Please HELP. Clarifying Double Baffle Box design

[casaestudio]

I have been reading a lot on baffle design and I've seen almost anything regarding duct sizing, number of baffles to use... So much that I'm confused now, and I'm about to make my own baffles so I want to be completly sure I'm going in the right direction.

I would like to clarify duct sizing and cross sectional areas in my own design so I just made a drawing to explain how it goes:

WhatsApp Image 2017-03-03 at 10.51.58 PM.jpg

This drawing shows how the extraction part would work, the fresh air input is not shown but it's pretty much alike.

So, from the inline extractor on the outside there is 6in ductwork (cross sectional area of 28in2) going through the outer wall and going in the "outter baffle box". Inside the outer baffle box the cross sectional area is 56in2. The connection to the "inner baffle box" keeps the same cross sectional area of 56in2.
Once in the inner baffle box the cross sectional area is 56in2 and gets in the live room with that same cross sectional area. Would this be correct?

I hope somebody could answer me
Thanks to all.

[Soundman2020]

Every time there is a sudden change in cross section, either increasing or decreasing, you get an impedance mismatch, which is good! With large impedance mismatch, sound waves tend to "bounce back" at the interface point, phase inverted, and cancel themselves. So you should have changes in cross section at BOTH ends of your silencer box. Air only flows one way through the box, but sound could be going either way...

Also, the change in cross-section does not have to be exactly double (or exactly half). It can be more. In fact, if you have the space to have a larger cross-section inside the box, that's even better.

The only time where I do not reduce the cross section at the "exit" end of the box, is where it goes through the wall to the register. At that location, I try to increase the area again, if possible, or at least keep it the same, to ensure that the air is flowing as slowly as possible. But for all other entry and exit points on silencers, I increase or reduce the cross section as much as possible.

The connection to the "inner baffle box" keeps the same cross sectional area of 56in2.

You don't need to do that. Just go back to 6" duct for that connection.

Once in the inner baffle box the cross sectional area is 56in2 and gets in the live room with that same cross sectional area

If you have 6" duct in the cavity, then the area would once again jump from 28 to 56 (or greater) at the entry to that box. Then it would stay the same (or maybe increase again, if possible) at the exit to the register. That final increase doesn't need to be double: even a small increase can help, to a certain extent.

- Stuart -

[casaestudio]

Veeeery clear. Thanks so much for your time and answer, Stuart!

I'll redo the connections between baffles so I use 6' insulated round duct.

Big hug,
Niko