Optimizing Acoustic Performance of Wood Buildings

[ Page 3 of 4 ]       
Sponsored by Think Wood

Acoustics and Green Building Standards

Green-building certifications such as LEED are beginning to incorporate acoustics more prominently. LEED v4 Building Design and Construction (BD+C) and Interior Design and Construction (ID+C) both address acoustics under the Indoor Environmental Quality (EQ): Acoustic Performance credit. This credit seeks to provide workspaces and classrooms that promote occupants’ well-being, productivity, and communications through effective acoustic design. One to two points are available under BD+C depending on whether it is applied to new construction, schools, data centers, warehouse and distribution centers, hospitality, or health care (1–2 points). Two points are available under ID+C Commercial Interiors or ID+C Hospitality. Some of the considerations, depending on the certification a project is seeking, are HVAC noise, sound transmission, speech privacy and sound isolation, background noise, acoustical finishes and site exterior noise, reverberation time requirements, sound reinforcement, and masking systems. Wood can contribute to optimal acoustic performance and can help projects earn points under the Indoor Environmental Quality category.

Source: CLT Handbook

Flanking path checklist.

California Green Building Standards has prescriptive mandatory acoustical control for some building elements. These include maintaining site noise levels at or above community noise equivalent level (CNEL) 65 dB within the noise contour of an airport as well as wall and roof-ceiling assemblies with a composite STC rating of at least 50 and exterior windows with a minimum STC of 40. There are also performance factors stating that wall and floor-ceiling assemblies separating tenant spaces and tenant spaces and public places shall have an STC of at least 40, and the wall and roof-ceiling assemblies should be constructed to provide an interior noise environment attributable to exterior sources that does not exceed an hourly equivalent noise level (Leq -1Hr) of 50 dBA (A-weighted decibels) in occupied areas during any hour of operation. There are also voluntary measures offered, such as that schools have a maximum background level of 45 dB (from HVAC noise).

Sound-Isolation Fundamentals

Acoustic design considers a number of factors, including building location and orientation, as well as the insulation or separation of noise-producing functions and building elements. According to the IRC, the most important factors affecting airborne and impact sound insulation of wall and floor/ceiling assemblies are:

  • Total weight per unit area: Heavier equates to better sound insulation, especially for low-frequency sound.
  • Stiffness: Stiffer equates to better sound insulation. It has been observed that very stiff wood-joisted floor/ceiling assemblies present greater low-frequency impact sound insulation.
  • Porosity: The less porous, the better the sound insulation.
  • Multi-layers with air space: The larger the airspace, the better the sound insulation.
  • Contacts between layers: The softer the contacts (such as neoprene isolators), the better the sound insulation.
  • Sound absorption: Sound-absorbing material in the air space of the cavity between layers is beneficial.
  • Floor surface hardness: The harder the surface, the poorer the impact sound insulation, especially with high-frequency impact sound.

Flanking Transmission

Flanking transmission is the sound along paths other than the direct path through the common wall or floor/ceiling assembly. Flanking always exists to some degree in buildings, allowing sound to bypass a wall or floor/ceiling assembly. Therefore, a high-performance floor/ceiling assembly or wall does not guarantee good isolation unless proper attention has been given to eliminating or minimizing flanking paths. The basics of flanking control are to seal the openings, to decouple the surfaces, and to discontinue the structural elements if it does not affect the structural safety and serviceability.

Typical flanking sound-transmission paths can include:

  • Above and through the ceiling (plenum) spaces
  • Through floor deck and floor joist space
  • Through windows and doors
  • Through fixtures and electrical outlets, light switches, telephone outlets, and recessed lighting fixtures
  • Shared structural building components, such as floor boards, floor joists, continuous drywall partitions, continuous concrete floors, and cement block walls
  • Perimeter joints at wall and floor, through wall and ceiling junctions
  • Through plumbing chases and joints between the wall and floor slab above or at the exterior wall juncture
  • Around the edges of partitions through the adjacent wall

Source: US CLT Handbook

Area of mass of some CLT elements for wall and floor applications.

Mass, Decoupling, and Air Space

A designer has numerous options for mitigating noise, each with its own advantages. Mass, decoupling, and air space are three fundamental building techniques for sound isolation. Sound travels through a wall by vibrating the walls. Increasing the mass of a building’s walls aids in sound isolation. Simply put, a heavier wall will not move as easily as a lighter wall. Therefore, a wall with greater mass will conduct less sound than a wall with less mass. Mass can be increased through multiple layers of material, whether gypsum wallboard, plywood, lightweight concrete, or CMU.

Sound can travel from one side of a wall to the other via the wall studs because wall studs act as a bridge for sound to vibrate both sides of the wall. Decoupling the wall studs restricts sound’s ability to pass from one side of a wall to the other side. Low-frequency decoupling can be achieved through resilient channels, acoustic clips, neoprene isolators, spring isolators, and independent studs. The advantages of resilient channels (STC 50) are that they are inexpensive and provide good acoustical performance for single-stud walls. But they are easily short-circuited acoustically and have a low load- carrying capacity. Acoustical clip (STC 55+) advantages are that they can be installed in retrofits and with fewer installation errors, they provide greater acoustical performance for a single-stud wall, particularly at low frequencies, and they have a higher load-carrying capacity. But acoustical clips are more expensive than resilient channels. High-frequency damping can be achieved with acoustical mat/underlayment.

While wall studs can increase the amount of noise transmitted between walls, air space in the wall assembly can mitigate noise transmission. That being said, a wood-finished building with wood studs is not as noisy as a complete steel or concrete structure. A staggered-stud (STC 55+) solution has a moderate cost and good acoustical performance, but fire rating can be problematic. Double-stud (STC 55+) solutions provide high acoustic performance, even at low frequencies. They are fire rated and have a high load-bearing capacity. However, double-stud construction results in loss of space.

Ceiling penetrations, soffits, outlet boxes, and other penetrations can be a source of acoustical breaches and should be considered as well.

Methods to Control Noise in Wood Building Systems

Wood is generally hard, flat, and smooth, making it inherently sound reflecting, with properties similar to a concrete slab system, so it is important to control noise and achieve optimal acoustics with wood building systems. The most basic acoustical detailing is to always ensure the wall or floor/ceiling cavity is insulated with batt insulation and sealed airtight, which can be achieved by floating a topping slab under the base of the gypsum board, by lapping the joints in the corner and taping, or with fire caulking/sealant. All holes, recessed light fixtures, plumbing penetrations, and outlet boxes should be sealed, because wherever air can flow, sound can travel.

Sound Insulation performance of bare CLT floors and walls. This table provides the measured STC and IIC values for some bare CLT walls and floors in various laboratories (reported by Gagnon and Kouyoumji, 2011) and the FSTC and FIIC values measured by FPInnovations on bare CLT floors and walls in CLT buildings (Hu, 2013a). Since flanking paths were possibly present for the field test results, the FSTC and FIIC presented provide an indication of the sound-insulating performance of bare CLT floors and walls in buildings.

Airborne sound can travel through wood systems for two reasons. First, the material may not have enough mass to mitigate airborne sound on its own, but also because it is composed of linear elements fastened together mechanically. Gaps can develop between laminations in mass timber elements, and any gap through which air can pass is also a path for airborne sound. Accordingly, wood without continuous plywood or OSB sheathing will not provide effective attenuation of airborne sound. Airborne sound also travels along the grooves on the surface of lumber, which can provide potential paths for airborne sound. Therefore it is important to pay special attention to interface details and apply acoustic sealants where necessary.

*Please refer to Chapter 9 of the CLT Handbook and Section 2.5 of the NLT Design and Construction Guide for additional guidance about detailing mass timber for optimal acoustics.

 

[ Page 3 of 4 ]       

Notice

Academies