OUTDOOR-INDOOR TRANSMISSION CLASS OF CONCRETE MASONRY WALLS
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 13-1C, Sound Transmission Class Ratings for Concrete Masonry Walls, and TEK 13-2A, 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 TEK 14-13B, Concrete Masonry Wall Weights (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).
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 13-1C, Sound Transmission Class Ratings for Concrete Masonry Walls, and TEK 13-2A, 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.
Table 1—Calculated OITC Ratings for Concrete Masonry Walls (ref. 6)
A Based on: grout density of 140 lb/ft³ (2,243 kg/m³); mortar density of 130 lb/ft³ (2,082 kg/m³) sand density of 90 lb/ft³ (1,442 kg/m³); unit percentage solid from mold manufacturer’s literature for typical units (4-in. (100-mm) 73.8% solid, 6-in. (150-mm) 55.0% solid, 8-in. (200-mm) 53.0% solid, 10-in. (250-mm) 51.7% solid, 12-in. (300-mm) 48.7% solid). Other unit configurations may have different OITC values. OITC values for grout-filled and sand-filled units assume the fill materials completely occupy all voids in and around the units. OITC values for solid units are based on all mortar joints solidly filled with mortar.
B Because of small core size and the resulting difficulty consolidating grout, these units are rarely grouted.
- Sound Transmission Class Ratings for Concrete Masonry Walls, TEK 13-1C. National Concrete Masonry Association, 2012.
- Noise Control With Concrete Masonry, TEK 13-2A. National Concrete Masonry Association, 2007.
- Standard Classification for Rating Outdoor-Indoor Sound Attenuation, ASTM E1332-10a. ASTM International, 2010.
- Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, ASTM E90-09. ASTM International, 2009.
- Standard Guide for Field Measurements of Airborne Sound Attenuation of Building Facades and Facade Elements, ASTM E966-10e1. ASTM International, 2010.
- Standard Method for Determining The Sound Transmission Rating for Masonry Walls, TMS 0302-12. The Masonry Society, 2012.
- Concrete Masonry Wall Weights, TEK 14-13B. National Concrete Masonry Association, 2008.
- 2003, 2006, 2009, and 2012 International Building Code. International Code Council, 2003, 2006, 2009, 2012.
NCMA TEK 13-4B, Revised 2012.
NCMA 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.