208 Building acoustics
radiating properties, i.e. the efficiency of the floor to radiate sound. One may consider
this process as analogous to a two-stage rocket. The vibration pattern set up in the floor
is determined by the excitation and the dynamic properties and the radiated sound energy
will be determined by how well this vibration pattern couples to the room below. This
does not exclude possible feedback in the system; the floor vibration may influence the
sources at the same time as the surrounding medium (air) may influence the movement of
the floor. The latter type of feedback is normally neglected when dealing with building
constructions; the inertia of the constructions is normally of quite another order of
magnitude compared with the corresponding ones for air. The situation would have been
quite different if the medium were water!
Figure 6.1 Machinery on the floor in an equipment room. Sound transmission to neighbouring room.6.2 Characterizing airborne and impact sound insulation
We shall introduce the quantities used to characterize sound insulation, quantities that
will be found in common building regulations and requirements for the sound insulating
properties of building elements and constructions. We shall derive the expressions used
when measuring these properties both in laboratories and in the field and we shall show
how these results may be converted to give a single number rating.
6.2.1 Transmission factor and sound reduction index
The transmission factor τ of a given surface is defined by the sound power, the ratio of
the transmitted power Wt and the power Wi incident on the surface:
t
i
.
W
W
τ= (6.1)The sound reduction index R is the corresponding logarithmic quantity defined as