The purpose of this manual is to provide members of the highway engineering field with the necessary tools to assess the noise insulation requirements of buildings. This analysis will enable the highway engineer to determine the effectiveness of existing buildings in insulating interior spaces against highway noise. It will also allow an evaluation of proposed modifications to the building to increase insulation effectiveness.
When noise strikes a structure such as a wall or a window, most of it is reflected, with the remainder being transmitted through by vibrating the structure, as shown in Figure 1. Since only a small portion appears on the other side of the structure, we say that the noise has been reduced.
Figure 1. Conceptual illustration of noise being transmitted through a structure. Incident noise vibrates the structure; some of the noise is reflected with the rest being transmitted through.
Two important properties of a wall that contribute to its ability to reduce noise are its weight per unit area, and its stiffness or resistance to bending when a force is applied. In general, the heavier a wall is, the better it will act to reduce noise, as shown conceptually in Figure 2.
Figure 2. Conceptual illustration of how heavy structures transmit less noise than lightweight structures. The concrete panel does not vibrate easily when noise strikes it so relatively little noise is transmitted through. The light plywood vibrates readily and so it transmits more noise than the concrete structure.
The term noise reduction has been used generally to this point to mean the decrease in level as noise passes through a wall. Once the noise manages to pass through the wall into a room, it may then be partially absorbed by soft materials such as drapes and carpets. Hence, noise reduction is really two separate mechanisms - one due to the blocking properties of the wall, and one due to the acoustic environment on the receiving side of the wall. These two mechanisms are shown conceptually in Figure 3. The exterior noise is first attenuated while passing through the exterior wall, attaining a level which is prevented from further buildup by the absorptive materials normally found in residential buildings. The first mechanism, which is the reduction due only to the physical properties of the wall, is termed Transmission Loss (TL). The second mechanism, interior absorption, is due to absorption of the noise materials inside the room.
Noise Reduction (NR) may now be defined as the total difference between noise levels existing on two sides of a wall. TL and room absorption are the two properties that contribute to NR.
In this manual, a simplified type of TL - called Exterior Wall Noise Rating (EWNR) will be used.* Thus, in general, NR will be equal to the EWNR adjusted to account for interior absorption.
The remainder of this manual is divided into four chapters. Chapter 2 deals with Noise Reduction Calculation Procedures and Chapter 3 contains Noise Measurement Procedures. Chapter 4 discusses Requirements for Ventilation and Energy, and, finally, Chapter 5 presents Procedures for Estimating the Costs of Noise Attenuation Modifications. The organization of the manual is shown graphically in Figure 4. A complete detailed flow diagram is presented in Appendix B along with step-by-step instructions concerning the procedures in the manual.
Following the last chapter, a series of worksheets are included for conveniently working the problems addressed in this manual.
*For a complete discussion of the Exterior Wall Noise Rating, see Appendix A.
