Building Acoustics

350 Building acoustics

not by the velocities. The consequences are that the reduction index R 13 will be equal to
Dv,13 but we shall have to correct R 12 to get Dv,12:

DRv,12=+⋅ 12 10 lg(ψχ). (9.40)

0.01 0.1 1 10 100

h 2 /h 1

0

5

10

15

20

25

30

Reduction index

(

dB)

R13 - 3 pl.

R13 - 4 pl.

R12 - 3 pl.

R12 - 4 pl.

Figure 9.19 Reduction index of bending waves at intersections involving three and four plates, respectively, as
a function of plate thicknesses. See Figure 9.18.

The idealized reduction indexes as shown in Figure 9.19 have limitations as to their
practical applications due to the fact that no account is taken of the generation of other
wave types at the intersection. Furthermore, the plates are of infinite extent and only
normal incidence is treated. However, the data have been useful, serving as a first
estimate in monolithic concrete building constructions. Comparing with the estimates
given in EN 12354 (see below), we shall find that they are not too different from
Cremer’s data. Certainly, the former is based on later work by Kihlman (1967), who
treated the case of random incidence on an intersection of four plates and Gerretsen
(1979, 1986), who also compared with measurement data.

9.3.2.2 Vibration reduction index Kij

This vibration reduction index is an attempt to establish a general invariant quantity
characterizing the transmission across a joint between finite size elements under diffuse
field conditions. Determining the index by measurements (see ISO 10848 series) implies
measuring space time averaged velocities and structural reverberation time of the actual
elements. The damping of the element, given by the reverberation time, is expressed by
the equivalent absorption length ai of the element i. The definition is then