For a room that requires soundproofing to an external noise source, which option would be least effective?

The least effective option for soundproofing against an external noise source is the 9" Concrete Hollow Block (CHB) wall. When evaluating soundproofing materials, the density, mass, and structure of the wall material are critical factors. A CHB wall typically has air gaps that can allow sound to travel through more easily compared to solid materials.

In contrast, the other options present more solid constructions with greater mass, which effectively dampens sound transmission. For instance, a 12" double brick wall offers the highest mass and density, providing superior sound isolation. Similarly, the 9" brick wall is solid and performs well, while the 7" concrete wall, while potentially less thick, still maintains the mass necessary to reflect sound effectively more so than a hollow block.

In soundproofing, the effectiveness is often related to the continuous mass and lack of air gaps, making solid brick and concrete walls more reliable for reducing external noise than those constructed with hollow block materials.