Acoustically, these products act like other wood-frame constructions with bare assemblies yielding STC ratings in the mid-to-low 40s and IIC ratings in the mid-to-high 20s with no ceiling and topping slab. Unlike traditional wood-frame construction, where sound control is addressed on both the floor and ceiling side of the assembly, one of the primary drivers in selecting mass timber is aesthetics and a desire to leave the ceiling side exposed. In these instances, sound reduction must be addressed on the floor side of the assembly. Given the starting point of mass timber, achieving code minimum sound control for residential construction takes an exceptionally robust system. A recent testing program commissioned at a third-party, accredited lab was performed on five-ply CLT (175 mm [6.875 in.]) in a variety of configurations. It was found a 122-kg/m2 (25-psf) topping slab with a minimum 27-mm (1.13-in.) entangled mesh and foam sound mat is needed to meet the code when there is no dropped ceiling.
In 2018, IBC adopted new codes to allow for tall mass timber structures. Depending on the overall building height, structures over 26 m (85 ft) will require covering some mass timber surfaces with gypsum board or underlayment. These structures have the opportunity to offer a higher level of occupant comfort (STC and IIC in the upper 50s and lower 60s) through the addition of dropped ceiling and the minimum 19-mm (3/4-in.) gypsum topping slab.
Commercial buildings do not have the same acoustic isolation code requirements as multifamily structures. However, as construction types change, it is important to understand the acoustic environment of current office building construction. Most commercial buildings are built on steel structures with a concrete slab on deck as the floor and a dropped acoustic ceiling tile. That floor/ceiling assembly performs in the low- to mid-50s STC and, with carpet flooring, an IIC of more than 60. Replacing that assembly with an exposed CLT ceiling and a hard floor surface would have a very significant impact on the acoustical performance of a commercial space. In mass timber construction, it is possible to maintain occupant comfort by incorporating a sound mat and 25 to 50 mm (1 to 2 in.) of gypsum concrete. This improves the sound isolation performance to the low-STC 50s and mid-IIC 40s—more commensurate with current commercial buildings. It is not just about keeping the office workers comfortable, as productivity is strongly linked to working in a distraction-free environment; hearing raised voices and footfall noise from adjacent floors can be a serious distraction.
Residential renovations
As residential buildings built 30 (or more!) years ago are being refurbished, either individual units or the full building, carpet is being replaced with luxury vinyl tile (LVT), engineered wood, and hardwood flooring. Since carpet provides a high level of impact isolation, it is unlikely the acoustic performance can be replicated with a hard floor finish. The decrease in impact isolation means the same level of activity can be more disturbing to the neighbors downstairs and can make the difference between meeting the minimum code or condo association requirements and being non-compliant post-renovation. Topical mats designed to be used directly under finished flooring can increase IIC, but their thickness (and resulting isolation performance) is typically limited by the existing door heights and floor elevations. Even with a topical mat, hard floor finishes typically perform at 10 or more IIC points below carpet. Understanding the limitations of an existing system and planning for additional floor thickness before embarking on a renovation project can save the cost and headache of remediating a non-compliant system.
New measurement metrics
In order to present a fuller picture of the impact isolation performance variation between sound control systems and the floorcoverings, new measurement metrics are being developed. The single number metrics for sound isolation (STC and IIC) are a useful approximation of the performance across the full frequency spectrum. However, due to the limitations in measurement techniques, neither STC nor IIC take into account frequencies below 100 Hz (e.g. the lowest notes of a cello or tuba or the rumble of a movie explosion). Additionally, a soft floor surface like carpet can improve the impact isolation of the high-frequency ‘click-click’ of high heels, but is ineffective at reducing the low-frequency ‘thump-thump’ of heavy footfalls or dropped objects. The new metrics for impact isolation performance analyze the low- and high-frequency content of impact testing separately. Low-frequency impact rating (LIR) will use the measured impact levels in the 50, 63, and 80 Hz third-octave bands, and high-frequency impact rating (HIR) will utilize the levels in the third-octave bands between 400 and 3150 Hz. These new metrics provide us with another tool to better predict and ensure the comfort level of building occupants.
Conclusion
While the world of construction and design constantly changes, the needs of building occupants do not. Appropriate sound isolating construction will raise the value of the buildings by increasing resident satisfaction and worker productivity.
Evelyn Way is the lead acoustic scientist of acoustics with Maxxon Corporation’s Research and Design Lab. Prior to joining Maxxon, she spent 15 years as an acoustic consultant for a wide variety of building types including residential, educational, and performing arts facilities. She is a member of the Acoustical Society of America (ASA) and the Institute of Noise Control Engineering (INCE). Way can be reached at eway@maxxon.com.
