Two leaf wall inside an existing brick wall structure

Hello, I'm new here. and starting to think about my home studio, I'm reading through Rod Gervais' book and am at the wall construction chapter. He stresses the uses of the two-leaf wall and I fear I have a problem. I have the groundplan attached of the building I will be constructing. We're keeping 3 brick walls of an old barn, inside these walls our architect drew a second wall, with 30cm isolation to reach the desired isolationvalue (it's a passive house). So if I understand Rod Gervais' book this 3rd inside wall is lowering the acoustical isolation. Is there a way around this ?

So if I understand Rod Gervais' book this 3rd inside wall is lowering the acoustical isolation.

Yes. That is a 3-leaf wall. It will REDUCE isolation on the low frequency end of the spectrum.

Is there a way around this ?

Yes. Don't build it like that! ONLY build a two-leaf wall, ever. Never one leaf, never three leaf. There are several other issues with your design that need major attention. I'd suggest you spend a few months reading over the forum to learn the basic of acoustics and studio design before you go any further, and also look for a new contractor: the one you have clearly doesn't know much about acoustics!