One of the more confusing aspects of acoustics to our clients is the difference between “laboratory” and “field” ratings when it comes to partition design. For example, the International Building Code (IBC) distinguishes between the two for assemblies around dwelling units in a multi-family building: why the distinction and what does that mean for the designer of the building? Read on to learn more about the difference between lab and field ratings.
Although there are many different types of acoustical tests that can be performed in a laboratory or in the field, we’ll focus on one of the most common: sound transmission ratings. Laboratory ratings of a wall or floor/ceiling assembly are obtained by constructing the partition in an acoustical test lab and inserting it into an opening between two test rooms. The partition is carefully built and well-sealed within the opening. In essence, this is as controlled a situation as one can achieve. Field ratings on the other hand are obtained in situ, and the construction is not as well controlled as in a laboratory. As you can imagine, there are many factors that can determine the acoustical performance of a field built assembly.
IBC states that demising assemblies around dwelling units must carry a minimum laboratory rating of sound transmission class (STC) 50 and a minimum field rating of 45. This relaxation of the requirement helps to account for performance limitations imposed by field conditions, such as by flanking, offsets, and room finishes.
A quick internet search can turn up hundreds of acoustical laboratory test reports for a myriad of assemblies. You might be tempted to grab the first one that looks like it’s composed very similarly to a wall you have on one of your projects. Looks can be deceiving, however, and factors such as increased stud spacing, using light-gauge studs, or subbing in dense type-C gypsum board can all result in higher ratings. While you may believe the test report is sufficient evidence of code compliance, an acoustician will take the same report with a generous pinch of salt and make doubly sure it truly represents the wall you’re trying to build.
A field test can also be used to demonstrate code compliance and provides several distinct advantages:
• Since the assembly in question is usually tested in situ, flanking paths that could prove troublesome to predict are reflected in the test result.
• Comparisons of a pre-remediation field test can easily be compared to a post-remediation field test (“apples-to-apples”).
• Field tests can be performed on real-world assemblies that haven’t ever been tested in a laboratory; think one-of-kind historical floor build-ups or a wall assembly whose composition cannot be verified.
Don’t fall into the trap of thinking that just because you’ve found a lab test report stating that a simple, economical wall assembly will be code compliant that you’re made in the shade: we’ve unfortunately seen too many instances in which these types of assemblies fall well short of code compliance, requiring substantial remedial work. Instead, let us put our experience and vast library of tests to work to get you an assembly you can rely on.