z60611 wrote:

Eric_Desart:

1a) I have the formula for 1DOF with one wall infinite mass.

I've been meaning to see if one of my books answers these questions:

1b) What's the formula for 1DOF with two normal masses?

2) What's the formula for 2DOF with two normal masses?

1a) That's true but incomplete, because

1b) you

DO have the formula 1) for normal masses. You quoted him regularly yourself, but until now didn't see the relationship. Which is what indeed surpriced me until now (not only from you).

You very recently entered Brian's summary at Studiotips about walls:

http://forum.studiotips.com/viewtopic.p ... ght=#31547In that you find:

Brian Ravnaas wrote:

rule #9 - the decoupling point rule. Calculate the depth of your air cavity by taking 1.4*thickness of insulation + thickness of dead air space. convert to millimeters. take [(mass 1 + mass 2)/(mass1*mass2*depth)]^0.5*1900. When you get that figure, multiply by 1.4 and you will have an estimate of the frequency at which your wall de-couples. Remember, de-coupling will make it worse below that point unless the wall is very heavily damped.

This formula is entered in different groups on several occasions already. I think you find it in the NRC/IRC docs also.

I wonder why nobody ever made this link. MSM is MSM whether that relates to walls or paneltraps. Just try that formula and put in 1 extremely heavy mass (to simulte infinite mass) as argument.

Forget about the 1.4 correction factor here and you come to exact the same result as the standard panel trap formula.
I make my answer much more complicated than necessary just to trigger your reasoning here. You've seen this formula a lot already and the mechanism it represents is EXACTLY the same as the one valid for a panel trap.

Bob this is related to respect I feel for you and your searching mind, not playing with you, just triggering what you clearly have and are.

I know you're going to translate this now in pictures for others to explain this difference between both formulas.

This magnitude and linearity of this 1.4 factor is still a question mark with unclear answers (confirmed by Brian himself somewhere else). If Brian does know more recent stuff about this I'm interested.

It's in order to check/feel how this factor is integrated in that program that I asked Andrew for those WinFLAG numbers here.

In as far this 1.4 is valid for walls it also should be integrated in the paneltrap formulas because we're speaking about exactly the same principles.

This also shows the limitations of those standard published panel trap formulas. Besides several other effects it clearly ignores the

a) effect of the mostly lacking infinite mass backing and

b) the effect of the standard applied mineral wool (or whatever absorptive) filling.

2) The 2-DOF: I never found that one in the traditional acoustic books either.

There are still lots of things I don't put on the net.