After a significant amount of time spent researching soundproofing solutions, one principle that kept coming up was the idea of a room within a room. This allows for extremely high levels of acoustic isolation by creating a Mass - Air - Mass system, with the air gap aiming to reduce the transference of resonance between the inner mass (the inner wall) and the outer mass (the outer wall), thus minimising the escape of sound.
I won't go into much detail about the physics of acoustics throughout this site as there is already plenty of information out there (this article by Sound on Sound is a great place to start). Having said this, the fundamental requirements for acoustic isolation can be summarised simply as follows:
- Mass
- Airtight construction
- Minimise flanking paths (i.e. physical connections between the inner and outer rooms)
The ideal way to achieve this would have been by creating a completely isolated inner skin using as dense a material as possible. Although an inner skin of blockwork would have been perfect in terms of mass, it would have been very difficult to isolate the two skins as they would have both sat on the same concrete slab. This, compounded with the difficulty of blockwork construction (for a complete construction novice), resulted in the choice of stud walls lined with plasterboard skin.
To maximise acoustic isolation, I decided to use two layers of acoustic grade plasterboard, with a compound called Green Glue sandwiched in between. This product creates a "constrained mass damping" layer, increasing the acoustic isolation by minimising the transmission of sound vibration between the two layers. It does this by transforming some of the incident sound energy into heat.
Thus the plan became to create a free standing inner stud-wall structure, isolated from the concrete slab by using some form of neoprene blocks, and complete with self supporting roof rafters that would need to bear the weight of two layers of acoustic grade plasterboard. The depth of the stud walls would be filled with insulation in order to minimise resonance in the air gap between the inner and outer structures.
In order to help visualise this construction (and being an engineer), I created "to scale" 3D CAD plans of the existing garage, as seen here:
The dimensions can then be seen below (in metres) in the plan view of the wall profiles, complete with the proposed positioning of the inner stud wall, shown as parallel lines. (Remember the wood shed section in the bottom right hand corner)
The proposed stud walls can then be seen in yellow in the view below, with the outer block wall hidden. The grey wall corresponds to the block wall that would need to be built to close up the existing double doorway and allow space for a smaller security door to be fitted.
Finally, the section view below shows both walls, and demonstrates the "room within a room" principle very effectively:
In order to effectively seal off the building, I decided to add a further two layers of plasterboard to the underside of the existing rafter structure, thus creating a new roof which would act as the lid of the blockwork (outer room) structure. This would be much more air-tight than the tiled roof, and would provide a much higher level of acoustic isolation, and can be seen in purple in the view below. I was concerned initially that doing so could actually be detrimental due to the dreaded "
Triple Leaf Effect", but decided on balance that the additional mass and air-tight 'ness of the "purple" roof would outweigh the negatives.
A front-facing cut-away view of the left hand side of the building shows the air gap between the proposed new roof of the outer structure (in purple), and the future roof of the inner stud wall structure (in green):
All the CAD was done using the free online CAD tool
OnShape, and although this is not an essential step, I would highly recommend it as it allows for a much better understanding of the space that will be created. Even just a simple plan view with dimensions annotated would be extremely helpful to be able to mark out services and key locations throughout the build.
