OR: (emphasis mine)
I suspect that's a typo, and was meant to be "and" or "plus" or "in addition". Either that, or Rod was talking about only low levels of isolation without silencers, and moderate / high levels with silencers, but somehow that part wasn't made clear.
It's a simple thing to do a practical test and see how much isolation you get from a duct alone: Get fours sheets of 5/8 drywall and seal two of those onto each side of a doorway to completely close it off (take off the door first to make it easier). So you'll have two sheets of 5/8, then an air gap, then 2 sheets of 5/8. Play loud music on one side of that, and measure the levels on both sides with a sound level meter, to see how much isolation you are getting. Now cut a 6" diameter hole in both "leaves", and thread a piece of flex duct through both holes. Play loud music like before, and measure again, like before... How much isolation will you have now?
I think you probably don't even need to do this test in real life: you can simply imagine how well that will work...
there is the label "Provide Soffit"
That suggests that he is talking about using a soffit around the duct, to get some extra isolation. That's certainly better than just duct, but it would only be good for low to moderate isolation.
Just remember that high volumes and low velocities are your friends in this endeavor, and don't forget to maintain your room isolation in the process.
Yup. With silencers!
It appears that Rod Gervais feels that relatively long lined ducting at low velocities (along with a soffit enclosure possibly?) is sufficient, maybe Rod is designing with lower isolation requirements, I'm not sure.
That would be my take: he probably wasn't talking about studios that need high isolation. I'm wondering if some part of his explanation was lost in the editing process, that would have clarified this.
Thinking logically, if a hole is cut in my isolated wall assembly, a silencer provides greater mass than a thin steel duct... whether this affects much I don't know,
Correct, and yes it does make a big difference. In the above "thought experiment", instead of using flex duct, use ordinary round metal duct, and see if you believe that it would make any substantial difference. You could even add to the experiment a bit, by bringing that duct through the drywall, turning it through an angle of 90°, and running it alone the wall a few feet before terminating it in a registers. Now play drums on that room, and go listen in the other room. The sound of the drums will be getting through, loud and clear. If you then put a SOFFIT around that metal duct, yes, that would reduce the level, but there would still be plenty of sound getting through.
considering that there won't be much of a spring for an MSM system within the silencers themselves, and mass law isn't a lot of help here.
Right, but do consider that you have one silencer on the inner-leaf penetration, another on the outer-leaf penetration, and a short section of flexduct joining them. Now you have MSM again. The flex duct doesn't really count here, and you are back to the mass of the silencers on each side with the spring of the air gap between the leaves...
To be honest looking at it from the (albeit limited) knowledge I have when it comes to the physics, I can't really see how they isolate much at all.
They do that in several different ways. First is mass (which is why you make them heavy), second is damping (which is why they are lined with duct liner), third is impedance mismatch (sudden, large changes in cross section at both ends), fourth is baffles (more mass in the direct path of the sound), fifth is multiple turns (the air can go around turns, but the sound waves can't), fifth is path length attenuation (the sound must take a much longer path through the silencer than just the length of the box, due to the multiple turns), and there are some other minor things going on in there too. Plus, of course, the plain old MSM thing, from the silencer box mass on each leaf being separate buy the air spring in between the leaves.
If it's simply down to impedance mismatch within the silencers/between two of them, then the same could be achieved with differing sized lined ducting without the need for silencers at all.
Right, but it isn't!
Impedance is only one of several mechanisms that all play their part.
I'm using silencers though, I'll let you know how it turns out.
If you do it right, it will turn out great! And silent...
Rod Gervais is stating a requirement for silencers (baffles) to maintain isolation:
Yup! My bet is that something happened when the publisher was editing what he wrote for the book, and they maybe left out something, or changed the wording in such a way that the meaning was lost. I can't imaging Rod suggesting that a straight-through duct would provide high levels of isolation.
- Stuart -
I should have some more progress to post on my threads over the coming week so stay tuned!