Dual 18" 2242H Cabinet Design

[Serge]

I know this is an easy one for some of you guys but I am a newbee so I need some expert advise to avoid wasting time and money.

I am building a dual 18" cabinet based on the SR7419x cabinet. I love the sound of this cabinet but I want to take it up a notch and put some 2242s inside of it and retune the ports. I will be building something from scratch so that I dont have to mess around with the origional cabinet.

I am also doing someting different though. I am using the shorter dimension of the cabinet as the baffle and the width of the SR as the depth of the cabinet. The box will have internal dimensions approx. (24"W x 48"H x 30"D). That's about 10ft^3 per woffer.

If I have a (6"x18") rectagular port (similar to the SR), what would be the ideal port length to provide a frequency responce down in the 30s?

Anyone have any experience building something like this or could point me in the right direction, I would appreciate it.

[Loren42]

I know this is an easy one for some of you guys but I am a newbee so I need some expert advise to avoid wasting time and money.

I am building a dual 18" cabinet based on the SR7419x cabinet. I love the sound of this cabinet but I want to take it up a notch and put some 2242s inside of it and retune the ports. I will be building something from scratch so that I dont have to mess around with the origional cabinet.

I am also doing someting different though. I am using the shorter dimension of the cabinet as the baffle and the width of the SR as the depth of the cabinet. The box will have internal dimensions approx. (24"W x 48"H x 30"D). That's about 10ft^3 per woffer.

If I have a (6"x18") rectangular port (similar to the SR), what would be the ideal port length to provide a frequency responce down in the 30s?

Anyone have any experience building something like this or could point me in the right direction, I would appreciate it.

To get the box tuned to the desired frequency I need to know the exact internal volume after you subtract the displacement of all internal braces and the displacement of the woofer.

JBL should give you the driver's displacement on their specification sheet, which I can look up as well, but I have no clue on the amount of bracing you plan to add. Can you calculate that volume for me?

The port displacement also needs to be known, but that is just an iterative approach to get that. My software does that automagically.

Without the data I am requesting I can only give you a guesstimate.

Also, why do you want to use a rectangular port? Round ports are easier to make, easier to tune, and there is no sonic benefit to a rectangular port.

All that assumes that there is room for a round port on your baffle.

[SMKSoundPro]

There is much info here! Everyone will add their own ideas. Read them and heed them. They do know what they are doing!

Scotty.

[Allanvh5150]

Hi Serge,

Just running the numbers that you have given, with the cabinet tuned to 32Hz the port length will be somewhere in the region of 5.25"

Allan.

[Loud & Clear]

Man! I need two of those!!! :)

[Loren42]

Man! I need two of those!!! :)

I want to know how he gets it out of the basement!

[bigyank]

What paint type did you use when you rolled the cabinets? Looking good!

Yank

[Mike Caldwell]

Don't forget about internal bracing, side to side front to back, stringers along the cabinet walls ect. Within reason the more the better. As noted before you need to subtract that from the cabinets overall volume.

Mike Caldwell

[4313B]

To get the box tuned to the desired frequency I need to know the exact internal volume after you subtract the displacement of all internal braces and the displacement of the woofer.

JBL should give you the driver's displacement on their specification sheet, which I can look up as well, but I have no clue on the amount of bracing you plan to add. Can you calculate that volume for me?

The port displacement also needs to be known, but that is just an iterative approach to get that. My software does that automagically.

Without the data I am requesting I can only give you a guesstimate.

Also, why do you want to use a rectangular port? Round ports are easier to make, easier to tune, and there is no sonic benefit to a rectangular port.

All that assumes that there is room for a round port on your baffle.

Don't forget about internal bracing, side to side front to back, stringers along the cabinet walls ect. Within reason the more the better. As noted before you need to subtract that from the cabinets overall volume.If one uses good old fiberglass insulation then it becomes a bit more complex. Basically 1" to 2" fiberglass is going to negate the volume displacement of most drivers, ports and any reasonable amount of bracing. This is where prototyping comes in where everything is dialed in pretty tight and total system loses are documented.

It's fairly easy to end up with a larger than optimal net effective volume tuned too low.

[SMKSoundPro]

What paint type did you use when you rolled the cabinets? Looking good!

Yank
I use a black premixed exterior paint from Walmart! It goes on deep limey green and dries to an oil-slick black. It works great and is cheap! I use it on all of my cabinets here at the nightclub.
scotty.

[SMKSoundPro]

I want to know how he gets it out of the basement!
Actually, my shop is above the main bar, so it's down hill all of the way, except for the turn at the bottom of the stairs. I can just barely get a sheet of 4x8 ply around the cornewr and up the stairs.

[SMKSoundPro]

Don't forget about internal bracing, side to side front to back, stringers along the cabinet walls ect. Within reason the more the better. As noted before you need to subtract that from the cabinets overall volume.

Mike Caldwell
Note the bracing.

[Loren42]

Actually, my shop is above the main bar, so it's down hill all of the way, except for the turn at the bottom of the stairs. I can just barely get a sheet of 4x8 ply around the cornewr and up the stairs.

Thanks for sharing your pictures. Your project looks like a lot of fun and from what I can see you are doing a bang up job.

I wish I had the room for a shop, even upstairs, which doesn't exist in my house.

However, having a bar just below you isn't a bad deal either. :cheers:

[Serge]

Thanks for all the great advise guys. Scotty's cabinets look dope. Great version of the SR4719A.

The volumetric displacement of the 2242's is 0.32 ft^3 and volumetric displacement of the bracing is approx, 1.3 ft^3. That's including the handles and connector plate. I am thinking of adding some minimal fiberglass dampening. Will that skew the offset to a lower frequency if the port is tuned to say 35Hz?

Thanks again and i'll take some pictures of the build once I get my design squared away.

[Loren42]

I am thinking of adding some minimal fiberglass dampening. Will that skew the offset to a lower frequency if the port is tuned to say 35Hz?


Yes, it will. This is one of the reasons I like round vents. They are easier to trim to size than rectangular.

I would start with the predicted vent length, then use a sine wave generator on a PC with an AC voltmeter to map the actual system resonance point. A good AC meter is a real help. If you can't resolve well enough with your meter, inset a non-inductive 1K Ohm resister in series with one of the speaker wires and measure with the voltmeter across the speaker terminals.

Just start the sine wave at a frequency below the tuned frequency and increment the frequency one Hz at a time. Log the AC voltage at the speaker input. As the impedance changes due to the resonance of the vent, you will see the voltage level change.

I logged mine into an Excel spread sheet and then did a graph. Look at my website HERE (http://web.me.com/mdebeque/Site/Pyramid_Speakers.html) to see two different plots I did at the bottom of the web page.

You can either alter the stuffing in the cabinet or change the vent length to meet your original requirement.

I would love to see your data when you do it.

[SMKSoundPro]

These 5 cabs are tuned at 40hz. I am using 2241h drivers driven off a seperate aux feed from the mixer into a rane 2-way crossover and only using the low ouput with the xover dialed in at 80hz. So, 80hz and down on a sperate gain knob on the desk. This way I can add or subtract the four double subs and balance with the fullrange mains.

I found 37hz a little low for my liking.
scotty.