Acoustics Basics Part 1: Sound Concepts - STC & IIC

AUTHOR:
Mike Raley, INCE Board Certified Acoustic Engineer

From the traffic outside, to the barking dog next door, to what sounds like a heard of elephants in the apartment upstairs, noise is all around us even when we’re in the supposed sanctuary of our own home. It doesn’t have to be that way. There are acoustic testing standards, guidelines, and regulations intended to make our world quieter. The basic test standards (STC and IIC) that we commonly use for apartments and condos have been around since the 1960’s, yet they are still often misunderstood and misused. In this blog post, we’ll review the first two basic acoustic test standards that apply to apartments and condos: STC and IIC.

Before getting into the first two standards and ratings themselves, here’s a quick overview of some basic sound concepts and how they relate to housing. When we talk about typical sounds in apartments, we can generally lump them into two categories: airborne sound and structure-borne sound.

BASIC CATEGORIES OF SOUND IN BUILDINGS:
1.)    AIRBORNE SOUND
Airborne sound encompasses sounds that begin in the air, things like TVs, stereos, barking dogs, people talking, and yelling kids. 
2.)    STRUCTURE-BORNE SOUND
Structure-borne sounds are those that start in the building structure, things like footsteps, dropped objects, cabinet door slams, and rolling objects (vacuum, suitcase, etc.). 
Ultimately, both types of sound travel through the air when they’re transmitted from a building surface (wall, ceiling, floor) to our ears, but it’s where they start that we use to make the distinction.



It’s important to understand the distinction between airborne and structure-borne sound, because the distinctions between the two determine which standards and guidelines apply,  which noise control measures make a difference, and which don’t. The first two acoustic test standards we’ll discuss are STC and IIC.

IIC (Measures Structure-Borne Sound Isolation)
IIC stands for Impact Insulation Class, and it measures the level of structure-borne sound isolation between two spaces. The test method is detailed and specific in an effort to produce reliable results and can be found in the ASTM E492 test standard. Here is an overview:

To conduct an IIC test, the following steps should be taken.
1.    Construct the assembly to be tested in the opening between the source (impact side) and receiving (listening side) rooms of the test lab
2.    Tap on the floor using a standardized tapping machine
3.    Measure the sound levels in the receiving room
4.    Use the measured sound levels to calculate the IIC rating using a special procedure


It is important to note that the “assembly” in Step 1 is the entire structure that is between two rooms. For instance, when we test a “floor-ceiling assembly” that would separate an apartment above from an apartment below, we construct the entire floor structure like the examples below.


All of the details in the assembly affect the results of the test and changing one element can have a huge effect on the overall IIC rating for the assembly.

Some factors that can affect the IIC rating of an assembly are:
1.    Mass of the assembly
2.    Stiffness of the assembly
3.    Presence of acoustic isolation/decoupling on the flooring side (typically an acoustical underlayment)
4.    Presence of acoustic isolation/decoupling on the ceiling side (typically resilient channel or isolation clips for wood structures, and wire hangers on concrete structures)
5.    Presence of batt insulation in the air cavity (when present) of the assembly

Examples of IIC ratings for some common assemblies with and without acoustic isolation at the floor and ceiling are shown below.


At Ecore, we’re often asked what the IIC rating is for one of our underlayments; but from the images above, it’s clear that the same flooring and underlayment can have widely varying IIC ratings depending on the rest of the structure. As an analogy, trying to give a single IIC rating to an underlayment is like saying I’m just as fast as Usain Bolt because I’m wearing the same shoes.

STC (Measure Airborne Sound Isolation)

STC stands for Sound Transmission Class, and it measures the level of airborne sound isolation between two spaces. The test method is detailed and specific in an effort to produce reliable results. Details are in the ASTM E90 test standard, but a brief overview is below.

To conduct an STC test, the following steps should be taken.
1.    Construct the assembly to be tested in the opening between the source (where sound is made) and receiving (where sound is heard) rooms of the test lab
2.    Play really loud sound in the source room
3.    Measure the sound levels in the source room
4.    Measure the sound levels in the receiving room
5.    Take the difference between the sound levels in the source room and the receiving room (this generates Transmission Loss values)
6.    Use the Transmission Loss values to calculate the STC rating using a special procedure


Similar to IIC ratings, STC ratings depend on all the details of the assembly. Like IIC ratings, STC ratings are affected by the mass and stiffness of the assembly, the presence of batt insulation in any air cavities, and the presence of acoustic isolation at the ceiling. However, unlike IIC ratings, STC ratings typically are not affected by the presence of acoustical underlayments, as seen by the assemblies and corresponding ratings given below.


SUMMARY
STC and IIC are the ratings most commonly used to specify and evaluate the level of sound isolation between spaces. In particular, STC and IIC ratings are included in the building code requirements for multi-family housing such as condos and apartments. Both ratings depend on the details of the floor/ceiling assembly being considered, things like the assembly’s mass, stiffness, and the presence of acoustic isolation. The IIC can be affected significantly by changes in flooring and underlayment, but STC ratings typically aren’t affected by flooring and underlayments.

When selecting flooring and underlayment for your particular assembly it is important to consider all the details of the assembly and to get acoustical test data for an assembly as close as possible to yours. Be wary of flooring and underlayments that publish ratings without any details of how they were tested. Remember, you will never be as fast as Usain Bolt just because you are wearing his shoes, and your assembly won’t be assured to get an IIC 70 just because you use an underlayment that tested that high on some unknown assembly.

For more information about acoustics, visit Ecore's Acoustic Page.