I didn't even know what you were talking about, then I found this gem of a website and did some reading. I learned a lot about soundproofing.

http://www.soundproofingcompany.com/soundproofing101/triple-leaf-effect/

A key point: "We know that an any cavity will resonate at some frequency, and poor sound isolation results. We can’t prevent resonance, but maybe we can MOVE the resonance point. Move it to a frequency that we don’t encounter very often. Such a frequency would be low, since low frequencies are less frequently encountered."

Half way down the page it discusses the triple leaf. "A triple leaf simply has three leaves, and therefore two air cavities. You may hear that a triple leaf is to be avoided at all costs. This is partly true. The real problem is that a triple leaf often has a small air cavity. A triple leaf wall that has two giant air cavities is not such a bad thing, but as we saw earlier in this article, a small air cavity can make things much worse. It is much better in most instances to remove the leaf in the middle to join the two smaller air cavities into one large one."

If you look at the diagrams it appears that block wall is a double leaf because there is an air gap inside. You can see the performance of the block wall near the bottom of that page. They also discuss the block wall in detail. "Bottom Line: For concrete walls and ceilings, bigger air cavities are better, but decoupling the mass is the real key. While lower frequencies still suffer, the great majority of the frequency spectrum is improved."

Thank you, Vith. In fact it is exactly because of that article that I asked the question. :) I wasn't sure whether the block wall counted as one leaf or two (given that it is sealed and somewhat hollow). I actually spoke to Ted at SoundproofingCompany today. EXTREMELY helpful and honest. In his opinion the block wall is a single leaf. . . and building the proposed wall in front of it will almost certainly be beneficial.

Sound transmission is reduced by mass, residency and absorption. You have taken care of the mass with the concrete masonry. You can add resiliency by attaching 1 or 2 layers of drywall on horizontal resilient channels, either the simple RC-1 type or the proprietary ones attached with resilient clips. Absorption can be added with a layer of fiberglass or mineral wool insulation in the cavity. Furring the channels out will allow thicker insulation but the insulation will only absorb sound that gets into the cavity and travels horizontally.

The ineffectiveness of triple leaf walls has been well documented for 50 years. Adding resilient system to the CMU is effectively a double leaf design. Adding isolated studs is effective if you have the space to waste but you should still use a resilient channel system, green glue or acoustic drywall for resiliency.

All of this is for nothing if the perimeter of the wall is not carefully sealed.

Ah good to know, thanks for sharing that it is considered single leaf.