How to predict the total TL of a 2-leaf MSM system

[OttoPus]

For the final equation, it gets a bit more complicated! Alpha and Beta are the real and imaginary parts of the propagation coefficient for the porous absorption (insulation)) inside your wall, and k is the wave number.

Fortunately, there's an easier way; by making several assumptions about typical properties here, you can round out all those terms to the constant "6".

So the third equation would then be simply:

R = R1 + R2 + 6

Thank you!! Another equation to add to my formulary.

That's probably not what you wanted to hear (no pun intended!), but it's reality.

- Stuart -

I know, thanks for the thorough explanation.

Marco, I think Bert is referring to single-number TL ratings. If an equation tells you that your wall will have a TL of 58.73 dB, then that is not very meaningful, since it doesn't tell you much about individual frequencies. You need to look at the entire TL graph to get a more realistic idea. Tl is a hell of a lot more useful than STC in this aspect, but still deficient.
- Stuart -

I figured it out a second after I posted my reply

Personally, I don't think it is even possible to have a meaningful single-number rating system for isolation.

I agree it can't.
In my humble opinion, it would be meaningful only if the isolation curve was linear and with a known slope. In that case you could extract a value from any point of the curve just knowing the central pivot.

BUT isolation system don't show such a curve, so...

Marco

[bert Stoltenborg]

Over here they use spectral weightings (like the dB(A)-, STC- or RW-correction) for popmusic, housemusic, filmmusic, traffic noise, plane noise and rail noise.
But as these weighting spectra are fairly old (from the 80's and 90's) and for example music production echniques changes, I agree that it is best to calculate with 1/3 or 1 octave spectra when you're designing isolation.

[OttoPus]

For the final equation, it gets a bit more complicated! Alpha and Beta are the real and imaginary parts of the propagation coefficient for the porous absorption (insulation)) inside your wall, and k is the wave number.

Fortunately, there's an easier way; by making several assumptions about typical properties here, you can round out all those terms to the constant "6".

So the third equation would then be simply:

R = R1 + R2 + 6

Sorry if I sound like a broken record, but I have a question about that formula.

Given a wall reported by lab test data to show a TL of 38 dB. (R1 = 38dB)
Given a second wall built the same way. (R2 = 38dB)

By that formula, the R should be:
R = 38dB + 38dB + 6

Since R1 and R2 are TL, so expressed in dB, should I apply the log math? So that:

R = 41dB + 6

R = 47 db?

Is it correct?

[bert Stoltenborg]

Yes.

[OttoPus]

Thank you!