Port size/type question - DIY Cab

[BassnBoa]

Short of it, I bought a '67 dualshowman head and cab. Head is legit but the cab is not. Trouble shooting a rattle i came across all sorts of issues because the cab itself appears to be an old DIY. The speakers are legit, JBL D140Fs with the big alnico magnets. They are in good functioning shape. I enjoyed the vintage sound I was getting, just the rattle was driving me nuts.

I'm building a new cab (one of the previous owners tried to fix the rattle with what I think is henrys roofing tar so this thing is trash in my eyes, and too shallow by design, 11" deep) and want to keep similar aesthetics. That said I opted to blow the dims out, I based my dimensions off Greenboys Dually 2x15 for what appears to be a simple design from what I can see. That said I can't find info on the port hole size for that cab. I also cant find any specs on the internal build which leads me to some questions I'd like input on. It's gonna be heavy, but i had a full sheet of 3/4" 13 ply baltic birch left over from a project I chose to use for this.

cab details - Dually

I isolated my 2x15 speakers with a divider.
I plan to add 2 - 4x12" rectangular ports dedicated to each section/15 woofer. I'm leaning towards this because I can make them myself and I also feel I could adjust the opening easier when it comes to tuning the cab.
How I plan to tune the cab:

This will be front ported, but I was thinking of building a channel 4x12 to the back 1/3 of the cabinet space. Why? Because port tubes seem to be "x" long and when looking at GB fearful 151566 diagram, I see he carries his ports to the back section of his cab. I'm assuming this keeps air moving more balance and keeps the speaker from drawing/pushing air over 1/2 itself, but really I have no idea. If anyone knows why or how these holes are size in proportion to the port openings let me know.

It looks like barefaced has their port channels flow to the back of the cab as well.

I'm wondering if when I go to tune the cab, if this tunnel will allow me to leave the front fully open and allow me to slowly cover the back of the channel, reducing it's size (from back access) until the cab is tuned appropriately?

I also noticed GB has a port divider in the fearful, but then drills holes between the divider towards the back of his port channels. I'm assuming this helps balance the tune of each individual speaker space to the other? Anyone know if the dually is sound dampened inside?

I plan to reuse the old fender grill, handles, corner gaurds and feet since it's trash. I'm not sure if I'll tolex it or use some Dura-tex to finish it off.

Thanks for any input.

P.S.
I know the DS isn't the most sought after bass rig, maybe the d140fs arent all the loved either but I enjoyed the sound I was getting. If I can get that sound without the rattle I'll be happy, but I do feel like the added volume to the cab space, could get me that same vintage sound with a little more balls for bass.

 

[agedhorse]

Port area is WAY too big, the port depth will be deeper than the cabinet IF you can eve get an acceptable tuning.

May I suggest that you learn a little more about speakers before cutting up wood? There are lots of threads on TB about this, but you will also see a lot of cautions and corrections due to bad info. THIS is the learning experience, and it's cheaper to learn from other people's mistakes.

 

[BassnBoa]

I hear ya @agedhorse . I've been reading around a bit and see a lot of conflicting information. My hope with the thread was to get a debate specific to this setup to help me narrow down and filter through more specific information.

The 4 x 12 I came across from a port suggestion on a similar cab (an old fender with 2x15s). The suggestion was two 4" ports by 6". Maybe I was thinking of it wrong. I thought the size would be large, but with a plan to essentially gate the back of the channel to fit the cabs needs when I go to tune it. I really don't have much understanding of it, maybe that will be an issue and cause some low frequency to be unbalanced or delayed. I'll keep reading around, but I do hope to get more specific information to this build so here are some dimensions for anyone who could help me out some.

Speakers 15" JBL d140F x2
Cab construction 3/4" baltic birch
Cab external dimensions
36" tall
24.5 wide
15" deep (front to back)

Center divider makes each 15" isolated to and internal space of 23x17x13.5 or 3.05 cubic feet per speaker

 

[agedhorse]

Because of the driver being used being suitable to a fairly wide range of tunings, optimization can maximize the performance compared with the typical use. I modeled for the K-140 which is a little more robust power-wise than the D-140 but plenty close for modeling

For a total of 6CF, you would need a total of 50 square inches of port and the depth would be the 3/4" thickness of the baffle. That would be one port for each driver that's 25 square inches each.

This is the result:

F3=56Hz, F6=50Hz, F10=44Hz
Sensitivity = 102dB/1W/1M
Mechanical Power Handling: 400 watts RMS (with a 2nd order HPF set at ~45hz)

This isn't too bad, BUT with modern drivers optimized around smaller boxes it's possible to get significantly better performance in a smaller, lighter box.

 

[BassnBoa]

Because of the driver being used being suitable to a fairly wide range of tunings, optimization can maximize the performance compared with the typical use. I modeled for the K-140 which is a little more robust power-wise than the D-140 but plenty close for modeling

For a total of 6CF, you would need a total of 50 square inches of port and the depth would be the 3/4" thickness of the baffle. That would be one port for each driver that's 25 square inches each.

This is the result:
View attachment 7515951

F3=56Hz, F6=50Hz, F10=44Hz
Sensitivity = 102dB/1W/1M
Mechanical Power Handling: 400 watts RMS (with a 2nd order HPF set at ~45hz)

This isn't too bad, BUT with modern drivers optimized around smaller boxes it's possible to get significantly better performance in a smaller, lighter box.

I'm very grateful for that information agedhorse, thank you! So I was going to oversize the heck out of the port by going with that channel idea, i was focused on opening size, not that actual volume of the port itself in comparison to the cab CF Is there any noticeable advantage to port depth with what I'm working with or should I just keep it simple and make a 2.5 x 10" slot in the 3/4 baffle, for each 3CF section?

Thanks again

 

[agedhorse]

Don't make it anymore complicated than it needs to be. There is zero advantage.

 

[crazycloud]

I've been reading around a bit and see a lot of conflicting information.

Only from people with little to no experience and / or who don't understand the math.

This isn't too bad, BUT with modern drivers optimized around smaller boxes it's possible to get significantly better performance in a smaller, lighter box.

D140s are still sought after in certain segments like some guitar players. If they're in good condition, sell them and buy some modern drivers that will work in the cab, have better power handling and less weight.

Or jump in the deep end and learn how to model and design your own - but that's a pretty big ask. You'll likely come out with a much better design for mess money in the long run, and places like diyaudio have tons of experienced and knowledgeable users that'll happily help you design something fairly basic like a BG cab.

Even easier would be to look at building an established design like one of greenboy's fEARfull cabs which come in a variety of configurations and are very highly rated. No design work involved, I think some flatpacks are available, but cut lists are so not to hard to build.

Repurposing old cabs has usually been a waste of time in my long experience. By the time you get around the constraints of using someone else's design, I could have had a new, better performing cab built from scratch.

 

[agedhorse]

Only from people with little to no experience and / or who don't understand the math.

D140s are still sought after in certain segments like some guitar players. If they're in good condition, sell them and buy some modern drivers that will work in the cab, have better power handling and less weight.

Or jump in the deep end and learn how to model and design your own - but that's a pretty big ask. You'll likely come out with a much better design for mess money in the long run, and places like diyaudio have tons of experienced and knowledgeable users that'll happily help you design something fairly basic like a BG cab.

Even easier would be to look at building an established design like one of greenboy's fEARfull cabs which come in a variety of configurations and are very highly rated. No design work involved, I think some flatpacks are available, but cut lists are so not to hard to build.

Repurposing old cabs has usually been a waste of time in my long experience. By the time you get around the constraints of using someone else's design, I could have had a new, better performing cab built from scratch.

I already gave him a solution with solid results, my experience, math and engineering skills are far beyond most DIY folks. He’s not repurposing his old cabinet, he’s building a new one.

 

[crazycloud]

I already gave him a solution with solid results, my experience, math and engineering skills are far beyond most DIY folks.

But far from all of them, myself included. There are a number of industry pros who post there regularly and whilst many are amateurs, some of them are very talented and experienced. Greenboy knew very little when he started and the design tools available then were quite basic compared to what's available today, yet they came out well and are highly regarded.

He’s not repurposing his old cabinet, he’s building a new one.

Apologies, I read the earlier posts whilst not paying a lot of attention and when I came back to reply I made some errors by not rereading from the beginning. I also am in no way disparaging your earlier input.

 

[Wasnex]

So I was going to oversize the heck out of the port by going with that channel idea, i was focused on opening size, not that actual volume of the port itself in comparison to the cab CF Is there any noticeable advantage to port depth with what I'm working with or should I just keep it simple and make a 2.5 x 10" slot in the 3/4 baffle, for each 3CF section?

The driver and cab must be designed to work together. I highly suggest you either study how speakers works, or simply take Agedhorse's advice on the cab's internal volume and port design.

I am by no means and expert, but hopefully this explanation is close enough to help a bit .

The driver interacts with the internal volume of the cab and the port. The exact interaction is determined by both the driver's Thiele Small (T/S) characteristic and the characteristic of the cab to include internal volume and port dimensions.

AFAIK as the frequency goes lower, at some point the driver will begin to cut off. The port is tuned to start becoming active around this point. As the frequency continues lower, the port become more active, produces more sound, and provides more suppression to driver excursion. This continues until the system reaches the tuning frequency (Fb). At Fb the port is providing most of the system output and also providing maximum suppression to driver cone excursion. Below Fb cone driver excursion increase rapidly and system output drops.

The relationship between port area and depth: Increasing the area of the port while holding depth constant raises Fb. Increasing depth while holding port area constant lowers Fb. So basically in regards to Fb, port area and port length have the opposite affect.

AFAIK an important benefit of increasing the area of the port is it reduces air velocity in the port to prevent chuffing. This relates to how much power the system can handle. A higher powered cab will require bigger ports to avoid chuffing. Let's keep in mind where talking about a cab to be used with a <135W amp. If you're not going to push the system super hard, the port size can be smaller.

Increasing port area to prevent chuffing means the port has to be made longer to prevent Fb from rising. If the area of the port becomes too large, it may need to be too long to actually fit in the cab. Also a bigger port takes up more internal volume of the cab, which in turn effects the frequency where the driver begins to cut off...or requires the cab to be made bigger.

Designing a cab is generally about choosing a preferred compromise rather than optimizing all parameters.

 

[BassnBoa]

Only from people with little to no experience and / or who don't understand the math.

D140s are still sought after in certain segments like some guitar players. If they're in good condition, sell them and buy some modern drivers that will work in the cab, have better power handling and less weight.

Or jump in the deep end and learn how to model and design your own - but that's a pretty big ask. You'll likely come out with a much better design for mess money in the long run, and places like diyaudio have tons of experienced and knowledgeable users that'll happily help you design something fairly basic like a BG cab.

Even easier would be to look at building an established design like one of greenboy's fEARfull cabs which come in a variety of configurations and are very highly rated. No design work involved, I think some flatpacks are available, but cut lists are so not to hard to build.

Repurposing old cabs has usually been a waste of time in my long experience. By the time you get around the constraints of using someone else's design, I could have had a new, better performing cab built from scratch.

I fully understand this point of view and appreciate the input. I dropped a few hundred bucks on the cab that turned to be bogus. I had the plywood so I figure I'll make a cab that hopefully works better than the bogus one and enjoy it for what it is at that point. I'll be out the tolex or paint but everything else I had or am repurposing. Ultimately I'm keeping my expectations realistic and just treating it as a fun project. If I fail, then I'll look into selling off the speakers for what I can get and move on.
One great thing about this has been in my looking around at desings is coming across Greenboy. I had never heard of the cabinets before but after looking through some of those greenboy threads, I'll be considering something like the fearful if I end up practicing/playing with a band again at some point.

 

[BassnBoa]

The driver and cab must be designed to work together. I highly suggest you either study how speakers works, or simply take Agedhorse's advice on the cab's internal volume and port design.

I am by no means and expert, but hopefully this explanation is close enough to help a bit .

The driver interacts with the internal volume of the cab and the port. The exact interaction is determined by both the driver's Thiele Small (T/S) characteristic and the characteristic of the cab to include internal volume and port dimensions.

AFAIK as the frequency goes lower, at some point the driver will begin to cut off. The port is tuned to start becoming active around this point. As the frequency continues lower, the port become more active, produces more sound, and provides more suppression to driver excursion. This continues until the system reaches the tuning frequency (Fb). At Fb the port is providing most of the system output and also providing maximum suppression to driver cone excursion. Below Fb cone driver excursion increase rapidly and system output drops.

The relationship between port area and depth: Increasing the area of the port while holding depth constant raises Fb. Increasing depth while holding port area constant lowers Fb. So basically in regards to Fb, port area and port length have the opposite affect.

AFAIK an important benefit of increasing the area of the port is it reduces air velocity in the port to prevent chuffing. This relates to how much power the system can handle. A higher powered cab will require bigger ports to avoid chuffing. Let's keep in mind where talking about a cab to be used with a <135W amp. If you're not going to push the system super hard, the port size can be smaller.

Increasing port area to prevent chuffing means the port has to be made longer to prevent Fb from rising. If the area of the port becomes too large, it may need to be too long to actually fit in the cab. Also a bigger port takes up more internal volume of the cab, which in turn effects the frequency where the driver begins to cut off...or requires the cab to be made bigger.

Designing a cab is generally about choosing a preferred compromise rather than optimizing all parameters.

This for sure helps me understand I'm in over my head in creating my own port channel haha. All jokes aside, it did help me build a better picture of the relationship between port/cab/speaker regarding port's function. I'll have to spend a lot more time researching to get a better grip on it, but in this case it sounds like since I'll be running a 85w tubed head, not pushing it so hard that I'll be running into chuffing issues, then I'm best off sticking with the 50 square inch baffle area per 6CF like agedhorse recommended. That's my plan going forward. It'll be a weekend project but I'll update when it's complete.

Thanks you all, for all the information and advice. I appreciate it

 

[agedhorse]

This for sure helps me understand I'm in over my head in creating my own port channel haha. All jokes aside, it did help me build a better picture of the relationship between port/cab/speaker regarding port's function. I'll have to spend a lot more time researching to get a better grip on it, but in this case it sounds like since I'll be running a 85w tubed head, not pushing it so hard that I'll be running into chuffing issues, then I'm best off sticking with the 50 square inch baffle area per 6CF like agedhorse recommended. That's my plan going forward. It'll be a weekend project but I'll update when it's complete.

Thanks you all, for all the information and advice. I appreciate it

it's 50 square inches of PORT area.

You are so far away from chuffing issues that you can stop worrying about this too. The cabinet will handle the full 400 watts without any chuffing issues at all.