Civil Engineering Reference
In-Depth Information
CHAPTER 6
Sound transmission. Characterization and
properties of single walls and floors
6.1
INTRODUCTION
In the preceding chapter, our concern was directed at the task of removing acoustic
energy from a medium (air) by transport into an absorber that effectively could convert
the energy into heat. Absorbing materials have a wide range of applications: as sound
absorbers in rooms, included in noise barriers along roads, in silencers for air-
conditioning systems etc. One could then envisage that e.g. a porous material could be
effective in reducing the transport of sound energy from an air-filled space to another,
i.e. act as an isolator for sound energy in the same way as the material works for heat
energy. Unfortunately, this is not the case. We normally demand that a dividing wall of
reasonably good quality between rooms has a sound reduction index in the range 40-50
dB. This corresponds to a transmission factor of 10
-4
-10
-5
(see definitions below). To
achieve this, our porous material must have an absorption factor of 0.9999 in the actual
frequency range, indicating that this is not a workable solution. Effective partition walls
are based on a major jump in the impedance, i.e. a very high reflection factor. Reducing
sound transmission between rooms is therefore based on reflecting the energy back as
opposed to trying to dissipate the energy in the partition.
These considerations apply to the transmission of airborne sound; we shall use the
concept
airborne sound insulation
when talking of the ability of a construction to isolate
against airborne sound. With the notion
structure-borne sound
is meant vibrations in
solid structures, which in turn may radiate sound in the audible frequency range. Human
walking or jumping is the major source of these vibrations in buildings, and one will find
in the literature the terms
impact sound
and
footfall noise
used for describing the radiated
sound. We shall use the first term,
impact sound insulation
as the corollary to airborne
sound insulation. It should be noted that here we strictly are concerned with the
transmission phenomena, i.e. not with the sound generated in the source room itself.
This chapter will be devoted to methods and techniques for prediction and
measurement of the sound insulation properties of partitions, i.e. the sound transmission
properties both by excitation of airborne sound and impacts. These properties may be
quite different due to the different type of excitation by these two sources. In the case of
airborne sound, a distributed pressure field will drive the construction, whereas the other
excitation will be by “point” forces. This, however, does not prevent us, subject to some
conditions, of calculating the airborne sound insulation when the impact sound insulation
is known and vice versa.
As an introduction to the subject we shall use a practical example involving both
types of excitation, in this case also a more general type of impact. A piece of machinery,
mounted on a floor as shown in
Figure 6.1,
may induce vibrations (structure-borne
sound) in the floor due to unbalanced forces. Furthermore, the machine will radiate
sound energy setting up a sound field in the room, which may in turn excite the floor.
Which one of these processes will dominate the sound energy being radiated into the
room below will not only depend on the dynamic properties of the floor, but also on its
