A contractor contacted me to test the sound transmission loss (STC value) at his newly built high-security conference room and confirm that the room's soundproofing meets the client’s requirements for STC 55. The test failed.
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The construction of the high-security conference room was completed without prior acoustic expert advice about room soundproofing. The contractor installed walls with STC 68. The assembly is listed in the Ontario Building Code as assembly no. S15a (Fig. 1).
Three identical walls were under test. One wall was an original, not affected by the construction.
Fig. 2 shows the space's layout. The green lines indicate the newly constructed walls and corresponding STC test results.
Figure 1 - Wall assembly
Figure 2 - Floor plan for the room soundproofing construction
The test results were STC 54, STC 47, and STC 28, all failing the project requirements of STC 55. The contractor was distraught; he thought he had built it for STC 68.
Soundproofing defects.
The contractor was unaware of the flanking noise paths and did not take the necessary steps to mitigate them. Most contractors make this mistake with room soundproofing, neglecting to consult an acoustic engineer before construction.
The defect causing STC 28 in room soundproofing
The wall has a door in it. Most standard doors, when perfectly sealed, have STC 30 to 35. No door is perfectly sealed. The door is the acoustically weakest part of the wall and dominates the test results, causing STC 28.
The defects causing STC 47 and 54
The conference room has a large rectangular duct in the plenum above its ceiling (dashed lines in Fig. 1). The duct penetrates both side walls, causing a large opening (Fig. 3). A steel rectangular duct has a low resistance to break-in noise, which exists on the other side of the wall as break-out noise. Additionally, the thermal insulation around the duct prevents the drywall from sealing correctly against the duct, further aggravating the problem.
Figure 3- Duct through the wall onto the room
Fixing the problem in the room soundproofing.
It is impossible to improve the door to achieve STC 55. The only solution is to replace it with a special acoustic door. This is a problem when the customer wishes to have all doors the same appearance.
Enclosing the duct in the plenum above the conference room in a drywall chase would mitigate the noise path through the duct openings in the walls.
Fixing a botched room soundproofing is always more expensive than doing it right the first time with the advice of an acoustic engineer.
