ribbon. So far the box size I'm using is 6w x 7.5d x 68.75h with 10 of the woofers. Did some more tweaking to it

Thanks a million!

Specifications:
*Power handling: 40 watts RMS/80 watts max
*VCdia: 1" *L1k: .60 mH
*Impedance: 6 ohms
*Re: 4.00 ohms
*Frequency range: 60-5,000 Hz
*Fs: 58 Hz
*SPL: 89 dB 2.83V/1m
*Vas: .20 cu. ft.
*Qms: 3.95 *Qes: .33
*Qts: .30 *Xmax: 3.5 mm
*Dimensions: Overall Diameter: 4-7/8", Cutout Diameter: 3-7/8", Mounting Depth: 2-3/8"

EDIT:

Thought I'd just throw them in my version of WinISD and let you know what I get. I up the volume to 23.8 liters to compensate for the speakers with a 78.5hz tuning. I have a ton of nice 1.5" x 4" ports here so I put in 1.5" diameter and it says four at 2.5 inches each. That would work. If you think going with 2-3 larger ports would be better I can just buy some, no biggie. Also, with 10 drivers I doubt I'll be pushing them more than 25% of their capacity so I doubt there will be a ton of air moving out of the ports.

Thanks!

[johninCR]

The way to get the proper port length for 4 ports is to divide the net volume by 4 and compute the port length for the 1.5" ports you have. The ports are not close to each other so you don't need to make adjustments for that interference.

[mazeroth]

john, you seem to be the only guy sticking with me in this post! Well, I did some figuring and also thought I'd post about it over at Parts Express's message board. I'll just copy and paste what I just posted over there. Thanks again, john!

In the process of designing my line arrays and working on the box. It will be 6" wide, 7.5" deep and 68.75" tall. They will resemble Dr. Jim Griffin's "Needles" array and use 10 5" speakers with a single ribbon. Here's where my problem is:

When I model the box in WinISD I get 0.9 ft3 tuning freq = 80hz. That's fine and dandy. What I'm inquiring is the port tube recommendation is a 4" port that's 3 inches long. Now, I will be using a 67.25" long box and with only one 4" port is this going to be a problem? I was thinking maybe 2-4 ports equally spaced would be better, BUT I can't get it to work out. In WinISD you need to take the 0.9 cubic feet volume, and if I wanted to try to use 2 ports, you divide by 2 and get 0.45 cubic feet. Then you model that box for a single port and just use two of them. Well when I do that I get these figures:

4" port = 8.6 inches long / mach vent = 0.12
3" port = 4.3 inches long / mach vent = 0.22!!!

Dual 4" ports would be fine but they need to be 8.6 inches long and it's only 6 inches internal from front to back of the box and I don't want to use curved ports. 3" would be better but I'm getting a mach vent of 0.22! If I go to 3 ports it just gets UGLY. I think I'm stuck with only a single 4" but want to make sure this would be fine? If so, put it at the bottom or middle? Will 3 inches from port to front baffle be long enough?

Sorry for the long post, but this is the only thing stopping me from cutting the MDF! =)

[johninCR]

You need to divide the net box volume by 4 and use a single 1.5" diameter port to get the length.

Port tuning is based soley on the volume of the air space, cross sectional area of the port, and port length, so if you still have all 10 of your drivers included it's going to show high port velocity which you can ignore.

It should give you a length slightly shorter than if you model full volume and one 3" port, since it's cross sectional area is right at 4 1.5" ports.

Don't you have a subwoofer? 80hz isn't low enough for rich full sound anyway. It's been my experience that porting in that region really gives you muddied up bass. I'd highly recommend going sealed and getting your bass another way.