In the first two parts of this series, we explored the fundamentals of sound transmission and its interaction with barriers, particularly in the context of firestop systems. Part 1 introduced ASTM E90, the standard test method for measuring airborne sound transmission loss (STL) across a barrier in a controlled laboratory environment, and how this STL data is then evaluated and transformed into Sound Transmission Class (STC), a single-number metric that summarizes a barrier’s ability to attenuate airborne sound, using the standard contour curve outlined in ASTM E413. Part 2 delved into the physical nature of sound, explaining how sound waves interact with solid barriers that break up the wave energy. Some energy is reflected, some is dissipated within the material through mechanisms like damping and vibration transforming sound energy into heat energy, and some is transmitted to the other side of the barrier at a reduced decibel level. And we learned that the difference in sound energy measured before and after the barrier equals the sound transmission loss that is quantified by ASTM E90 across a spectrum of frequencies spanning 5 octaves. In this third installment, we address the complex challenge of integrating firestop systems into barriers that must meet both fire-resistance and sound-attenuation requirements, ensuring safety and acoustical performance are achieved without compromise.
Ratings for Systems, not Products
Very often, it is mistakenly asked, what is the rating of your firestop product? And if you have ever attended a level 1 firestop training course, within the first 10 minutes you know the answer to that question. Of course, the firestop product is not rated. To illustrate the point as to why, consider that a product may be tested in a 1-hour wall penetration, but the very same product may be tested in a 4-hour floor penetration, sometimes at the same application thickness. Does the product carry a 1-hour rating? a 4-hour rating? The answer is neither, because performance is based on several interdependent elements working together, and ratings vary based on individual system performance. The product is only one part of that system. The answer is no different when someone asks what the STC of a firestop product is. Similar to the previous example, what if the product is tested in an STC 50 wall? What if it is tested in an STC 70 wall? You don’t build entire sound barriers out of firestop materials. You build the barrier, and test how the treated deviation in the barrier affects performance. Just as it is with firestop systems, the outcome is dependent on the interaction of multiple components that make up the system. As with firestop systems, the sound performance system carries the rating, not the product. This distinction is critical for designers and specifiers to understand, as it underscores the need to evaluate firestop systems holistically when addressing both fire and sound performance.
Designing for Dual Fire and Sound Performance
Designing barriers to meet both fire-resistance and STC requirements involves distinct but somewhat complementary approaches. In firestop design, the process begins with a barrier that meets the required fire-resistance rating (e.g., a 2-hour-rated wall per ASTM E119). When a penetration or joint breaches the rated barrier, a firestop system is installed to restore the original fire-resistance rating completely. The system must withstand the fire test duration equal to the barrier rating without failure (e.g., no flame passage before 2 hours for a 2-hour rated barrier). If the system fails prematurely, even if only by 1 minute, the firestop system is inadequate and must be improved upon and retested until it meets the barrier rating. That is the only choice the designer has because you cannot do anything to the barrier construction to bolster the fire-resistance performance of the penetration or joint.
Acoustical design, however, requires a nuanced approach due to the nature of sound transmission. The process starts with a barrier of a known STC rating, established through ASTM E90 testing. Penetrations and joints introduce flanking paths, which are alternative routes for sound to travel. This happens because the inclusion of the penetration or joint alters the original barrier and may incorporate deviations such as air gaps or even undamped vibrations that may travel through the penetrating item from one side of the barrier to the other. These can affect the acoustical performance of the system, often degrading the STC rating to a certain degree. Unlike firestop systems, where you must design the firestop to restore the exact fire-resistance rating of the barrier, acoustical design only needs to make accommodations to account for potential STC degradation. Herein lies the difference. Suppose a firestop system is installed in an STC 50 wall and results in an STC 47 assembly rating due to flanking losses. If the project requirement is an STC of 50, the designer has two options. Either they can select a firestop system that minimizes flanking paths to maintain the STC 50 rating, or they can design a barrier with a higher STC rating (e.g., STC 53) to compensate for the 3-point degradation. Either approach ensures the final assembly will meet the required STC value. The only thing the designer needs to consider, if the 2nd choice is chosen, is whether or not the barrier still meets the fire-resistance rating. Typically, any alterations made to the barrier to improve sound transmission performance will not detract from fire-resistance performance.
The Critical Role of Air Leakage Control
As discussed in Part 2 of this series, the most effective way to enhance STC performance is to minimize air leakage across the barrier, as sound waves travel efficiently through air gaps. This principle aligns with firestop design, where airtight seals are essential for smoke control, as tested per UL 1479’s air leakage (L-rating) test. Firestop products that create robust, airtight seals are critical for maintaining both STC and smoke performance. For example, SpecSeal® caulks, sealants, and sprays are designed to fill gaps and provide a hermetic seal in order to resist transmission of smoke. SpecSeal® SSP Firestop Putty and putty pads, being dense mastic materials, not only seal air gaps but also dampen vibrations from penetrating items, reducing flanking losses caused by vibrational energy transfer. Similarly, SpecSeal® Firestop Blocks provide effective sealing and vibration damping, making them suitable for both fire and sound applications. Another excellent option is SpecSeal® CID (Cast-In-Place) Devices, which incorporate water- and airtight seals, ensuring minimal air leakage. As well, SpecSeal® Composite Sheet paired with SpecSeal® sealants, especially when installed on both sides of a barrier, provide both fire-resistance and sound attenuation by trapping air space and providing airtightness.
Besides looking simply at products, the designer can improve STC performance by incorporating firestop systems with low L-ratings, as verified by UL 1479. These are particularly effective, as they demonstrate proven resistance to air flow. The L-rating, expressed in cubic feet per minute per square foot (cfm/ft²) at a specified pressure, quantifies a system’s air tightness which directly correlates with improved smoke migration protection which indirectly improves STC performance. When selecting firestop systems, designers should prioritize systems with low L-ratings to optimize both sound and smoke mitigation.
Mass Firestop Materials and Vibration Control for Sound Attenuation
Beyond sealing air gaps with mastic materials, certain firestop materials enhance STC performance by adding mass to the barrier, which reduces sound transmission by reflecting sound waves. SpecSeal® SSM Mortar, for example, contributes to sound attenuation by filling the opening around a penetration with the dense mortar and it also seals penetrations effectively. While the base barrier (e.g., gypsum board, concrete) primarily reflects sound energy due to its surface properties, the added mass of these firestop materials strengthens the overall system’s performance.
The choice of firestop material depends on the penetration type and barrier construction. For metallic pipes, which can transmit vibrations, mastic materials like putty or foam blocks are particularly effective at damping vibrational energy. For non-metallic penetrants, such as plastic pipes, sealants or cast-in devices will do well. If a collar or wrapstrip product is used with plastic pipe penetrations, they would need to be used in conjunction with a SpecSeal® sealant in accordance with the firestop design. Designers should consult ASTM E90 test reports that incorporate firestop materials in representative systems to verify their impact on STC performance in the intended assembly.
Optimizing Annular Space for Sound Control
Annular space, the gap between the penetrating item and the barrier, can play a critical role in STC performance. Larger annular spaces increase the potential for air leakage and flanking paths, potentially degrading the barrier’s STC rating. Firestop systems that minimize annular space, or firestop devices such as EZ Firestop® Grommets or smaller EZ Path® devices, provide smaller potential for STC degradation by reducing air gaps and by occupying a smaller footprint on a barrier surface. These devices have been shown to perform well in ASTM E90 tests due to their ability to limit flanking losses. When designing penetrations, minimizing the size of the opening is a practical strategy. Ultimately, specifiers should review UL 1479 and ASTM E90 tested systems to balance fire-resistance and acoustical requirements while minimizing annular space.
Practical Considerations and Best Practices
To achieve optimal fire and sound performance, consider the following best practices:
- Select Airtight Systems: Use firestop products like SpecSeal® caulks, sealants, putties, or Intumescent Foam Blocks that seal air gaps effectively, as verified by low L-ratings in UL 1479 tests.
- Dampen Vibrations: Choose materials that reduce flanking losses by damping vibrations, such as SpecSeal® SSP putty or Intumescent Foam Blocks, especially for metallic penetrants.
- Add Mass: Incorporate dense materials like SpecSeal® SSM Mortar to enhance the barrier’s sound attenuation properties by reflecting sound waves more effectively.
- Minimize Annular Space: Employ reduced annular spaces and smaller devices when possible and avoid point contact to minimize vibrational transfer from the penetrating item to the barrier frame.
- Consult Test Data: Review ASTM E90 and UL 1479 tested systems to ensure the selected firestop system meets project-specific STC and fire-resistance requirements.
Not all firestop applications require STC ratings, and not all sound barriers need fire-resistance ratings. However, when both are required—such as in multifamily housing, commercial buildings, or healthcare facilities—Specified Technologies offers a range of tested systems designed to meet these dual demands. For detailed performance data, consult STI’s technical resources, including UL 1479 and ASTM E90 tested systems, to select the appropriate system for your project.
Conclusion
Integrating firestop systems into sound-rated barriers requires a thorough understanding of both fire-resistance and acoustical principles. By prioritizing airtight seals, vibration damping, and mass addition, designers can ensure that firestop systems maintain the barrier’s STC rating while meeting fire-safety requirements. Specified Technologies provides a comprehensive portfolio of products and systems, backed by rigorous testing, to address these challenges. Whether you’re sealing a pipe penetration in a residential wall or a complex duct system in a commercial building, STI’s solutions offer the reliability and performance needed to contain fire, smoke, and sound effectively.
This article, by Tim Mattox, Senior Manager of Systems & Testing Development, originally appeared in the Spring Wrap-Up 2025 edition of The Burn.