25 Jun Modular Buildings and Acoustics: 3 Key Principles
Compared to traditional construction, modular buildings offer shorter build times, less wasted material, and lower overhead. These benefits have popularized modular construction for residential and commercial development projects in recent years.
However, modular buildings also present unique acoustical challenges that, if overlooked, can substantially affect the amenity of the space. This post looks at three key acoustics principles: flanking transmission, mass-air-mass resonance, and structural transmission. Each plays a significant role in determining a building’s acoustical environment, which in turn influences the living and working comfort for end users.
Photos courtesy of Horizon North Logistics
What is a modular building?
It’s a series of pre-fabricated building components that are manufactured off site. Often, these components consist of fully-framed cubic units, such as private offices and sleeping areas. Once delivered, the modules “stack and stick” together on site.
After the units are aligned and pieced together, the layout of the internal building elements (such as walls, floors, and ceilings), is limited by the pre-constructed framing itself. The floor plans, wall cavity depths, distances between supporting beams, and connection details of common junctions (such as where demising walls and separating floors meet) restrict available options for acoustical treatment.
The way in which noise and vibration travels throughout modular buildings is very different than in conventionally constructed buildings. To ensure a comfortable acoustical environment for occupants, we recommend that modular building designers pay close attention to the following three factors:
Flanking (Indirect) Sound Transmission
Flanking transmission describes the transfer of sound between adjoining spaces through sound paths other than common separating partitions. The most typical flanking paths include floors/ceilings and their respective junctions with internal and external walls. Poor control from flanking can result in substantially poorer levels of sound isolation, which the revised version of the building code has recently addressed. In short, the acoustical performance of a partition will be compromised without sufficient attention to the construction of all peripheral elements.
To control internal flanking, it’s important to assess the external building envelope, mechanically decouple floors, walls, and ceilings as much as practicable, and identify potential weaknesses in the building services design. Often, additional layers of internal gypsum lining at the external walls, and, full-height demising partitions are required to control flanking noise within modular buildings.
To effectively control direct sound transmission between common partitions, consider wavelength. As low-pitched sounds (bass) have longer wave cycles (known as frequencies, which are measured in cycles per second or hertz [Hz]) than high-pitched sounds (treble), low-pitched sound events contain more energy. The more energy a sound event has, the more challenging it is to attenuate. Generally, building elements require sufficient mass and generous air cavities between layers to sufficiently mitigate low-frequency energy. When walls and floors, for example, do not provide sufficient density or air cavities, occupants can experience low-frequency disturbances.
Modular buildings, however, are often designed to include rigidly mounted internal boarding between studs and joists for structural integrity. Within walls, these systems are often called ‘shear’ walls. In these cases, shear walls become a series of small cavity partitions sandwiched together (as a triple-stud system), rather than two generously separated linings (as a double-stud system). The result? Air spring and coupled resonance. Residual sound energy transmitted through the first layer of a shear wall can resonate within the first stud cavity. The width of this cavity determines a natural frequency, at which the resonating sound contains most of its energy. Without mechanical decoupling of these wall layers, or, without extended air cavities and internal insulation to dampen this resonance, it is often transmitted into next air cavity, turning the entire partition system into a resonant spring. This concept also applies to party floor systems.
This acoustic weakness is called mass-air-mass resonance.
It is therefore important to dampen the energy transmitted into partition cavities before adding more rigid mass into a system. Simply adding rigid mass between separate studs in a party system has been known to reduce the overall sound transmission class (STC) rating of the whole system by 5 to 15 points in some cases. Therefore, where possible, internal shear layers should be minimized as much as practicable. Further control solutions can include mechanical decoupling of common separating partitions and their adjoining assemblies, and/or extending the depth of these (insulated) partitions.
When a building element, such as a floor, wall, or a supporting structural beam, receives vibrational excitation due to impacts (from footfall or mechanical operations), this vibrational energy is propagated into it. As modular buildings often require rigid connections on all sides of the pre-fabricated units, this design approach provides extended opportunities for horizontal and vertical vibration transfer. When walls and floors are excited, they can oscillate and act like large springs. Through these oscillations, structural transmission can be transduced into audible airborne noise within adjoining spaces.
Structural transmission control solutions can include the effective use of mechanical decoupling, vibration isolation, flooring finish underlay, minimizing partition oscillations (by stiffening elements), and use of resilient mounts.
While today’s modular homes and offices can be built faster and for less than conventional construction, they do introduce unique acoustical challenges. The good news is that with planning and a bit of help from an acoustical consultant, you can deal with these concerns early in the project.
At BKL, we have performed acoustical tests on modular homes and offices, and have worked closely with designers and architects to find solutions to issues like flanking transmission, mass-air-mass resonance, and structural transmission. If you want to learn more about acoustics and modular buildings, contact our office. We are available to provide comprehensive assessments of any modular project.
Written by David Stepanavicius