Originally Posted by stockmonkey2000
I would also like to use my riser as a trap but its hard to get any answers on this topic.
After I read the question laid out in this thread yesterday, I did an hour or so worth of searching. I agree with you entirely: there is not much conclusive in forums about this.
What I gleened from my reading is that people have a lot of trouble tuning resonators. Especially if you read at gearslutz, you'll find a number of people who have built tubes intended to work as Helmholz resonators and many of them have failed. It seems a lot of people conclude that the physics just doesn't play out to work in the real world. I don't personally buy that; I think they've just done a bad job tuning the thing. There are two or three things that I haven't figured out the impact of in designing a tuned resonator for bass trapping. I know that if you're looking at multiple opening or slits in your riser the formulae are different, but this was what I could find most readily.
Here's a conceptual description of how they should work: http://hyperphysics.phy-astr.gsu.edu...es/cavity.html
Here's what seems to be a good article with some important technical details and tips for construction: http://www.audioholics.com/education...onant-absorber
Some of the things people suggest matters, though I haven't quite worked out how, are cavity volume (because it's impractical to tune a resonator by changing the volume in most cases - unless you build it like a trombone I suppose), positioning (within a room), and the shape of it. Also, as an observation, the physics is clear that port length compensation must be carried out. If I remember correctly, you can find discussion of it at hyperphysics, and it amounts to adding about 73% of the port diameter to the actual length when calculating the functional length.
I would assume for bass, the larger volume the better to start with.
People suggest placing the absorber where the sound is loudest. I think that makes good sense.
It seems that shape and wadding or insulation both act to change the tuning of the absorber through it's Q. A Helmholtz resonator can have a very high Q, with an effective range of only 5 or 10 Hz, but if the shape is more angular or convoluted or if insulation is added the Q comes down rapidly. The maximum potency decreases and the bandwidth increases at the same time.
So it seems like if you want to use the space ocuppied by your riser for broadband trapping, you want as much free movement of air into it as you can get and you should just fill it loosely with fluffy pink insulation. Ethan Winer would probably endorse this approach, as he seems to always believe that more bass trapping is better. On the other hand, if you have one particular room mode you need to deal with, tuning a resonator is not for the faint of heart.
For my personal take-home message so far: If you build a riser with open sides or edges, it can be used as a bass trap just like a soffit or corner trap. On the other hand, for Helmholtz applications a riser is a foolish choice.
For recommendations to the OP based on my reading: I'd suggest opening the sides (the riser, in stair terms, not the tread) as much as possible, and leave the covering as thin as possible - definitely no carpet pad. If you fill it with insulation, it will tune the same way as a corner or wall trap - the larger and thicker the better.