Soundman2020 wrote:Assume that the mass of the I-beam is infinite, and use the normal 2-leaf MSM equations to find out out how big the gap needs to be to isolate the lowest frequency that you are concerned about.
Ok, so I'm having a hard time with the equation, and I might need a little help.
Here is what I am using for the data:
density of air: 1.225 Kg/m3
speed of sound: 343 M/s
surface density of 5/8" plywood: 590 Kg/m3
surface density of I-beam: 8000Kg/m3
depth of cavity: .127m (5")
Using these numbers, I came up with 46.37, but that is before multiplying the 1 over 2pi. After the 1 over 2pi, my answer was 7.38.
The first answer makes more sense than the second one, but I'm not sure if I'm doing it right.
That's the DENSITY of plywood, not the SURFACE density! Note that the units are kg / CUBIC meter in that one, whereas surface density is in kg / SQUARE meter.
Surface density of 5/8" plywood is more like 9 kg/m2.
So, I figured that a 5" cavity should bring it down to around 57Hz, and a 7" cavity down to 48Hz.
I was wondering if the same applies to the bottom section of the I-beam. If I choose to use the 7" cavity depth, I'll drop the verticals 7" away from either side of the I-beam, but do I need to also drop them 7" below the bottom of the I-beam?
Ok, so here's what I'm going to do to correct the I-beam problem.
I didn't include it in the sketch, but there will be a ladder frame going up to the floor joists on the left side as well.
