Using Lab Tests Instead of Field Tests: Which Type of Test Applies?
Code considerations are important when making product selection decisions. For noise control, national building codes require that the design utilizes laboratory tests in order to pass plan check. For code minimums, field tests must perform within five points of 50 for both STC and IIC. It does not allow for field tests to be used for design purposes. Field test reports provided by material suppliers should be eliminated for plan checking. Of course, this elimination begs the question "why?" in anyone's mind. There is a compelling answer.
First, field tests have many more variables than a laboratory. Flanking paths are eliminated in a laboratory. Mostly though, the "other" factors that are adjusted for in the ASTM standard are much more stable in the lab. Those factors include:
1. Room size
2. Sound absorption of the room
3. Background noise
The measurement of sound in small rooms gets a positive adjustment to results. Using a room with significant absorption adjusts results negatively. A large amount of background noise doesn't mean that the test can't be conducted; it just means that the results need to take into account the outside noise factor. That means that small closets, galley kitchens and typical bathrooms – the areas where we use hard surfaces – might have a more positive result than a living room or bedroom. Therefore, testing during construction tries to make up for the excess noise in the background.
It is also not proper to ADD 5 points to a field test, since the code allows for a 5-point reduction from the laboratory test. Some lab tests are always lower than field tests, and there is no strong correlation that indicates that all lab tests are 5 points greater than field results on the same assembly. I’ve seen STC results in the lab that are 7 to 10 points better when tested in the field.
If you are rehabbing an old mill, it behooves you to field test the actual building, since none were built to a UL standard. Other than those types of projects, look for laboratory tests that are close to the design. Ask yourself a question about a submitted field test also – how many field tests did it take to come up with these large numbers? If there is only one field test provided, remember that the other 200 bathrooms might have been totally different. Look for similar assemblies with laboratory results to draw a good assessment of performance. If it looks too good to be true, it most likely isn’t what you’ll see in your project.
Sounds good, doesn’t it?