Building Acoustics and Vibration

The following sections are included:

• Preface

• Contents

This chapter provides a general introduction to building acoustics and vibration, explaining the practical principles and concepts of acoustics and building acoustics, and describing some typical applications.

The goal of this chapter is to review the basic principles of theoretical building acoustics. Rather than delving deep into the mathematics, it focuses on imparting a working knowledge of the common methods employed to analyse building acoustics problems. Along with covering the more theoretical aspects of building acoustics, the chapter addresses some typical application associated with the noise control and vibration propagation.

In this chapter, the fundamental terms and common measures used in building acoustics to quantify sound and the acoustical properties of systems and materials are described. Single number rating systems have been used successfully to select and assess building acoustics products for many years, e.g., sound insulation of building elements such as walls and floors.

In sound insulation, one must consider the many paths that sound can take from one room to another: (a) direct path: sound passes through common partition; (b) flanking: sound passing around common partition, e.g. along roof rafters; and (c) structure-borne: direct vibration of structure by source (e.g. boiler pumps), sound passes through structure. One procedure for noise control is to provide an acoustic barrier or partition to reduce the transmission of sound. For design purposes, one must be able to predict the transmission loss (sound reduction index) for the partition over a wide range of frequencies. In this chapter, it is attempted to examine the general case of airborne sound transmission through a panel or partition; particularly, the direct path of airborne sound transmission. The other paths will be further examined is the subsequent chapters.

Impact noise is usually structural borne sound, which can travel up the walls or directly through the floor whereas airborne sound penetrates dividing structures. Impact sound, in particular, footstep noise, is one of the most difficult forms of acoustical energy to control. There are probably more variations in the acoustical design of floors than there are for walls, because in the design of floors, both impact and airborne noise must be controlled. This chapter extends the work carried out in Chapter 4 by considering the case of structure-borne impact transmission of sound through floor constructions. Moreover,it is discussed the airborne sound insulation of floor constructions and the relation to impact sound isolation.

The sound reduction index of partitions, whether it is measured in acoustical laboratories or predicted, using the methods in Chapter 4, considers that the sound transmission from the source room to receiving room occurs only directly through the partition under concern. The same fact applied also to the case of impact sound level of a given floor, as treated in Chapter 5. However, ifthe same partition constitutes a part of a building, then sound can be transmitted through many paths. The apparent sound reduction index or field transmission loss is defined by taking all the flanking paths along with the direct path. By definition, flanking transmission is the structural transmission of sound energy from one room to another by any path other than the nominally separating wall or floor, as illustrated in Fig. 1.23. This chapter investigates the theoretical and experimental methods used to determine flanking transmission via flanked building components.

Because of their low surface density and the gaps around their edges, doors and windows are acoustically weak elements of a partition. Apart from glazing,doors are a common weak link in terms of sound insulation performance of walls and partitions. This is mainly due to poor seals around the door perimeter, at the head and jamb. Acoustic seals on doors rely on compression of flexible rubber to form an airtight seal around the perimeter of the door. On the other hand, sound transmission through windows is governed by the same physical principles that affect walls. Practical noise control measures are influenced by the properties of the glass and the characteristics of the window assemblies. From acoustical point of view, the overall sound insulation of a construction is greatly reduced by the presence of areas of poor insulation. When a “weak” element, such as window or door, is used in a construction, the composite sound reduction index for the combination is usually closer to that of the “weaker” element. This chapter investigates common practical aspects of sound insulation of doors, windows, and façades.

Noise from transport has become a major pollutant of the outdoor urban environment. The levels of environmental noise cannot be any longer ignored by highway engineers, developers and local authorities. High levels of noise may lead to a substantial change in the proposed construction plans and require noise control measures such as barriers, absorbing road surfaces, and building insulation. These measures should be carefully examined and justified in terms of their acoustic performance, costs, effectiveness, durability, visual intrusion and safety. At a very early stage of the design of a building next to a busy street, the architect or building engineer need to be able to estimate the sound levels being generated by the traffic or any other outdoor noise source and come up with some solutions for the control of the resulting noise. This chapter will outline propagation models used to determine outdoor noise. Simple calculation procedures are presented to provide data that are useful for site and building planning purposes.

There are two basic environments in which one makes predictions and measurements of sound and noise: outdoors and indoors. The outdoor acoustic environment has been treated in Chapter 8. The indoors acoustic environment, also referred to as room acoustics, introduces boundaries which reflect sound. If the boundaries completely reflect all incident sound without any absorption then the resulting sound field is termed diffuse or reverberant. In a diffuse sound field the time average of the mean square sound pressure is the same everywhere through the enclosure. The flow of energy is equally probable in all directions. Consequently, room acoustics describes how sound behaves in an enclosed space and how to design a room with respect to acoustical quality. A desired acoustical environment varies according to the type of activity, that is to take place in that environment. Recording, mastering, stereo or surround mixing, speech and music playback can require different or variable acoustic environments.Room acoustics is the practice of studying and designing these environments. This chapter attempts to examine the fundamental methods of room acoustics that are interrelated with building acoustics applications.

Noise radiating from piping and ductwork can be a serious problem in modern building construction. New buildings, additions and remodelling often involve a heating, ventilating, and air-conditioning (HVAC) equipment that creates high noise levels. Low mechanical system noise from HVAC system, low intrusive noise, and low reverberation are acoustical basics for most residential spaces. In this context, the new trends in building construction materials cause piping and ductwork noise to be more of a concern today than in the past. Lightweight PVC plastic pipe is now frequently used to replace cast iron, copper or mild steel tubing. Sheet metal ducting becomes thinner and lighter in weight in order to minimize the initial material cost. These light construction materials have worsened the noise problems in residential areas since they are more easily set into motion due to air and fluid flow. From an acoustical viewpoint, it is often necessary to estimate the sound pressure level that may be expected to be produced from these noise sources. In the absence of experimental data for the noise source, predictive relationships are required. In this chapter, methods and procedures that may be used to predict the noise emitted from air distribution system in buildings are examined. In many cases in practice, the overall noise level as well as the noise spectrum (sound pressure level in each octave band) is equally important to be predicted in order to check the agreement with noise regulations under concern.

The most common source of vibrations in buildings is caused by mechanically induced vibrations from mechanical equipment and HVAC systems such as air distribution system. The structure-borne sound may also be produced when vibratory motion or forces are transmitted from the machine to its support structure, through connecting ductwork or piping and radiates into the occupied space, as discussed in Chapter 10. In addition, human-induced vibrations due to regular activities such as walking, etc. needs to be considered by the designer to ensure that the vibration is not perceptible to the occupants, especially in longspan, lightweight floors in steel-concrete composite construction, which are potentially susceptible to vibration problems. The vibration reduction may be achieved by isolating the vibrations of the machine from the connected elements.Floor vibration can also be reduced with vibration isolators. For the case of structure-borne sound from a vibrating panel on the machine, noise reduction may be achieved by using damping materials on the panel to dissipate the mechanical energy, instead of radiating the energy into the surrounding air. As the vibration of residential buildings could be air-borne or ground-borne and could be caused by both internal and external sources, the scope of building vibration is very broad and it cannot be all covered in this book. The main focus will be put on machine and human-induced floor vibrations. A discussion on vibration isolation of both sources of vibrations is also reviewed.

The following sections are included:

• Appendix 1 Summary of Building Acoustic Measurements

• Appendix 2 Sound Insulation of Common Constructions: ISO and ASTM Ratings

• Appendix 3 Sound Reduction Indices (Transmission losses) of Some Common Constructions

• Bibliography

• Index