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Understanding Measurements and Ratings

When it comes to sound attenuation, particularly in multi-family residential applications, there are typically two areas of concern. The first is commonly known as "Airborne Noise", such as from stereos, televisions, human conversation and ringing phones. The level of airborne noise that passes through either a floor/ceiling or wall assembly at a specific location is identified by what is known as the Sound Transmission Class (STC).

The second type of sound transmission occurs due to "Impact Noise" transferring through a floor-ceiling assembly. Examples of this are footsteps, furniture movement or coins and other objects hitting the floor above. These sounds are annoying and disruptive to a unit owner below. To determine how well a floor/ceiling assembly inhibits the transfer of these types of sounds, acoustical tests are performed and the assembly is assigned an Impact Insulation Class (IIC) rating.

Hard floor surfaces such as tile, stone, marble, wood and engineered laminates have become increasingly popular in multi-family buildings. The problem is they are very percussive and impact noise has become a major concern. As a result, the International Building Code (IBC-2003) has mandated that all floor/ceiling assemblies between dwelling units have a minimum 50 IIC when tested in accordance with ASTM E492 or a minimum 45 FIIC (Field Impact Insulation Class) rating of not less than 45 when tested in accordance with ASTM E1007.

Testing Products vs. Floor/Ceiling Assemblies

The IIC rating is an actual measurement of how well all of the floor/ceiling components work together. Quite frequently sound mats are advertised as having IIC ratings as high as 70! The rating advertised is a result of not just the product but of a complete floor/ceiling assembly including the sound mat. The performance of that single material cannot be extrapolated from this number. You should not assume how effective that material will be in reducing impact sound solely based on this rating.

Laboratory vs. Field Tests

When determining IIC ratings for floor/ceiling assemblies, there are two different methods: in a laboratory environment or in the field. A lab test will produce an IIC (Impact Insulation Class) rating while a field test will produce an FIIC (Field Impact Insulation Class) rating.

Laboratory tests are conducted in a controlled environment. Field tests are conducted at the project site. Even in a laboratory, there can be wide range of acoustic results. The chart below illustrates how a wide array of IIC ratings can result for the same thickness concrete slabs.

Concrete Slab Thickness
No Sound Rated Ceiling
Sound Rated Suspended Ceiling*
In Lab IIC
In Field IIC
6"
X
 
26 to 30
24 to 32
8"
X
 
28 to 32
25 to 35
6"
 
X
45 to 52
33 to 48
*Suspended sound rated ceiling composed of 7" plenum, 3" of insulation, resilient channels, 5/8" Type X gypsum wallboard panels.
:

By conducting a certified field test, one can take into account any flanking paths (see picture below) and sound leaks which affect the amount of sound transfer. These are quite common due to uncaulked wall perimeters, ducts, piping, fixtures, electrical outlets, etc. As opposed to a laboratory test, these can only be accounted for in the field.

Table courtesy of The National Research Council Canada

Different Assemblies = Different Concerns

When testing, measurements are typically taken over a frequency range of 100 to 3150 Hz. The measured noise levels in 16 standard frequency bands spanning this range are then compared to a reference contour and the final IIC rating is produced.

Whether it is a concrete or a wood floor assembly, sounds transmitted at the high or low frequency can both be very disturbing.

When considering a concrete foundation it is at the higher frequencies that we must be concerned. (Figure 1). These types of sounds can best be described as "clicking" and "clacking" such as from a person walking with high heels.

One will notice that when looking at wood truss or joist assemblies' sound performance, it is at the lower frequencies below 250Hz or even quite frequently below the 100Hz level that the highest levels of sound are still being transmitted. (Figure 2) Noise at these "lower" levels can best be described as "thumping" or "thudding" such as a person walking in socks on a hard surface floor.

CONCRETE
 
WOOD
 
Figure 1: IIC contour fitted to impact sound pressure levels for 150-mm thick concrete slab. The cross-hatched areas at high frequencies show where the levels lie above the contour and determine IIC.   Figure 2 : IIC contour fitted to impact sound pressure levels for the basic wood joist floor. The cross-hatched areas at low and high frequencies determine IIC

Figures 1 and 2 courtesy of A.C.C. Warnock Controlling the Transmission of Impact Sound Through Floors - National Research Council Canada, Construction Technology Update No. 35, December 1999.

Suggestions for Further Reading

Reference: Warnock, A.C.C.and Birta, J.A. Summary Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission Class and Impact Insulation Class Results. Institute for Research in Construction, Council of Canada, Internal Report 766, April 1998.

 

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