Firestop and Sound Transmission Performance

Author: Joshua Close

Sound transmission and It’s Relation to Firestop

Building construction has evolved rapidly over the past decade. New materials have been developed, and existing materials have evolved, expanding the limits of design for architects and engineers. These modern buildings have a variety of occupants and uses, and sound transmission through wall and floor assemblies within these buildings becomes an important design consideration to accommodate occupant needs. Though various wall and floor designs are available with tested Sound Transmission Coefficient (STC) ratings, complexity arises when these assemblies are also required to maintain fire resistance ratings. Fire rated separations are often required between occupants, and the STC rating and fire rating of these assemblies are compromised when plumbing, electrical, and mechanical penetrations are present. Dynamic joints created between wall and floor assemblies also compromise these ratings. Firestop systems, which provide passive fire protection, are called upon to maintain the STC and fire ratings of the assemblies.

How STC Ratings Are Determined

 
Sound transmission testing consists of difficult test parameters used to determine material performance. The American Society of Testing and Materials (ASTM) developed several test standards to determine the acoustical properties of different products and materials. A frequently used standard is ASTM E90 Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. This standard outlines the method and procedures for measuring sound loss through assemblies and calculating the overall STC rating. Much like a fire test, the standard evaluates the performance of the whole system (including the assembly itself, joints, gaps, and penetrations within the assembly) to measure and determine the Sound Transmission Loss (STL) and the resulting Sound Transmission Classification (STC) rating. 

The test method defined by ASTM E90 uses two separate rooms that are divided by a partition, as shown in Figure 1. One of the rooms is designated as the source room and the other room is designated as the terminating room. The test specimen is installed within the dividing partition and the test apparatus is designed so that sound is only transmitted through the test specimen.

Figure 1: Typical ASTM E-90 Configuration

During this test, materials are evaluated to verify that they can maintain the desired rating of the assembly. When it comes to fire resistance ratings, if an assembly is required to maintain a 2-hour F-rating, then the materials as installed must be capable of maintaining that rating. The same is true when STC ratings are required. If a separation wall is designed to provide an STC rating of 50, then the complete assembly, including materials used to seal openings and joints, must be capable of maintaining the STC rating of 50. Note that an STC rating, like an F-rating, does not apply to the product itself, but is instead provided for a specific assembly condition in a specific wall or floor construction.

Determining Basis of Design for Fire and Acoustically Rated Assemblies

When evaluating the sound performance of any firestop material, it is recommended to understand the baseline* performance of the wall that the material is being tested in.

*Baseline wall performance is defined as the wall’s acoustical STC performance before a joint or penetration is introduced.
 
Looking for a single point STC value for a firestop material that matches the design intent of a particular wall assembly is often not possible, since the introduction of a proper dynamic joint system, for example, will result in a lower STC value than that wall’s baseline.

For example, if the product data sheet for a firestop sealant lists an STC performance of 56, and the wall specification is STC 56, this may seem to be a viable product for your given project. However, after evaluation of baseline data of the test assembly of the firestop sealant, you may find that the baseline value was much higher, for instance, STC 61. This firestop product allowed for a 5 STC point degradation of the baseline assembly, and only with this information can the true acoustical performance of a product be evaluated. Hilti has conducted these baseline STC tests for many firestop products which are used to protect openings around penetrations and to seal construction joints created between two assemblies.

Lab Testing Available for Hilti Firestop Products

Hilti has gone beyond single point ASTM E90 tests and has also tested products in various assembly configurations and construction types. Additional acoustical testing consisted of measuring the Sound Transmission Class for head-of-wall joint systems in accordance with ASTM E90. This evaluates the joint between gypsum framed wall assemblies and flat concrete deck floor/ceiling assemblies. One tested product example is CFS-TTS Top Track Seal, a preformed firestop, smoke, and sound seal. This solution is installed at head-of-wall joints, between the flat concrete slab and the top track and gypsum wall assembly.

The test program allowed for comparison of two main methods for filling head-of-wall joints: sealant and pre-formed devices. Testing found that both methods performed similarly and achieved parity with a sound-isolated wall. The results of the test have been summarized below in Table 1, the STC deviation reporting table. This data allows specifiers and contractors to evaluate the products performance compared to the baseline of the original assembly. Figures 2 and 3 show the test setup.

Table 1: STC Deviation Reporting Table

Figure 2: CFS-TTS Top Track Seal Test Setup 1

Figure 3: CFS-TTS Top Track Seal Test Setup 2

The full lab test report summary is available here.

Field Testing to Further Evaluate Sound Attenuation

 
Lab testing according to ASTM E90 can be used to evaluate product performance in laboratory conditions, but these conditions, in most cases, do not translate to real world jobsite conditions. To further understand CFS-TTS Top Track Seal’s acoustic performance, it was important to evaluate it under field conditions, when installed on an actual project. To accomplish this, Hilti contracted Veneklasen Associates, to complete field acoustical measurements of demising walls within the subject project (Figure 4) to evaluate the acoustical performance of the top-of-wall joint, as shown in Figure 5.

Figure 4: Field Acoustical Measurement Project Exterior

Figure 5: Field Acoustical Measurements of CFS-TTS Top Track Seal

The project used the Hilti CFS- TTS Firestop Top Track Seal as the top-of-wall closure, with no additional acoustical sealant. Veneklasen measured the field acoustical performance of guestroom demising wall assemblies using two methods to quantify sound transmission through the top-of wall detail: Normalize Noise Isolation Class (NNIC) ratings and sound intensity measurements. The report in Figure 6 documents their measurements, results, and evaluation: 

Table 2: Field Acoustic Test Summary Report – Acoustical Field Testing Results

The full Field Acoustic Test Summary report can be found here.

Field Acoustical Test Summary

 
The field acoustical tests concluded that:
·        The wall construction with the Hilti CFS-TTS closure provided sufficient sound attenuation to maintain the acoustical performance of the tested wall construction as the same assembly with a sealant closure for this project and QA/QC procedure.
·        The measured single-number ratings for the demising walls were consistent with the statistical performance for the same wall construction using acoustical sealant at the top-of-wall closure, based on the historical test results. 
·        The CFS-TTS closure provided consistent performance with the average performance for that wall type with a traditional sealant closure.

Learn More:

Learn more about achieving high performing construction joints in our upcoming webinar: FIRESTOP 102: JOINT DESIGN FOR FIRE AND SOUND CONSIDERATIONS

Learn more about Hilti’s prefabricated joint firestop solutions, designed as a faster, simpler, and no-mess solution to provide excellent fire, smoke, and acoustic ratings. Minimize installation errors and waste while saving money:
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