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Allowable Stress Design of Pier and Panel Highway Sound Barrier Walls

  • August 2018

INTRODUCTION

Sound barrier walls are increasingly being used to reduce the impact of traffic noise on properties abutting major urban traffic routes. Because concrete masonry possesses many desirable features and properties—excellent sound resistance, low cost, design flexibility, structural capability and durability, it is an excellent material for the design and construction of highway sound barrier walls.

Aesthetics is also an important consideration. Noise barriers significantly impact a highway’s visual impression. Visual qualities of noise barriers include overall shape, end conditions, color, texture, plantings and artistic treatment.

The variety of concrete masonry surface textures, colors and patterns has led to its extensive use in sound barrier walls.

Various types of concrete masonry walls may be used for sound barriers. Pier and panel walls are relatively easy to build and are economical due to the reduced thickness of the walls and the intermittent pier foundations. In addition, the piers can be offset with respect to the panels to achieve desired aesthetic effects. Pier and panel walls are also easily adapted to varying terrain conditions and are often used in areas that have expansive soils.

This TEK presents information on the structural design of concrete masonry pier and panel sound barrier walls. Requirements and considerations for reduction of highway traffic noise are discussed in TEK 13-03A, Concrete Masonry Highway Noise Barriers (ref. 2).

DESIGN

Building Code Requirements for Masonry Structures, ACI 530/ASCE 5/TMS 402 (ref. 1) includes requirements for allowable stress design, strength design and prestressed approaches. The allowable stress design approach was used to develop the designs in this TEK. Allowable stresses were increased by one-third, as permitted for load combinations which include wind or seismic loads. Allowable Stress Design of Concrete Masonry, TEK 14-07C (ref. 4), describes the basic design approach.

Materials and Workmanship

Since concrete masonry sound barrier walls are subject to a wide range of load conditions, temperatures and moisture conditions, the selection of proper materials and proper workmanship is very important to ensure durability and satisfactory structural performance. Accordingly, it is recommended that materials (concrete masonry units, mortar, grout and reinforcement) comply with applicable requirements contained in Building Code Requirements for Masonry Structures (ref. 1).

Lateral Loads

Design lateral loads should be in accordance with those specified by local or state building and highway departments. If design lateral loads are not specified, it is recommended that they conform to those specified in Minimum Design Loads for Buildings and Other Structures, ASCE 7 (ref. 3). Wind and earthquake loads required in this standard are briefly described in the following paragraphs.

Design wind loads (F) on sound barrier walls may be determined as follows:

For the wall designs in this TEK, G is taken as 0.85 and C as 1.2. The minimum wind load specified in ASCE 7 is f 10 psf (479 Pa). For basic wind speeds of 85 mph (minimum), 90 mph, 100 mph, and 110 mph (53, 145, 161, and 177 kmph), the corresponding wind loads are listed in Table 1.

Earthquake loads (F ) on sound barrier walls may be p determined as follows, considering the wall system as a reinforced masonry non-building structure (ref. 3):

Seismic loads for a range of conditions are listed in Table 3.

Table 1—Wind Loads for Sound Barrier Walls
Table 2— Design Assumptions for Tables 4, 5, and 6
Table 2—Seismic Loads for Sound Barrier Walls

Deflections

Deflection considerations typically govern wall design for long spans and taller walls with greater lateral loads. Deflections are imposed to limit the development of vertical flexure cracks within the wall panel and horizontal flexure cracks near the base of the pier. The design information presented in this TEK is based on a maximum allowable deflection of L/240, where L is the wall span between piers.

DESIGN TABLES

Design information for pier and panel walls is presented in Tables 4 through 7. Tables 4 and 5 provide horizontal reinforcing steel requirements for 6 in. and 8 in. (152 and 203 mm) panels, respectively. Horizontal reinforcement requirements can be met using either joint reinforcement or bond beams with reinforcing bars.

Table 6 provides pier size and reinforcement requirements for various lateral loads. Table 7 lists minimum sizes for pier foundations, as well as minimum embedment depths. These components of pier and panel walls are illustrated in Figure 1.

When pier and panels are used, walls are considered as deep beams, spanning horizontally between piers. Walls support their own weight, vertically, and also must resist lateral out-of-plane wind or seismic loads. The panels are built to be independent of the piers to accommodate masonry unit shrinkage and soil movement. For this design condition, wall reinforcement is located either in the horizontal bed joints or in bond beams. Wall reinforcement is based on maximum moments (M) and shears (V) in the wall panels, determined as follows:

The wall panels themselves are analyzed as simply supported beams, spanning from pier to pier.

In addition to the horizontal reinforcement, which transfers lateral loads to the piers, vertical reinforcement in the panels is required in Seismic Design Categories (SDC) C, D, E and F. Building Code Requirements for Masonry Structures (ref. 1) includes minimum prescriptive reinforcement as follows. In SDC C, vertical No. 4 (M #13) bars are located within 8 in. (203 mm) of the wall ends, and at 10 ft (3.0 m) on center along the length of the wall; minimum horizontal reinforcement requirements are satisfied by the primary reinforcement listed in Tables 4 and 5. In SDC D, E and F, vertical No. 4 (M #13) bars are located within 8 in. (203 mm) of the wall ends, and at 4 ft (1.22 m) on center along the length of the wall.

Table 6 shows pier size and vertical reinforcement requirements. Piers are designed as vertical cantilevers, not bonded with the walls, and pier reinforcement is based on maximum moment and shear, determined as follows:

Design assumptions for the pier and panel walls are given in Table 2. Note that allowable stresses were increased by one-third, as permitted for load combinations which include wind or seismic loads (ref. 1).

Requirements for concrete foundations supporting the concrete masonry piers are given in Table 7. These foundations can be constructed economically by drilling. The concrete foundation piers should contain vertical reinforcement (same as shown in Table 6) which should be properly lapped with vertical reinforcement in the concrete masonry piers. The embedment depths given in Table 7 are based on an allowable lateral passive soil pressure of 300 psf (14.4 kPa).

Table 4—6 in. (152 mm) Panel Wall Reinforcement
Table 5—8 in. (203 mm) Panel Wall Reinforcement
Table 6—Pier Size and Reinforcement
Table 7—Pier Foundation Requirements, Minimum Embedment/Diameter

DESIGN EXAMPLE

A pier and panel highway sound barrier is to be designed using the following parameters:

  • 6 in. (152 mm) panel thickness
  • 10 ft (3.05 m) wall height
  • 14 ft (4.27 m) wall span
  • open terrain, stiff soil
  • basic wind speed is 90 mph (145 km/h)
  • SS = 0.25, SDC B

From Table 1, the design wind load is 14.1 psf (674 Pa) for a basic wind speed of 90 mph (145 km/h) and exposure C. Using Table 3, the design seismic load is determined to be 2.8 psf (0.13 kPa) for a 6 in. (152 mm) wall grouted at 48 in. (1219 mm), or less, on center, for SS = 0.25. Since the wind load is s greater, the wall will be designed for 14.1 psf (674 Pa).

Using Table 4, minimum horizontal panel reinforcement is either W1.7 (MW 11) joint reinforcement at 8 in. (203 mm) on center, or bond beams at 48 in. (1220 mm) on center reinforced with one No. 5 (M #16) bar. At the bottom, the panel requires a beam 16 in. (406 mm), or two courses, deep reinforced with one No. 5 (M # 16) bar (last column of Table 4). Because the wall is located in SDC B, vertical reinforcement is not required to meet prescriptive seismic requirements.

The minimum pier size is 16 x 18 in. (406 x 460 mm), reinforced with four No. 4 (M #13) bars, per Table 6. The pier foundation diameter is 18 in. (457 mm), and should be embedded at least 7.5 ft (2.29 m), per Table 7.

NOTATIONS

Af    = area normal to wind direction, ft² (m²)
Cf    = force coefficient (see ref. 3)
d     = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
Em  = modulus of elasticity of masonry in compression, psi (MPa)
Es   = modulus of elasticity of steel, psi (MPa)
F     = design wind load, psf (Pa) (see ref. 3)
Fa   = acceleration-based site factor (at 0.3 second period) (see ref. 3)
Fm  = allowable masonry flexural compression stress, psi (Pa)
Fp   = seismic force, psf (Pa) (see ref. 3)
Fs   = allowable tensile or compressive stress in reinforcement, psi (MPa)
Fv   = allowable shear stress in masonry, psi (MPa)
f’m  = specified compressive strength of masonry, psi (MPa)
G    = gust effect factor (see ref. 3)
H    = wall height, ft (m)
I     = importance factor (see ref. 3)
Ip   = component importance factor (assume equal to 1.0 for sound barrier walls) (see ref. 3)
Kd  = wind directionality factor (see ref. 3)
Kz  = velocity pressure exposure coefficient (see ref. 3)
Kzt = hill and escarpment factor (see ref. 3)
L    = wall span, ft (m)
M   = maximum moment at the section under consideration, in.-lb (N-mm)
n    = ratio of elastic moduli, Es/Em
P    = applied lateral force, lb (N)
qz   = velocity pressure, psf (Pa) (see ref. 3)
= 0.00256K KzKztKdv²I
R    = response modification coefficient (see ref. 3)
Rp   = component response modification factor (equal to 3.0 for reinforced masonry non-building structures) (see ref. 3)
SDS = design short period spectral acceleration =⅔(FaSS), where SS varies from less than 0.25 to greater than 1.25, and Fa is dependent on SS and soil conditions at the site (see ref. 3)
Ss    = mapped maximum considered earthquake spectral response acceleration at short periods (see ref. 3)
V     = shear force, lb (N)
v      = basic wind speed, mph (km/h) (see ref. 3)
Wp   = weight of wall, psf (Pa)
w      = wind or seismic load, psf (Pa)

REFERENCES

  1. Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
  2. Concrete Masonry Highway Noise Barriers, TEK 13-03A. Concrete Masonry & Hardscapes Association, 1999.
  3. Minimum Design Loads for Buildings and Other Structures, ASCE 7-02. American Society of Civil Engineers, 2002.
  4. Allowable Stress Design of Concrete Masonry, TEK 14-07C Concrete Masonry & Hardscapes Association, 2002.

 

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.

Concrete Masonry Highway Sound Barriers

  • August 2018

INTRODUCTION

As urban areas continually expand, a large percentage of high volume, high speed roadways are located within metropolitan areas, resulting in a great number of people being exposed to high levels of roadway noise. Controlling this noise is often a required part of highway construction or suburban expansion. Although the perception of unwanted sound varies from individual to individual and from one activity to another, objective criteria have been established to help designers determine when noise abatement is required, and the levels of noise reduction that will relieve the problem.

Noise abatement measures should be individually evaluated for each project, based on the project’s noise reduction goal and budget, the community needs, and longer-term maintenance and durability issues. Alternatives for reducing traffic noise include:

  1. land use planning: separating noise-sensitive uses from highways,
  2. controlling noise at the source: such as by reducing speed limits or prohibiting truck traffic, and
  3. attenuating noise by modifying the horizontal or vertical alignment of the highway, using larger right-of-ways, or shielding the noise with a barrier.

Sound barriers are solid obstructions built between the noise source and the receiver – they are often chosen as the most expedient and effective method to reduce highway traffic noise. Although earth berms can act as sound barriers, solid walls are more often used. An effective barrier can significantly reduce the level of unwanted noise, while providing an attractive durable element in the community.

Concrete masonry construction successfully fills all of the requirements for effective sound barrier walls, providing excellent noise insulation and a wide choice of aesthetic styles, excellent stability, strength, durability, and low maintenance. These benefits are well recognized; concrete masonry sound barriers represent over five times the wall area than the next popular choice, wood post and plank (ref. 4).

This TEK covers acoustic requirements for concrete masonry highway sound barriers. For structural design considerations, the reader is referred to Allowable Stress Design of Pier and Panel Highway Sound Barrier Walls (refs. 3).

NOISE REDUCTION REQUIREMENTS

The Federal Highway Administration (FHWA) publishes two criteria for determining when highway noise abatement is required. Values are expressed in dBA which is defined as a time weighted average sound level when measured on the A-scale of a standard sound meter at slow response. The two criteria are:

  1. when predicted noise levels exceed FHWA’s noise abatement criteria (see Table 1), or
  2. when predicted noise levels represent a substantial increase over existing noise levels.

The term “substantial increase” is defined by state highway agencies, but is most often taken as a 10 to 15 dBA increase in noise levels (ref. 2). Once either one of these criteria triggers the need for abatement considerations, the designer is required to provide a substantial reduction in noise levels, typically defined as a 5 to 10 dBA reduction (a 10 dBA reduction will generally be perceived as halving the noise; a reduction less than 5 dBA would probably not be noticeable).

The noise abatement criteria (NAC) in Table 1 are not meant as a design goal nor to represent desirable noise levels. If predicted levels exceed the NAC, abatement measures must be taken to reduce the predicted level by 5 to 10 dBA, not just to the level indicated in Table 1. Likewise, if predicted noise levels are 15 dBA above current levels, noise abatement must be considered even if the predicted level is below the NAC.

Table 1–FHWA Noise Abatement Criteria (NAC)a,b (ref. 5), A-Weighted Sound Levels (dBA) when Abatement Must Be Considered

BARRIERS FOR NOISE REDUCTION

Total noise reduction by a barrier is commonly referred to as insertion loss. Simply defined, insertion loss is the difference in sound level before and after a barrier is placed next to a highway.

Insertion loss has five components:

  1. barrier attenuation due to the diffraction of sound waves over and around a barrier placed in the line-of- sight between the source and receiver,
  2. transmission loss of sound through the barrier,
  3. reductions in barrier attenuation resulting from multiple reflections caused by double barriers,
  4. shielding attenuation from other barriers between the source and the receiver, and
  5. loss of excess attenuation already received from soft ground cover.

For most highway applications, the first two of these components are by far the most significant.

Barrier Attenuation

Barrier attenuation due to sound wave diffraction is controlled by designing the barrier to be both tall enough and long enough to adequately shield the receiver. The noise barrier should be tall enough to break the line of sight between the highway and the receiver – noise barriers adjacent to the highway do very little for homes on a hillside overlooking that highway. Sound travelling around the ends of sound barriers can degrade the barrier performance. To avoid these end effects, one rule of thumb is to extend the barrier four times as far in each direction as the distance from the receiver to the barrier. If this is not possible, the sound barrier can often be combined with the natural terrain, such as knolls. Another alternative is to provide returns on the ends of the barrier back towards the community to reduce the noise level near the end of the barrier.

Barrier Physical Characteristics

Transmission loss through the barrier is controlled by the barrier material (see next section) and by eliminating holes and other openings in the barrier. FHWA and the American Association of State Highway and Transportation Officials (AASHTO) both recommend that the transmission loss be at least 10 dBA above the attenuation resulting from diffraction over the top of the barrier (refs. 1, 2).

Barrier insertion loss can be compromised if there are holes or openings in the barrier. For large openings, sound energy incident on the barrier will be directly transmitted through the opening to the receiver. When the opening is small, an additional phenomenon occurs: upon striking the barrier wall, the sound pressure can increase, effectively amplifying the transmitted sound. Maintenance openings and doorways can be placed behind a baffle or recessed wall area to minimize the effect of the opening. When sound walls terminate at an earth berm, care should be taken to ensure that there are no gaps between the wall and berm.

Barrier location also impacts effectiveness. For a given barrier height, moving the wall closer to the receiver, or closer to the source, will provide additional noise reduction (ref. 1). Barriers placed on hilly terrain above the highway can typically be shorter and still provide the same noise reduction as a taller barrier placed closer to the highway. Changes in wall height impact the barrier’s acoustic performance. Abrupt height changes of 2 ft (0.61 m) or more can significantly reduce effectiveness. A better alternative is to gradually step the height down to the lower level. This is particularly easy to accomplish with concrete masonry, as the modular size facilitates such changes.

Barrier Reflection

Recent research has clarified the impact of acoustic reflections, particularly between two parallel barriers, and in the case where a barrier is placed on only one side of the highway. In theory, multiple noise reflections between two parallel walls can reduce the effectiveness of the individual barriers and contribute to overall noise levels. To avoid reducing the performance of parallel barriers, it is suggested that the ratio of the distance between barriers to the average height of barrier above the roadway be at least 10:1 (see Figure 1). In these cases, measured increases in noise levels due to reflected noise have been below the threshold of normal human hearing (ref. 1).

Similarly, when a barrier is constructed on only one side of the highway, highway noise levels on the opposite side of the highway are not significantly affected. Measurements made to try to quantify the increase in noise level have shown maximum increases of 1 to 2 dBA, an increase not detectable to the average human ear (ref. 2).

Figure 1—Recommended Minimum Distance Between Parallel Barriers to Avoid Amplification by Reflected Noise

Barrier Types

Many types of materials are used to construct highway noise barriers. For maximum performance, however, the material should be rigid and of sufficient density to provide a transmission loss of 10 dBA greater than the expected reduction in noise due to diffraction over the top of the barrier alone (ref. 2). The preferred method of rating a material’s ability to transmit noise is by the transmission loss (TL), which is related to the ratio of incident acoustical energy to transmitted acoustical energy. For highway noise sources and their typical sound spectral content, the transmission loss of common barrier materials increases with increasing surface weight of the material.

For many common heavyweight materials used in barrier construction, such as concrete masonry, transmission loss values are usually more than adequate. For less massive materials, such as steel, aluminum, and wood, transmission loss values may not be adequate, especially where large insertion losses are required. Typical TL values for common materials are given in Table 2.

Under certain conditions, vegetation can provide sound attenuation. AASHTO suggests that vegetation at least 15 ft (4.5 m) tall, and 98 ft (30 m) deep with sufficient density to completely block the line of sight can achieve a noise reduction of up to approximately 5 dBA (ref. 1). However, since it is usually impossible to plant enough vegetation to achieve a substantial noise reduction, the FHWA does not consider vegetation to be a noise abatement measure (ref. 2). Vegetation can, however, provide visual interest and relief, thus providing a psychological benefit, if not an acoustic one.

Table 2—Transmission Loss Values of Common Barrier Materials (ref. 6)

REFERENCES

  1. Guide on Evaluation and Abatement of Traffic Noise 1993. American Association of State Highway and Transportation Officials, 1993.
  2. Highway Traffic Noise Analysis and Abatement: Policy and Guidance. Federal Highway Administration, Washington, D.C., 1995.
  3. Allowable Stress Design of Pier and Panel Highway Sound Barrier Walls. TEK 14-15B, Concrete Masonry & Hardscapes Association, 2004.
  4. Highway Traffic Noise in the United States: Problems and Response. Federal Highway Administration, Washington, D.C., 1997.
  5. Procedures for Abatement of Highway Traffic Noise and Construction Noise. Code of Federal Regulations, 23CFR Part 772, U. S. Government Printing Office, 1997.
  6. Noise Barrier Design Handbook, FHWA-RD-76-58. Federal Highway Administration, Washington, D.C., 1976.

TEK 13-03A, Revised 1999. 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.

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.

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.