The Science of Sound Part VII: Beating Flanking Paths
Perhaps the most practical way to state the acoustical performance of movable partitions already installed is Noise Isolation Class (NIC). This is a number describing the performance of ALL building elements in isolating one room from another. It is found from the Noise Reduction (NR) figures of the same 16 frequencies used in the laboratory STL. The same steps are used in the measurement and calculation, except the test is done in the field, and no effort is made to measure or use the absorption in the calculation.
In this type of testing on the overall isolation of one room from another, so no attempt is made to measure flanking paths or the effect of room absorption on the results. In effect it tells what the users of the room will experience.
Caution: It is very difficult to transfer the NIC obtained at an existing installation to a new facility. Very seldom are two buildings identically designed, and almost never do the same workmen build them. There are often major differences in ductwork, return air plenums, floor levelness, wall plumbness, etc. The only sure way to guarantee the results of a new installation is to require a field test for NIC upon completion.
Also, one can expect NIC values to be 5 to 10 dB lower than the corresponding laboratory STC values for the same product.
WHY THE BIG DIFFERENCE BETWEEN LAB AND FIELD RATINGS?
STC is sort of like the EPA gas-mileage ratings for cars: you can use it for comparison but your actual mileage may vary.
That is because real-life buildings are not as well built as laboratories. The floors are not as level, the permanent walls are not as plumb, the joints are not as well sealed, the structure is not as heavy, etc. In a lab, test specimens tend to fit perfectly because the lab is built just for that purpose. Also, in the real-world, it’s not just about the operable wall as a product–it is about the operable wall as a system. For example, you might have a panel with a high STC rating but if there are gaps between the panels or the panels and the floor, then your STC rating isn’t going to matter because the system as a whole isn’t working together as it should.
WHAT TO LOOK FOR IN TESTS
When acoustical tests are performed in labs, they are conducted under strict ASTM standards. Only a handful of labs across the U.S. have received accreditation from the National Bureau of Standards under the National Voluntary Laboratory Accreditation Program (NVLAP). When requesting lab tests for STC and STL, you should look for a NVLAP-accredited lab or at the very least, one with an excellent reputation.
In most situations, the same sensitive equipment used in the lab can be carried to the job site for field testing. Some on-site preparation is necessary to determine the noise source location and proper microphone paths. To ensure a completely unbiased test, the procedure should be overseen by an independent acoustical consultant.