Home / Technical Resources / multifamily housing

Resources

Noise Control With Concrete Masonry

  • August 2018

INTRODUCTION

Sound control is an important design consideration in most buildings. Sound control involves two important properties: sound transmission and sound absorption, as depicted in Figure 1. The International Building Code (IBC, refs. 1, 2) contains minimum requirements for sound transmission in certain situations (see Sound Transmission Class Ratings of Concrete Masonry Walls, TEK 13-01D, ref. 3). However, the IBC does not contain minimum requirements for sound absorption, although proper control of sound reflected back into the room is a very important design function in many buildings as well, such as concert halls, gymnasiums, places of assembly, rooms containing loud equipment.

Concrete masonry is an ideal noise control material for both properties: it can act as a barrier by diffusing incident noise over a wide range of frequencies; and it can be an effective sound absorption material for absorbing noise generated within a room. This TEK discusses the sound absorption and sound transmission properties of concrete masonry, and provides general design guidance to help provide a good acoustic environment.

Figure 1—Sound Attenuation Characteristics of Concrete Masonry

MAXIMIZING SOUND ABSORPTION

Sound absorption control involves minimizing sound reflection, so that the noise generated within the space is not echoed back into the space. Sound absorption is most important in applications like assembly areas or concert halls. The extent of control provided by a particular surface depends on that surface’s ability to absorb rather than reflect sound waves. This ability is estimated by the surface’s sound absorption coefficient: an indication of its sound absorbing efficiency. A surface which can theoretically absorb 100% of incident sound would have a sound absorption coefficient of 1. Similarly, a surface capable of absorbing 45% of incident sound has a sound absorption coefficient of 0.45.

Because the sound absorption coefficient typically varies with the frequency of the incident sound, the sound absorption coefficients measured at various frequencies are averaged together to produce an overall absorption coefficient. Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM C423 (ref. 4) prescribes the test method and calculations. Traditionally, sound absorption has been reported in terms of the noise reduction coefficient (NRC), determined by taking a mathematical average of the sound absorption coefficients obtained at frequencies of 250, 500, 1,000 and 2,000 Hertz. More recently, the Sound Absorption Average (SAA) has been added to ASTM C423. Although the SAA is very similar to NRC, it is determined by averaging the sound absorption coefficients obtained at the twelve one-third octave bands from 200 through 2,500 Hz. ASTM C423 requires that both NRC and SAA be reported. Experience in the concrete masonry industry has shown that the new SAA values and the old NRC values vary little and generally are within 1 or 2 percentage points of each other.

Sound absorption values depend primarily on the surface texture and porosity of the material under consideration. More porous and open-textured surfaces are able to absorb more sound and, hence, have a higher value. This is reflected in the concrete masonry NRC values listed in Table 1. Note that painting a concrete masonry wall closes small surface openings, and hence decreases the wall’s sound absorption value.

Table 1—Approximate Noise Reduction Coefficients

MINIMIZING SOUND TRANSMISSION

Sound insulation, as between dwelling units, is accomplished by designing walls to minimize sound transmission. For this purpose, effectiveness primarily depends on wall weight, rather than on surface texture. In general, the heavier a concrete masonry wall is, the more effectively it will block sound transmission.

The sound transmission class (STC) rating provides an indication of how effectively a given wall prevents sound transmission across a range of frequencies. STC ratings for concrete masonry walls are determined using Standard Method for Determining the Sound Transmission Class Rating for Masonry Walls, TMS 0302 (ref. 5). TEK 13-01D, Sound Transmission Class Ratings of Concrete Masonry Walls, contains a complete discussion on determining STC ratings, applicable building code requirements, as well as tabulated values for various concrete masonry walls.

WALL SELECTION

When used for noise control, concrete masonry walls should be evaluated for both surface texture and density. Sound transmission is reduced by using heavier walls, but may be increased by using units with a very open surface texture. Transmission loss characteristics of unpainted, open-textured units can be increased by plastering or painting, although this will also result in a corresponding reduction in the sound absorption (SAA or NRC) of the block.

In some cases, the designer may wish to use both the transmission loss and absorption properties of concrete masonry to advantage. For example, using open textured units in a cavity wall with back plastering on the inside face of one or both wythes provides sound absorption on both sides of the wall as well as sound transmission reduction. Another option for providing both effective sound absorption and sound transmission loss is the use of acoustical concrete masonry units, such as those shown in Figure 2. These units typically have an opening molded into the face shell, to allow sound energy to readily enter the masonry cells. The cells are designed to incorporate systems such as metal septa and/or fibrous fillers to dissipate the sound energy and minimize sound transmission.

Figure 2—Examples of Acoustical Concrete Masonry Units (viewed from bottom)

DESIGN AND CONSTRUCTION

Early in the design, a detailed noise survey should be conducted to determine the outside noise level and the anticipated background noise level in the various building areas. A building layout can then be developed which will help reduce noise transmission from one area to another. Effective sound control depends on proper layout and wall selection as well as good construction techniques.

Sound will be easily transmitted through any opening in a wall. An improperly fitted corridor door is a prime source of sound leakage, as well as openings around ducts, piping and electrical outlets which are improperly fitted or sealed. A crack just 0.007 in. (0.178 mm) wide along the top of a 12½ ft (3.8 m) wall will allow as much transmitted sound as a 1 in.² (645 mm²) hole. Hence, it is very important to seal all cracks, joints and gaps to maintain the acoustical integrity of the wall.

Building design and layout can also impact the building’s acoustic effectiveness. Planning early in the design process can help alleviate potential problem areas farther down the line. For example, individual spaces should be planned to minimize common walls whenever possible (see Figure 3), and to place adjacent spaced such that quieter areas (such as bedrooms) abut each other, and noisy areas (such as kitchens) abut similar noisy areas (see Figure 4).

When considering building layout, also note that sound travels most effectively in straight lines. Every time sound energy changes direction, some of it is absorbed and some diffused, hence reducing the amount that is transmitted. For example, Figure 5 shows that simply offsetting hallway doors can decrease the sound transmitted from one space to another through the doors. Separating windows will have a similar effect (see Figure 6).

Any wall penetration will potentially transmit sound. Therefore, plan to eliminate penetration whenever possible (see Figure 7). When unavoidable, partial wall penetrations such as electrical boxes should be completely sealed with joint sealant. Through-wall openings should be completely sealed, after first filling gaps with foam, cellulose fiber, glass fiber, ceramic fiber or mineral wool. See Sound Transmission Class Ratings of Concrete Masonry Walls, TEK 13-01D, for a more complete discussion of minimizing sound transmission through wall penetrations.

Finally, building heating and cooling ducts offer a potential noise pathway throughout a building. There are many ways to absorb or dissipate this noise, including acoustic linings and splitters to help break up and disperse the sound energy (see Figure 8). Any changes to the building’s ductwork will also potentially impact heating and cooling distribution. These effects should be considered during the HVAC system design.

Figure 3—Plan the Building Layout to Minimize Common Walls
Figure 4—Use Mirror Floor Plans
Figure 5—Offset Doors Opening onto the Same Hallway
Figure 6—Move Windows Farther Away from Common Wall
Figure 7—Minimize Wall Penetrations
Figure 8—Acoustic Controls for Heating and Cooling Ducts

REFERENCES

  1. 2003 International Building Code. International Code Council, 2003.
  2. 2006 International Building Code. International Code Council, 2006.
  3. Sound Transmission Class Ratings of Concrete Masonry Walls, TEK 13-01D. Concrete Masonry & Hardscapes Association, 2012.
  4. Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM C423-07. ASTM International, 2007.
  5. Standard Method for Determining the Sound Transmission Class Rating for Masonry Walls, TMS 0302-07. The Masonry Society, 2007.

TEK 13-02A, Revised 2007. CMHA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.

Balanced Design Fire Protection for Multifamily Housing

  • August 2018

INTRODUCTION

The application of balanced design principles is particularly important in multifamily residential buildings. The risk of fires in apartments and condominiums is high because of the numerous kitchens, furnaces, hot water heaters, and other causes of residential fires. In addition, there is always a potential for sleeping occupants, children, and others who may be at a disadvantage for evacuating quickly. Fire safety in multifamily housing requires an understanding of the hazards involved, so that both the potential for fire and the threat to life-safety during a fire are minimized. The purpose of balanced design is to protect life and property during a fire. Death and injury from fire is caused by asphyxiation from toxic smoke and fumes; burns from direct exposure to the fire; heart attacks caused by stress and exertion; and impact due to structural collapse, explosion and falls. Life safety in multifamily housing is influenced by the design of the building, its fire protection features, and by the materials, building contents, quality of construction, and maintenance.

BALANCED DESIGN

Balanced design relies on three complementary life safety systems to reduce the risk of death due to fire:

  • a detection system to warn occupants of a fire
  • a containment system to limit the extent of fire and smoke
  • an automatic suppression system to control the fire until it can be extinguished.

Each of these essential systems contributes to lowering the risk of death and injury from fire in multifamily housing. The three balanced design components complement each other by providing fire protection features that are not provided by the other components. Some features of each balanced design component are intended to be redundant, so that in the event that one system is breached, or fails to perform, the other components continue to provide safety.

Although not a tangible element in fire protection, a strong education program should be an integral part of any good fire protection plan in addition to the physical components of a balanced design system.

DETECTION

Accurate early warning is the first line of defense against the slow smoldering fire, typical in dwelling units with a low heat release rate that doesn’t activate a sprinkler head. Detectors that respond to light smoke are important from a life-safety standpoint to alert occupants so they can evacuate.

Other detection or alarm systems may be used to notify the fire department, thus decreasing response time, expediting rescue operations, and limiting the resulting fire spread and property damage. Detectors wired to a central alarm and installed in corridors and common areas notify all building occupants, permit timely and orderly evacuation, and decrease the potential for injury and death.

The National Fire Protection Association (NFPA) National Fire Alarm Code, NFPA 72 (ref. 3), covers minimum performance, location, installation, testing and maintenance of detectors. The Standard includes heat, smoke, flame and gas sensing fire detectors.

The most common detector used in multifamily housing is the smoke sensing fire detector. Detectors should be wired into a continuous power supply. Their location in multifamily housing is determined in accordance with the building code.

Each dwelling unit should be equipped with detectors in all sleeping rooms and in areas adjacent to all sleeping rooms, and on each level of the building, including the basement. The amount of air movement and obstructions within the space, such as partitions, ceiling height, and other factors, will guide the fire protection engineer in the proper selection of the detector location.

The performance of detectors is vulnerable to unpredictable malfunctions, among which are lack of maintenance due to human error and neglect, faulty power supply, and even acts of sabotage by arsonists. Young children, the disabled, elderly, and the deaf may not be able to respond to auditory alarms.

Tests on smoke detectors (ref. 4) indicate the need for a regularly scheduled maintenance and testing program, as well as the periodic replacement of some components.

SUPPRESSION

The function of automatic sprinkler systems is to control a fire at the point of origin. While not designed to extinguish a fire, residential sprinklers have been shown to be very reliable and effective in controlling a fire in the room of origin until it can be extinguished. Sprinklers reduce the likelihood of flashover. Flashover, the rapid ignition of volatile gasses, is particularly hazardous in exit corridors. Suppression of a fire allows access to the building to permit rescue and fire fighting efforts to proceed. Sprinklers are credited with preventing multiple deaths in fires.

The NFPA maintains minimum standards for the design and installation of sprinkler systems. Sprinkler systems for multifamily housing more than four stories in height are covered by NFPA Standard 13 (ref. 1). NFPA Standard 13R (ref. 2) covers the installation of sprinkler systems in multifamily housing up to and including four stories in height. When the interior construction or building contents contain a large amount of combustibles, sprinkler systems should meet the NFPA 13 Standard, regardless of height, to ensure protection in attics, closets, and other concealed spaces built with combustible materials, and to provide additional suppression in all areas due to the higher fuel loadings.

The NFPA Standards cover the design, installation, acceptance testing, and maintenance of sprinkler systems. In addition, there are specific spaces which are not required by NFPA 13R to be sprinklered. The Standards require an adequate water supply and a piping system designed to deliver sufficient water to the sprinkler head. Sprinkler head requirements ensure proper water coverage based on the room dimensions, area to be covered, and fuel loading. The Standards also list exceptions for specific spaces which are not required to be sprinklered. Once installation is complete, the Standards require inspection and acceptance of the system’s piping, valves, pumps and tanks. Testing also includes verification of adequate water flow to the sprinkler heads. After the sprinkler system is in use, it must be maintained; however, specific maintenance requirements and frequency of maintenance are not specified by the Standards.

Performance of automatic sprinklers can be vulnerable to system failures due to inadequate maintenance and inspection, or inadequate water supply. Sprinklers are not intended to control electrical and mechanical equipment fires or fires of external origin, such as fires from adjacent buildings, trash fires, and brush fires. Fires in concealed spaces, including some attics, closets, flues, shafts, ducts, and other spaces where sprinkler heads are not required to be installed, can compromise life safety due to the spread of toxic fumes and smoke. Inadequate water supply can occur due to low pressure in the municipal water system, broken pipes due to earthquakes or excavation equipment, explosions, freezing temperatures, closed valves due to human error, arson or vandalism, corrosion of valves, pump failure due to electrical outage, and lack of system maintenance.

COMPARTMENTATION

Compartmentation contains a fire until it can be brought under control by fire fighters as illustrated in Figure 1. Compartmentation limits the extent of fire by dividing a building into fire compartments enclosed by fire walls or fire separation wall assemblies, and by fire rated floors and ceilings. Compartments also minimize the spread of toxic fumes and smoke to adjacent areas of a building. Conflagrations beyond the fire compartment are prevented by limiting the total fuel load contributing to the fire. Compartmentation provides safe areas of refuge for handicapped, young, elderly, incapacitated and other occupants who may not be capable of unassisted evacuation. Compartmentation also provides safe areas of refuge for extended periods, where evacuation is precluded due to smoke filled exit ways or blocked exits. Compartmented construction provides protection for fire and rescue operations. Highly hazardous areas, such as mechanical, electrical or storage rooms, can be isolated by fire walls from other occupied areas of a building. Fire separation walls and floor/ceiling assemblies between dwelling units in multifamily housing afford protection from fires caused by the carelessness of other occupants.

Each compartment is enclosed by fire resistive components. Floor and wall elements forming the boundaries of each compartment should have a fire resistance rating of at least two hours, and should be constructed of noncombustible materials that are capable of preserving the structural integrity of the building throughout the duration of the fire.

Openings through compartment boundaries should be protected openings. Doors should be self closing when fire or smoke is detected. In multifamily housing, each dwelling unit should form a separate compartment. In addition, interior exit ways, as well as storage, electrical, and mechanical rooms, should be separate compartments. Exterior walls should be fire rated to form a barrier to the penetration of exterior fires and to contain interior fires.

The value of compartmentation could be reduced when joints between floors and walls, typically exterior curtain walls, or between walls and ceilings, are not properly fire-stopped. Damage caused by equipment, abuse, or the installation of utilities which are not properly sealed, can allow the passage of smoke and gas. Unsealed openings around penetrations can also allow the convective spread of smoke. Self-closing mechanisms on doors in compartment walls may fail if not maintained, or if blocked open.

PROPERTY PROTECTION

The initial cost of providing fire safety can be significant; however, balanced design offers advantages which offset initial costs. The higher level of protection for both the structure and its contents limits the potential loss due to fire. Immediate and long term savings will be reflected in lower insurance rates for both the building and its furnishings. Balanced design limits both fire and smoke damage of the building’s contents to the compartment of fire origin. Noncombustible compartment boundaries limit damage to the structure itself and reduce repair time following a fire. Repair is generally nonstructural, but may include the replacement of doors and windows; electrical outlets, switches and wiring; heating ducts and registers; and floor, wall and ceiling coverings.

SUMMARY

Fire protection engineering is as much an art as it is a science. The number of unknowns and potential fire propagation scenarios are numerous. Fire protection is, therefore, generally based more on risk assessment than precise calculation. Currently, building code prescriptive criteria, (ref. 5, 6) along with an understanding of the science of fire protection, guides the designer in addressing fire safety.

Some of the more significant fire safety issues requiring consideration are listed in Table 1, along with the effectiveness of each component of balanced design. As shown by the table, there may be more than one component which is considered effective in mitigating a particular hazard. Since none of the components are fail-safe, overlapping functions are needed to provide the required level of safety. In addition, there are some functions listed in the table which are addressed by only one component of balanced design. The appendix of NFPA Standard 13R (ref. 2) also recognizes the need for compartmentation and detection, along with sprinklers, to ensure a reasonable degree of life-safety protection.

There is a general agreement among the engineering and scientific communities that the desired approach to improving fire safety in buildings is through computer modeling. Limited success to date has been achieved in developing a system (ref. 7) that will assess risk and design protection levels for each of the three components of balanced design. Future efforts will be directed through the government and the private sector in a cooperative effort to develop this tool.

REFERENCES

  1. Installation of Sprinkler Systems, NFPA 13, National Fire Protection Association, 2007.
  2. Standard for the Installation of Sprinkler Systems in Residential Occupancies up to and Including Four Stories in Height, NFPA 13R, National Fire Protection Association, 2007.
  3. National Fire Alarm Code, NFPA 72 National Fire Protection Association, 2007.
  4. Why We Need to Test Smoke Detectors, Leon Cooper, Fire Journal, National Fire Protection Association, November 1986.
  5. Life Safety Code, NFPA 101, National Fire Protection Association, 2006.
  6. Standard on Types of Building Construction, NFPA 220, National Fire Protection Association, 2006.
  7. Fire Hazard Assessment Method, Hazard I, NIST Handbook No. 146, National Institute of Standards and Technology, U.S. Department of Commerce, 1989.