Sounds About Right!
by Marcy Marro | February 1, 2022 12:00 am
Site Selection and Building Envelope Noise Control
In the early 1970s, research published by the Environmental Protection Agency’s former Office of Noise Abatement and Control determined that outdoor sound levels of 55 A-weighted decibels (dBA, a non-linear scale of relative loudness) or lower were a requisite for protecting “public health and welfare with an adequate margin of safety”. The World Health Organization confirmed the 55 dBA goal, stating that 40% of the population of European Union countries lived in areas that exceeded this goal; and worse, 20% lived in areas that exceeded 65 dBA (which is perceived as twice as loud). It’s amazing to think that the Romans identified this issue a thousand years before!
A development site with sound levels of 55 dBA or lower avoids the need to mitigate excessive noise and allows inclusion of outdoor amenity spaces for recreation and relaxation. These sites also support the design of natural and mixed-mode ventilation systems that enhance energy efficiency, occupant comfort, and resilience (passive survivability during power outage).
However, selection of a site with these ideal acoustic conditions is not always possible. Often, affordable, developable land is in areas impacted by the clamor of roadways, railways, airports or industry. Development of these site is possible but requires attention to thoughtful building envelope design and detailing, window system selection, and structural isolation. Before discussing these mitigations, let’s review standards for improved acoustic comfort:
- LEED provides design guidance for interior sound levels to promote improved acoustic conditions. Initially, LEED for Schools adopted the ANSI S12.60 goals for reducing exterior noise intrusion to classroom spaces, to ensure that students can hear teachers. Later, LEED for Healthcare adopted the Facility Guidelines Institute (FGI) guidelines requiring an assessment of site conditions for exterior noise concerns and mitigation of potentially intruding noise when present. Beyond these two specific project types, LEED v4.1 BD+C now includes a pilot credit that aims to protect the occupants of residential projects from exterior noise. Meeting these goals requires keeping outside noise out and interiors well isolated.
- WELL Building v2 includes Sound Feature S02 Maximum Noise Levels, which requires limiting background noise levels, including the intrusion of exterior noise. The criteria includes A-weighted sound levels, which represent human sensitivity to sound. Achieving an average interior sound level of 45 to 55 dBA (depending on the indoor activity category) earns one point, while three points can be earned for average interior sound levels between 35 and 45 dBA.
These rating systems provide important goals for aural comfort and encourage best practice acoustic design.
For sites with excess noise, control of airborne noise and ground-borne vibration starts with a building envelope that reduces exterior noise transmission, primarily through airtightness, porous insulation, mass, and noise-reducing windows. Thankfully, energy standards for building envelopes (airtightness and insulation) may also reduce sound transmission. Airtightness helps ensure that the sound, which travels through the air, does not have small cracks for entering the building.
Insulation materials that provide increased thermal resistance may not also provide acoustic benefits. Thicker applications of porous insulations (fiberglass, mineral wool, cellulose, etc.) are extremely effective at reducing sound transmission. However, rigid foam insulation boards often provide limited noise reduction benefits beyond their contribution to controlling air leakage.
Rainscreen cladding systems can create building envelopes with superior thermal and moisture performance, but because they are lightweight, they lack the mass required to block excess noise. For sites with adverse noise exposure, heavier cladding systems, such as masonry, are superior at quieting the din for those within. Alternatively, a double-stud exterior wall can provide both acoustic benefits and avoid thermal bridging, with less mass and embodied carbon.
Just as with envelope thermal performance, fenestrations are the weak point for exterior sound transmission. The double-pane (insulting glass units) windows meeting commercial fenestration standards for air leakage are a good start. Interestingly, triple-pane windows often perform only slightly better than double-pane windows, despite their increased thermal performance. Optimal acoustic upgrades for window systems (double- or triple-pane) involve using glass panes and/or airspaces of different thicknesses that resonate at different frequencies, reducing noise transmission. Laminated panes of glass can further enhance the acoustic performance.
The above strategies are focused on airborne noise mitigation. In some cases, developments occur adjacent to or overtop of railways and roadways (surface, underground or elevated). These transportation systems generate high levels of ground-borne vibration that introduces noise inside buildings through the foundation and building structure, bypassing building envelope mitigation. For this reason, it is critical to perform studies to assess the magnitude of the ground-borne noise and vibration to determine whether isolation of the building foundation or structure is needed.
Selecting a quiet site is the easiest way to ensure the acoustic comfort of occupants, but this is often not a choice. A timely, initial site assessment of the type and intensity of ambient noise and vibration is critical to identifying and integrating cost-effective sound isolation strategies to ensure the well-being and performance of occupants. Practical acoustic upgrades are readily available and compliment energy efficiency and moisture control strategies, and high-performance building programs are increasingly recognizing that these design enhancements help to improve indoor environments and make buildings comfortable and supportive of their programmatic function. And we’d say that sounds about right.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. Jeffery Fullerton, INCE Bd. Cert., LEED AP BD+C, is an acoustical consultant with 26 years of experience and a Department Manager with Intertek Building Science Solutions in Boston. To learn more, follow Scott on Twitter @alanscott_faia[1].
- @alanscott_faia: https://twitter.com/alanscott_faia
Source URL: https://www.metalarchitecture.com/articles/alan-scott-jeffery-fullerton-site-selection-noise-control/