Sound transmission 215
for the sound reduction index of the actual façade element. Tabled data for Lj covers not
only the usual frequency range of 100–3150 Hz, but also the extended range 50–5000
Hz.
A corresponding procedure is used to calculate the adaptation term C, which is used
for the internal sound insulation in buildings. The only difference is that here one uses a
pink noise spectrum instead of a traffic noise spectrum. It is recommended in ISO 717 to
state the performance of building elements by adding the adaptation terms to the
calculated weighted sound reduction index in the manner shown in the following
example:
RCCwtr(; ) 41(0;5)dB.= −
Several countries are now using the sum of the relevant single number quantity and the
appropriate adaptation term in their requirements for sound insulation, e.g. between
dwellings
RRCw′′(limit)= w+ 50 5000−. (6.20)
6.2.2 Impact sound pressure level
Sound insulation against impact sound is normally concerned with isolation of sound
energy generated by footfalls. A hammer apparatus, normally called a tapping machine,
for testing of floors was as far back as 1960 standardized by ISO; see ISO 140 Part 6 for
specifications. In spite of many criticisms over the years, complaints that measured data
do not rank the floors in accordance with the subjective impression etc., no international
agreement has been reached concerning another test source. We shall treat this tapping
machine in greater detail later on. At this stage we shall just point out that it has five
hammers, each weighing 0.5 kg, arranged in such a way that they fall freely against the
test object (the floor) twice per second, i.e. the tapping frequency is 10 Hz. The quantity
actually measured is the sound power radiated into the room in question. This is
commonly the room below the floor but there may as well be situations as with rooms
sharing a common floor, transmission to a room from footfalls on a staircase etc. The
measure in question, reflecting the sound power, is the normalized impact sound
pressure level Ln. Under laboratory conditions, it is defined by
(^) ni
0
10 lg ,
A
LL
A
⎛⎞
=+⋅⎜⎟
⎝⎠
(6.21)
where Li is the sound pressure level in the receiving room due to the tapping machine.
The room has a total equivalent absorption area A, and the reference area A 0 is 10 m^2. In
a building we will define in a similar way an apparent normalized impact sound pressure
level
(^) ni
0
''10lg ,
A
LL
A
⎛⎞
=+⋅⎜⎟
⎝⎠
(6.22)
where L'i is the sound pressure level in the receiving room. In this level, there may now
be contributions from other surfaces than the primary floor. These two situations,
measurements in a laboratory and in the field are illustrated in Figure 6.6.