The CMHA Concrete Masonry Sound and Assemblies Properties Calculator is a spreadsheet-based calculator tool to aid in determining a variety of properties of concrete masonry units. The calculator can be used to determine section properties such as net area, moment of inertia, radius of gyration and more based on user-defined inputs. Wall weights are also calculated based on units, grouting schedule, and more. In addition. the calculator can determine Sound Transmission Class (STC) and Outdoor-Indoor Transmission Class (OITC) based on wall weight using the methodology contained in TMS 302, Standard Method for Determining the Sound Transmission Class Rating for Masonry Walls.
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
2003 International Building Code. International Code Council, 2003.
2006 International Building Code. International Code Council, 2006.
Sound Transmission Class Ratings of Concrete Masonry Walls, TEK 13-01D. Concrete Masonry & Hardscapes Association, 2012.
Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM C423-07. ASTM International, 2007.
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.
Unwanted noise can be a major distraction, whether at school, work or home. Concrete masonry walls are often used for their ability to isolate and dissipate noise. Concrete masonry offers excellent noise control in two ways. First, it effectively blocks airborne sound transmission over a wide range of frequencies. Second, concrete masonry effectively absorbs noise, thereby diminishing noise intensity. Because of these abilities, concrete masonry has been used successfully in applications ranging from party walls to hotel separation walls, and even highway sound barriers.
Sound is caused by vibrations transmitted through air or other mediums, and is characterized by its frequency and intensity. Frequency (the number of vibrations or cycles per second) is measured in hertz (Hz). Intensity is measured in decibels (dB), a relative logarithmic intensity scale. For each 20 dB increase in sound there is a corresponding tenfold increase in pressure.
This logarithmic scale is particularly appropriate for sound because the perception of sound by the human ear is also logarithmic. For example, a 10 dB sound level increase is perceived by the ear as a doubling of the loudness.
The speed of sound through a particular medium, such as a party wall, depends on both the density and stiffness of the medium. All solid materials have a natural frequency of vibration. If the natural frequency of a solid is at or near the frequency of the sound which strikes it, the solid will vibrate in sympathy with the sound, which will be regenerated on the opposite side. The effect is especially noticeable in walls or partitions that are light, thin or flexible. Conversely, the vibration is effectively stopped if the partition is heavy and rigid, as is the case with concrete masonry walls. In this case, the natural frequency of vibration is relatively low, so only sounds of low frequency will cause sympathetic vibration. Because of its mass (and resulting inertia) and rigidity, concrete masonry is especially effective at reducing sound transmission.
DETERMINING SOUND TRANSMISSION CLASS (STC) FOR CONCRETE MASONRY
Sound transmission class (STC) provides an estimate of the acoustic performance of a wall in certain common airborne sound insulation applications.
The STC of a wall is determined by comparing sound transmission loss (STL) values at various frequencies to a standard contour. STL is the decrease or attenuation in sound energy, in dB, of airborne sound as it passes through a wall. In general, the STL of a concrete masonry wall increases with increasing frequency of the sound.
Many sound transmission loss tests have been performed on various concrete masonry walls. These tests have indicated a direct relationship between wall weight and the resulting STC—heavier concrete masonry walls have higher STC ratings. A wide variety of STC ratings is available with concrete masonry construction, depending on wall weight, wall construction and finishes.
In the absence of test data, standard calculation methods exist, which tend to be conservative. Standard Method for Determining Sound Transmission Ratings for Masonry Walls, TMS 0302 (ref. 1), contains procedures for determining STC values of concrete masonry walls. According to the standard, STC can be determined by field or laboratory testing in accordance with standard test methods or by calculation. The calculation in TMS 0302 is based on a best-fit relationship between concrete masonry wall weight and STC based on a wide range of test results in accordance with the following:
Equation 1 is applicable to uncoated fine- or medium- textured concrete masonry and to coated coarse-textured concrete masonry. Because coarse-textured units may allow airborne sound to enter the wall, they require a surface treatment to seal at least one side of the wall. At least one coat of acrylic latex, alkyd or cement-based paint, or plaster are specifically called out in TMS 0302, although other coatings that effectively seal the surface are also acceptable. One example is a layer of drywall with sealed penetrations, as shown in Figure 2. Architectural concrete masonry units are considered sealed without surface treatment for the purposes of using Equation 1.
Equation 1 also assumes the following:
walls have a thickness of 3 in. (76 mm) or greater,
hollow units are laid with face shell mortar bedding, with mortar joints the full thickness of the face shell,
solid units are fully mortar bedded, and
all holes, cracks and voids in the masonry that are intended to be filled with mortar are solidly filled.
Calculated values of STC are listed in Table 1.
Because the best-fit equation is based solely on wall weight, the calculation tends to underestimate the STC of masonry walls that incorporate dead air spaces, which contribute to sound attenuation. See the following section for the effect of drywall with furring spaces on STC.
For multi-wythe walls where both wythes are concrete masonry, the weight of both wythes is used in Equation 1 to determine STC. For multi-wythe walls having both concrete masonry and clay brick wythes, however, a different procedure must be used, because concrete and clay masonry have different acoustical properties. In this case, Equation 2, representing a best-fit relationship for clay masonry, must also be used. To determine a single STC for the wall system, first calculate the STC using both Equations 1 and 2, based on the combined weight of both wythes, then linearly interpolate between the two resulting STC ratings based on the relative weights of the wythes. Equation 2 is the STC equation for clay masonry (ref. 1):
For example, consider a masonry cavity wall with an 8-in. (203-mm) concrete masonry backup wythe (W = 33 psf, 161 kg/m²) and a 4-in. (102-mm) clay brick veneer (W = 38 psf, 186 kg/m²).
The installed weight of concrete masonry assemblies can be determined in accordance with CMU-TEC-002-23 (ref. 10). When STC tests are performed, the TMS 0302 requires the testing to be in accordance with ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements (ref. 2) for laboratory testing or ASTM E413, Standard Classification for Rating Sound Insulation (ref. 3) for field testing.
CONTRIBUTION OF DRYWALL
Drywall attached directly to the surface of a concrete masonry wall has very little effect on sound attenuation other than the same benefit as sealing the surface. Adding ½ or ⅝ in. (13 or 16 mm) gypsum wall board to one side of the wall with an unfilled furring space will generally result in a slight increase in STC. However, when placed on both sides of the wall with a furring space of less than 0.8 in. (19 mm) a reduction in STC is realized due to mass-air-mass resonance similar to the action of drum. Better results are realized when the furring space is filled with sound insulation. Sound insulation consists of fibrous materials, such as cellulose fiber, glass fiber or rock wool insulation, are good materials for absorbing sound; closed-cell materials, such as expanded polystyrene, are not, as they do not significantly absorb sound (refs. 1, 7). Note that most of these materials are susceptible to moisture so care must be taken when applying these types of insulation to exterior walls.
Equations to determine the change in STC when adding drywall are as follows (Table 2 lists calculated values of ΔSTC based on Equations 3 through 6):
For drywall on one side of the wall with no sound absorbing material in the furring space:
For drywall on both sides of the wall and no sound absorbing material in the furring spaces:
For drywall on one side of the wall with sound absorbing material in the furring space:
For drywall on both sides of the wall and sound absorbing material in the furring spaces:
In addition to this TEK, CMHA has generated a calculator for determining the sound transmission class (STC) of a user defined assembly. See CMU-XLS-003-19, CMU Sound and Assemblies Properties Calculator (ref. 8).
BUILDING CODE REQUIREMENTS
The International Building Code (ref. 4) contains requirements to regulate sound transmission through interior partitions separating adjacent dwelling units and separating dwelling units from adjacent public areas, such as hallways, corridors, stairs or service areas. Partitions serving the above purposes must have a sound transmission class of at least 50 dB for airborne noise when tested in accordance with ASTM E90. If field tested, an STC of 45 must be achieved. In addition, penetrations and openings in these partitions must be sealed, lined or otherwise treated to maintain the STC. Guidance on achieving this for masonry walls is contained below in Design and Construction.
The International Residential Code (ref. 5) contains similar requirements, but with a minimum STC rating of 45 dB when tested in accordance with ASTM E90 for walls and floor/ceiling assemblies separating dwelling units.
DESIGN AND CONSTRUCTION
In addition to STC values for walls, other factors also affect the acoustical environment of a building. For example, a higher STC may be warranted between a noisy room and a quiet one than between two noisy rooms. This is because there is less background noise in the quiet room to mask the noise transmitted through the common wall.
Seemingly minor construction details can also impact the acoustic performance of a wall. For example, screws used to attach gypsum wallboard to steel furring or resilient channels should not be so long that they contact the face of the concrete masonry substrate, as this contact area becomes an effective path for sound vibration transmission.
TMS 0302 includes requirements for sealing openings and joints to ensure these gaps do not undermine the sound transmission characteristics of the wall. These requirements are described below and illustrated in Figures 1 and 2.
Through-wall openings should be completely sealed, After first filling gaps with foam, cellulose fiber, glass fiber, ceramic fiber or mineral wool. Similarly, partial wall penetration openings and inserts, such as electrical boxes, should be completely sealed with joint sealant.
Control joints should also be sealed with joint sealants to minimize sound transmission. The joint space behind the sealant backing can be filled with mortar, grout, foam, cellulose fiber, glass fiber or mineral wool (see Figure 2).
To maintain the sound barrier effectiveness, partitions should be carried to the underside of the structural slab, and the joint between the two should be sealed against sound transmission in a way that allows for slab deflection. If the roof or floor is metal deck rather than concrete, joint sealants alone will not be effective due to the shape of the deck flutes. In this case, specially shaped foam filler strips should be used. For fire and smoke containment walls, safing insulation should be used instead of foam filler strips.
Additional nonmandatory design and building layout considerations will also help minimize sound transmission. These are covered in detail in TEK 13-02A (ref. 6). The design of exterior walls for the mitigation of outdoor-indoor sound transmission is covered under TEK 13-04B (ref. 9).
Table 1—Calculated STC Ratings for Concrete Masonry Walls (ref. 1)
Table 2—Increase in STC Ratings Due to Furring Space and Drywall (ref. 1)
Figure 1—Sealing Wall Intersections & Control Joints
Figure 2—Sealing Around Penetrations & Fixtures
NOTATIONS
ΔSTC = the change in STC rating compared to a bare concrete masonry wall d = the thickness of the furring space (when drywall is used on both sides of the masonry, d is the thickness of the furring space on one side of the wall only), in. (mm) STC = Sound Transmission Class STL = Sound Transmission Loss W = the average wall weight based on the weight of the masonry units; the weight of mortar, grout and loose fill material in voids within the wall; and the weight of surface treatments (excluding drywall) and other components of the wall, psf (kg/m²)
REFERENCES
Standard Method for Determining Sound Transmission Ratings for Masonry Walls, TMS 0302-12. The Masonry Society, 2012.
Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, ASTM E90-09. ASTM International, 2009.
Standard Classification for Rating Sound Insulation, ASTM E413-10. ASTM International, 2010.
2003, 2006, 2009, and 2012 International Building Code. International Code Council, 2003, 2006, 2009, 2012.
2003, 2006, 2009, and 2012 International Residential Code. International Code Council, 2003, 2006, 2009, 2012.
Noise Control with Concrete Masonry, TEK 13-02A. Concrete Masonry & Hardscapes Association, 2007.
Controlling Sound Transmission Through Concrete Block Walls, Construction Technology Update No. 13. National Research Council of Canada, 1998.
Outdoor-Indoor Transmission Class of Concrete Masonry Walls, TEK 13-04A, Concrete & Masonry Hardscapes Association, 2012.
TEK 13-10D, Revised 2012. 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.
