Canadian Funding Corp Concerned for Privacy and Well-Being of Residents
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Canadian Funding Corp knows that sound control in multi-unit residential construction is one of the key factors affecting occupant privacy and well-being. Insufficient sound isolation can lead to lawsuits against architects and builders. This is part one of a two part feature post on reducing the negative impacts of sound.
This CMHC better building information posted by Canadian Funding Corp. explains how sound moves through buildings, and how designers can control airborne, impact and flanking noise transmission. It suggests ways to control sound in wood-frame, multi-family buildings and best practices for acoustic isolation.
Architects must consider many interacting factors in designing a building, including structural integrity, fire safety, energy efficiency and noise control. Understanding and implementing sound control has a major impact on occupants’ comfort, concentration and happiness, not to mention neighbourly relations. Sound control is particularly important in multi-unit residential construction. This post focuses on wood-frame construction in many Canadian multi-unit residences and briefly discusses masonry party walls.
Canadian Funding Corp. finds it interesting that the rigid assemblies required to resist high wind loads or earthquakes make noise control difficult because they are pathways for sound vibrations. On the positive side, the sealing of penetrations to resist the spread of fire also blocks movement of certain kinds of sound. Some sound details reduce the spread of odours and pests and improve comfort and durability.
This post examines airborne sound, such as loud music and impact sound, such as the thud of a dropped object. It describes the hidden routes that sound vibrations follow, such as leaks and flanking paths.
The article summarizes information from the Best Practice Guide: Fire and Sound Control in Wood-Frame Multi- Family Buildings, published by Canada Mortgage and Housing Corporation (CMHC).1 This guide brings together years of research from numerous sources, including the National Research Council of Canada’s Institute for Research in Construction (NRCC/IRC) and the experience of acoustic consultants.
Sound transmission can be effectively controlled in multi-family buildings by isolating neighbouring units from one another with proper walls and floors between suites. An architect should select wall or floor assemblies in multi-family housing for both resistance to fire spread–the fire-resistance rating–and resistance to the transmission of sound vibrations.Three indices–sound transmission class (STC), impact insulation class (IIC), and outdoor-indoor transmission class (OITC)–help designers select the suitable assemblies.
However, choosing a good floor or wall assembly doesn’t guarantee acoustic privacy.To ensure the desired level of sound reduction, designers and builders must avoid any leaks and flanking paths, which are the gaps or short-circuits through which sound may travel.
Airborne Sound
Sources such as voices and music produce airborne sound. It can originate in neighbouring rooms or from outside from sirens, machinery or traffic. Heavy layers that are not solidly connected at any point and that are separated by the thickest cavity possible effectively attenuate airborne sound in wall and floor assemblies. Attenuation improves when the cavity is filled with sound-absorbing material, such as insulation.
The effectiveness of a wall or floor assembly in attenuating airborne sound transmission depends on the frequency of the sound. Most assemblies attenuate high-frequency sounds more effectively than low-frequency sounds.
An assembly’s sound transmission class (STC) is a single numerical rating derived from sound attenuations at various frequencies, which approximately describe a wall’s ability to attenuate the typical frequency content of speech. STC is a measure of the average noise reduction in decibels for speech- like sounds that pass through an assembly. An assembly with a high STC rating has good sound attenuation characteristics.Table 2 shows how the STC ratings for walls relate to their ability to attenuate different types of sound.
Impact Sound
Good STC ratings do not necessarily satisfy occupants. Mechanical vibrations and impacts, such as foot traffic, dropped objects or objects sliding across a hard floor, also cause sound to move through construction materials. Although the code does not actually specify levels for impact ratings, it is important that architects design to reduce impact noise effects.
A floor or wall being vibrated by direct mechanical contact or impact transmits impact sound.The vibration spreads along or through the wall or floor into the assembly and its cavities, ultimately becoming sound in adjoining spaces.
Floor vibrations can also be transmitted through the structure to walls and be re-radiated as airborne sound into adjoining spaces. Impact sounds on concrete slabs finished with a hard surface, such as ceramic tile, are often described as “clacking” or “tapping.” Most of such sound energy occurs at high frequencies. Impact sounds on lightweight joist floors–the result of low-frequency sound waves–are usually described as “booming” or “thudding.”
Although airborne and impact sound have some things in common, impact transmission is far more complicated to measure and control.The character and level of impact noise generated in a living space below depends on many factors, including the nature of the object striking the floor, the force of the blow, the rigidity of the floor assembly and the resilience of the floor covering. NRC is considering including impact noise requirements in future versions of the NBCC.
BARRIERS TO THE PASSAGE OF SOUND
This section describes the main, proven-effective techniques for reducing airborne, impact and flanking sound transmission. A sound barrier must be impervious to air, non-porous, solid and reasonably heavy–materials such as gypsum board, glass, plywood, OSB and concrete are commonly used to reduce sound transmission in buildings.
Heavy materials
The density of materials has a large affect on sound transmission. Heavier, denser, building materials tend to block sound better than lightweight building materials. A double-layer of gypsum wallboard or floor sheathing, for instance, provides better sound isolation than a single layer.
Because wood-frame construction is relatively light, a combination of techniques is required to control sound. An STC of 50 cannot be obtained with a single layer of gypsum board on each side of 89-mm wood studs because the combination of mass per unit area, cavity depth and stud stiffness is not sufficient to produce the required transmission loss. Doubling the mass of gypsum board on both sides of the stud wall increases the STC by about 9 dB.
Absorptive materials
The cavities inherent in wood-frame construction are a convenient space for absorptive material, such as fibrous insulation. Installing sound-absorbing material that fills at least two-thirds of the wall cavity can increase sound transmission loss by 8 to 10 dB if the surface layers are properly isolated.The insulation is only effective in absorbing sound waves within the cavity, so other measures are needed to reduce the sound passing through the framing.
Absorptive materials are normally quite porous.They interact with sound passing through them, converting the vibrations into heat. Absorptive materials are not sound barriers.They reduce sound energy in an enclosed cavity as sound repeatedly reflects between the enclosing surfaces and passes through the sound-absorbing material many times. Each pass-through causes a small decrease in energy, with the cumulative effect being significant. A single pass provides very little sound attenuation unless the absorbent material is very thick.Thus, adding a carpet or acoustic tiles directly to a surface will not significantly improve the sound insulation of the separation.
Canadian Funding Corp presents a BBC video on sound absorption.
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