The Science of Sound Part V: Field Testing & NIC

Sep
21
2017

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.

The specific test procedure is defined by  ASTM E336, while the NIC value is found  exactly the same way as STC, using ASTM E413, except the Noise Reduction figures are  plotted, rather than STL.

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.

 

 

 

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