Fibre fill and box tuning
 

When you know you'll be using fibre fill and expect a virtual box size approximately 25% larger, would you tune the port based on the virtual size or the actual size? I would appreciate expert answers only, as i'm well aware of mid level box design and techniques, but i've never experimented with this particular issue. Thanks in advance.

Example: 1 ft^3 ported box with 1lb of fibre fill= approx 1.25 ft^3 virtual volume.
so, would tuning be for 1 or 1.25 ft^3 box?

I've actually tried this a few times as i was curious as well. What i found was empirical as i don't have an rta or spl meter, but should be of some help to you. Basically, i built a 1 cube box tuned to approx 40hz using a 3"x10" port...i then listened carefully to test tones and marked the range it was loudest....this was approx 42-48 hz. I then stuffed the box with 1 lb of polyfill fiber and retested...this yielded a loudest range of approx. 37-42hz. It appeared that the stuffing caused the tuning to drop about 3-5 hz, so next i removed the stuffing, cut the port tube to reflect a 45 hz tuning or about 7.25 inches, re-stuffed and listened. What i found was that the loudest range was almost exactly the same as the original 1 cube box with the 40 hz tuning. My conclusion based on this is that for a specific tuning you should consider the virtual volume when designing the port...unfortunately the virtual volume is hard to know as it requires the box to be built and tested. Stuffing a box changes with volume so it's not a linear relationship. For boxes under 3.0 cubes, it appears that the fiberfill adds approx 25% virtual volume, so if this is used you'll get close to your target tuning. Since that test many years ago, I've always used this model to tune stuffed enclosures with predictable results and I've since run it on winisd as well, which seems to reflect my findings. hope that helps.