Civil Engineering Reference
In-Depth Information
CHAPTER 8
Sound transmission through multilayer
elements
8.1
INTRODUCTION
Using the methods presented in Chapter 6, we may, with reasonable accuracy, predict the
sound transmission properties of walls and floors characterized as single homogeneous
elements. The complexity increases quickly when the elements are composed of several
layers, also mechanically coupled in ways that are difficult to specify.
This chapter deals with these types of multilayer element but will, as in Chapter 6,
treat the specific elements as being disconnected from their boundary elements.
Certainly, this imposes a limitation as other connected structures certainly will influence
the vibration wave field of the element in question, and more so if the mechanical
couplings are strong. We shall postpone further discussion of this question to Chapter 9,
which deals with the theme of flanking transmission, i.e. the prediction of sound
transmission between rooms with several interacting structures.
We shall use the term multilayer to characterize a building element made up of two
or more homogeneous layers. These may contain mechanical coupling elements, such as
ties, studs or elastic layers. A double wall is a pertinent example, where the leaves are
mechanically coupled or uncoupled depending on a common or separate system of studs.
Another example is a floating floor construction where the top layer is coupled to the
base floor through either a continuous elastic layer or by elastic point supports.
The level of coupling may be difficult to ascertain. Two common boards
(plasterboard, chipboard etc.) placed in contact with another or even screwed together
are, acoustically speaking, “weakly” coupled. With boards of identical thickness, the
critical frequency of the combination will be approximately equal to the one for a single
board and the sound reduction index will ideally increase by 6 dB due to the increase in
weight by a factor of two. Gluing the boards together, however, we end up with a
sandwich element
, where the coupling depends on the type of glue and the element itself
acquires some quite different properties than in the first mentioned case.
We shall, according to the terms used in the standard EN 12354, use the term
element when speaking of a building component like a partition wall, a floor, a door etc.,
even when these are composite structures. In many cases one should perhaps better use
the term “construction”, a collection of elements. Hopefully, the reader should not be
confused finding both terms being used throughout the topic.
8.2
DOUBLE WALLS
With the term double walls we are to understand constructions having two independent
wall elements separated by an air-filled cavity that may contain a porous absorber. As
