There is no real general ratio for 4th orders. There are too many variables involved in the design. 4th orders differ from other designs (even 6th) in that they naturally have a higher efficiency at a specific frequency. We all know that, but what many done realize is that regardless of the chamber volume, due to the on and off response of a 4th order, naturally, the vehicle makes the final determination in how it will sound. I will rarely design a 4th without the vehicle in mind. If someone asks for a basic design without vehicle calculations, I tend to draw myself to a more simplified physical form and guarantee proper output performance, such as that with tlines and BRs. SO, the 3:1, 1.5: etc all has its own effect on the performance, just as it should. Sometimes minimal, sometimes COMPLETELY different sounds can be heard. The most important part of the 4th order lies in the control of the driver. As mentioned above, using a sealed box recommendation volume for the sealed area, then another mentioned using a standard ported calculation for the ported section......these are great ideas to follow if you do not figure for every factor and just want to design something from simplistic means of calculation. I personally do not use a general ratio. I let the sub tell me what it wants. Without regard to dimensional space limitations, I also do not trend the "port is to big" quote either, as the port cannot ever be to big, but we ARE limited on space, so that does put some limitations on that as well. But I always go with an optimum port area UNLESS I do not have the room to use it, then go with my calculated minimum area and use a larger compression volume. But in doing this, again you maintain control. Control is key in 4th orders, and without knowing a specific ratio for your subs, its a toss up. There are many people that will design and build many of these time after time to get it right, and those will argue with me that sometimes that works best. But to not waste your time and money, you have to involve all aspects of the design to come up with any volumes to utilize effectively from the start in these designs.
The best idea that was given thus far was whoever said use the conventional ported recommended specs for the ported side. This will ensure at the least that some control is given BEFORE the sealed side is even figured in. The sealed side is, and can be actually tougher to design sometimes as the cross-sectional area may limit the depth of the sealed area for excursion capabilities based on a specific volume, so I guess as mentioned, the recommended sealed volume can be utilized for this part, but I would say try to stay UNDER that at the most. I believe that is what was said from the other member as well. Two of the best, "generalized" comments on 4th orders you can use thus far. That all considered, you should be very close to having a great response with those two design ideas considered within the same design.
Just remember, you will never be able to make the box too big as long as all aspects of the design are done right. You can however make it too small, so make sure to give enough room for it, or output will suffer greatly even if you never mechanically blow the sub(s). Mechanical limits may force some to exceed thermal ones without knowing it. This is why I would never design a BP without proper calculation, but if you want generalized input from me, I would go with what the other two member mentioned that I recited here. Hope that helps
