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Outdoor-Indoor Transmission Class of Concrete Masonry Walls

  • August 2018

INTRODUCTION

Providing a quality indoor acoustic environment is becoming a higher priority in many cases; particularly in urban environments where noise from traffic and other outside sources can be a significant distraction, especially in schools, homes and the workplace. Concrete masonry walls provide excellent noise control due to their ability to effectively block airborne sound transmission over a wide range of frequencies.

The ability of a wall to insulate a building interior from outdoor noise can be indicated by the wall’s outdoor-indoor transmission class (OITC), with higher OITC values indicating better sound insulation.

OITC is one rating system available to help compare the acoustic performance of various wall systems. Others include the sound transmission class (STC) and the noise reduction coefficient (NRC). Both OITC and STC indicate a wall’s ability to block the transmission of sound from one side of the wall to the other. OITC differs from the STC rating in that the OITC was developed specifically to indicate transmission of noise from transportation sources. STC was developed primarily for indoor noise sources, such as human speech. Unlike OITC and STC, NRC indicates the ability of a wall to absorb sound, which is useful for controlling sound reverberations within a room.

This TEK presents OITC values for a variety of common concrete masonry exterior walls. STC and NRC values for concrete masonry walls can be found in TEK 13-01D, Sound Transmission Class Ratings for Concrete Masonry Walls, and TEK 13-02A, Noise Control With Concrete Masonry (refs. 1, 2), respectively.

OUTDOOR-INDOOR TRANSMISSION CLASS

The OITC is a rating intended for exterior building facades, and is an estimate of a wall’s or window’s ability to reduce typical transportation noise. It is determined in accordance with ASTM E1332, Standard Classification for Rating Outdoor-Indoor Sound Attenuation (ref. 3). E1332 presents a standard procedure to calculate OITC based on tested sound transmission loss (TL) across the wall or wall element at specific frequencies from 80 to 4,000 Hz. These TL values are measured either in the laboratory or in the field using ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, or ASTM E966, Standard Guide for Field Measurements of Airborne Sound Attenuation of Building Facades and Facade Elements (refs. 4, 5).

OITC is calculated using these tested TL values along with the sound spectrum of a reference sound source. This reference sound spectrum is an average of typical spectra from three transportation noise sources: aircraft takeoff, freeway and railroad passby. The reference sound spectrum is A-weighted to better correlate to human hearing (A-weighting is a frequency response adjustment that accounts for the changes in human hearing sensitivity as a function of frequency).

Although higher OITC values indicate more effective sound insulation from noises similar to the reference sound spectrum, it should be noted that the accuracy of the rating depends on the actual exterior noise spectrum and the surface area of the wall, as well as the acoustic performance of other building elements, such as windows and doors. The OITC is intended to be used to compare various facades, rather than as a predictor of performance.

The OITC can be applied to walls, doors, windows, or combinations thereof. As presented in this TEK, the OITC values apply to the masonry portion of the wall only, without windows or other openings.

CONCRETE MASONRY OITC VALUES

OITC Values Based on Test Data

Many ASTM E90 sound transmission loss tests have been performed on a wide variety of concrete masonry walls. OITC values for some of these walls have been calculated in accordance with ASTM E1332 from E90 test data, and are presented in Table 1. In general, for concrete masonry walls, heavier walls have higher OITC values.

Note that the ASTM E1332 OITC calculation requires transmission loss (TL) test data from 80 Hz to 4,000 Hz, while ASTM E90 test reports often do not include TL values at 80 Hz. Test reports which do include 80 Hz show that the TL value of concrete masonry walls at 80 Hz is typically about the same or higher than that at 100 Hz. For the purposes of this TEK, where TL values at 80 Hz were not reported, the 80 Hz TL was assumed equal to the 100 Hz TL.

OITC values can also be determined by field testing, using test data from ASTM E966, then calculated in accordance with E1332.

Estimated OITC in the Absence of Test Data

Ideally, OITC should be based on tested transmission loss values. In recognition that this data is not always available, however, the information in Figure 1 is presented as a tool to help designers estimate OITC values.

It has been well established (ref. 6) that the STC of concrete masonry walls is directly related to wall weight. Using this knowledge and the calculated OITC values in Table 1, a correlation between concrete masonry wall weight and OITC has been developed for walls at least 3 in. (76 mm) thick:

where 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 plaster, stucco and paint, psf (kg/m²). The weight of drywall is not included.

Table 1 contains calculated OITC values for various concrete masonry walls, based on Equation 1.

For multi-wythe walls where both wythes are concrete masonry, the weight of both wythes is used in Equation 1. 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 OITC for the wall system, first calculate the OITC using both Equations 1 and 2, using the combined weight of both wythes, then linearly interpolate between the two resulting OITC ratings based on the relative weights of the wythes. Equation 2 is the OITC equation for clay masonry (ref. 1):

Tabulated wall weights for concrete masonry walls can be found in CMU-TEC-002-23, Weights and Section Properties of Concrete Masonry Assemblies (ref. 7).

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).

Table 1—Calculated OITC Ratings for Concrete Masonry Walls (ref. 6)
Figure 1—OITC Estimates Sound Insulation From Common Traffic Sources

OITC REQUIREMENTS

Although not currently required by the International Building Code (ref. 8), designers sometimes include an OITC requirement in the construction documents, particularly for buildings close to railroads, airports and highways.

DESIGN AND CONSTRUCTION

In addition to transmission class values for walls, other factors also affect the acoustical environment of a building. Seemingly minor construction details can 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.

Through-wall openings, partial wall penetration openings and inserts, such as electrical boxes, as well as control joints should be completely sealed.

The reader is referred to TEK 13-01D, Sound Transmission Class Ratings for Concrete Masonry Walls, and TEK 13-02A, Noise Control With Concrete Masonry (refs. 1, 2) for more detailed information on the above as well as additional design and building layout considerations to help minimize sound transmission.

REFERENCES

  1. Sound Transmission Class Ratings for Concrete Masonry Walls, TEK 13-01D. Concrete Masonry & Hardscapes Association, 2012.
  2. Noise Control With Concrete Masonry, TEK 13-02A.  Concrete Masonry & Hardscapes Association, 2007.
  3. Standard Classification for Rating Outdoor-Indoor Sound Attenuation, ASTM E1332-10a. ASTM International, 2010.
  4. Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, ASTM E90-09. ASTM International, 2009.
  5. Standard Guide for Field Measurements of Airborne Sound Attenuation of Building Facades and Facade Elements, ASTM E966-10e1. ASTM International, 2010.
  6. Standard Method for Determining The Sound Transmission Rating for Masonry Walls, TMS 0302-12. The Masonry Society, 2012.
  7. Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.
  8. 2003, 2006, 2009, and 2012 International Building Code. International Code Council, 2003, 2006, 2009, 2012.

TEK 13-04A, 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.

Sound Transmission Class Ratings for Concrete Masonry Walls

  • August 2018

INTRODUCTION

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:

  1. walls have a thickness of 3 in. (76 mm) or greater,
  2. hollow units are laid with face shell mortar bedding, with mortar joints the full thickness of the face shell,
  3. solid units are fully mortar bedded, and
  4. 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

  1. Standard Method for Determining Sound Transmission Ratings for Masonry Walls, TMS 0302-12. The Masonry Society, 2012.
  2. Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, ASTM E90-09. ASTM International, 2009.
  3. Standard Classification for Rating Sound Insulation, ASTM E413-10. ASTM International, 2010.
  4. 2003, 2006, 2009, and 2012 International Building Code. International Code Council, 2003, 2006, 2009, 2012.
  5. 2003, 2006, 2009, and 2012 International Residential Code. International Code Council, 2003, 2006, 2009, 2012.
  6. Noise Control with Concrete Masonry, TEK 13-02A. Concrete Masonry & Hardscapes Association, 2007.
  7. Controlling Sound Transmission Through Concrete Block Walls, Construction Technology Update No. 13. National Research Council of Canada, 1998.
  8. CMU Sound and Assemblies Properties Calculator, CMU XLS-003-19, Concrete & Hardscapes Association, 2019,
  9. 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.