by Josh Jonsson, CSI
The sound transmission class (STC) and impact insulation class (IIC) are ASTM-derived single number ratings that try to quantify how much sound a stopped by partition being tested.
Laboratory testing involves an ideal setting for the floor/ceiling assembly—it is isolated from the walls, and there are no penetrations for HVAC, plumbing lines, sprinklers, can lights, or electrical boxes. In the field (i.e. F-STC and F-IIC), the floor/ceiling assembly often sits on load-bearing walls, is connected to the structure, and contains many ceiling and floor penetrations for the items just mentioned. Consequently, the code allows for a lower rating for field scores over those in the lab.
The STC rating essentially tells how much noise is stopped from going through a wall. The test involves blasting loud noise at all the measured frequencies in a room. A Level 1 sound meter measures this exact noise in that room level at all frequencies, as well as the sound in the room on the other side of the partition. These two different levels are then essentially subtracted from each other, with some corrections made for background noise.
The IIC rating is not a comparative test like the STC. Rather, it uses an ASTM-specified tapping machine that sits directly on the floor—more specifically, directly atop the finished floorcovering. (Consequently, an IIC rating always lists the floorcovering with which it was tested.)
The machine has five steel hammers that spin on a cam shaft, falling onto the floor from the same height, no matter what or who is testing. These hammers put a consistent energy into the floor. The sound level meter is taken downstairs below the tapping machine and the sound level is measured at all the frequencies called out in the ASTM standard. These sound levels are plugged into the equations in the standard; a single number is generated summarizing how much sound was stopped.
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