Which wall assembly would produce the highest STC rating?

The Sound Transmission Class (STC) rating of a partition is most significantly influenced by its mass, the depth of the air cavity, and the presence of sound-absorbing insulation. Option A specifies two layers of 5/8" [15 mm] gypsum board on each side of the studs. This double-layer construction provides substantially more mass than the single-layer assemblies in options B and C. According to the mass law principle in acoustics, doubling the mass of a partition can increase its sound insulation capability by approximately 5 STC points. While metal studs are less stiff and can offer better acoustic performance than wood studs in similar single-layer assemblies, the significant mass increase in option A will provide the highest overall STC rating.

B. This assembly has the least mass (single layer of 1/2" gypsum board) and the shallowest stud cavity (2-1/2"), resulting in a significantly lower STC rating compared to the other options.

C. Although this assembly uses a deeper stud and heavier gypsum board than option B, its single-layer construction provides far less mass than the double-layer assembly in option A.

1. Gypsum Association. (2021). GA-600-2021 Fire-Resistance and Sound Control Design Manual.

Reference for A: Assembly WP 1350 (2x4 wood studs

16" o.c.

3-1/2" mineral fiber insulation

two layers 5/8" Type X gypsum board each side) has a tested STC rating of 50-54. (p. 238)

Reference for B: Assembly WP 3321 (2-1/2" steel studs

16" o.c.

2-1/2" glass fiber insulation

one layer 1/2" Type X gypsum board each side) has a tested STC rating of 45-49. (p. 272)

Reference for C: Assembly WP 3510 (3-5/8" steel studs [comparable to 3-1/2"]

16" o.c.

3" glass fiber insulation

one layer 5/8" Type X gypsum board each side) has a tested STC rating of 45-49. (p. 278)

2. Egan

M. D. (2007). Architectural Acoustics. J. Ross Publishing.

General Principle: "The sound transmission loss of a barrier is related to its mass per unit area... For a single-leaf partition

the STC rating increases by about 5 for each doubling of the surface weight." (Chapter 10

p. 169)

Specific Examples: Table 10.2 shows that a wood stud wall with a single layer of 5/8" gypsum board and insulation has an STC of 39

while adding a second layer to each side increases the STC to 51. (p. 176)