Building Acoustics

Multilayer elements 297

Figure 8.17 gives an example, which shows that the difference in the reduction index,
using a single wall of 150 mm thickness as compared with a double wall, 150 mm and
100 mm thick, is negligible as long as one uses a common frame. It should be noted that
the main part of the curve lies above the critical frequency, which for the 150 mm thick
lightweight concrete partition is approximately 250 Hz. Both partitions have, however, a
thin layer of plaster added which probably makes the effective critical frequency slightly
lower.

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Frequency (Hz)

20

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50

60

70

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100

Sound

reduction index (dB)

150 mm

150 +100 mm

150 + 100 w/gap

150 + 100 w/ binders

18 dB/octave

Figure 8.17 Sound reduction index of lightweight concrete walls showing the effect of coupling along the
boundary. A double wall with a common vs. a separate frame (with gap). Effect of binders in the case of a
separate frame. See Figure 8.18. Data from Homb et al. (1983).

In these laboratory measurements the elements were mounted on a foundation or
frame in the form of a niche between the measuring rooms. In fact, there are two niches,
one in each room structurally separated by a gap (see Figure 8.18). As shown in Figure
8.18 b) this makes it possible to mount the elements separately, each on its side of the
gap. The distance between the elements gets larger (220 mm) but this is of minor
importance as compared with the minimizing of the frame coupling. It should be noted
that the dimensions of the boundary constructions is much larger than one normally finds
in real buildings. As evident from Figure 8.17, we obtain a huge increase in the reduction
index by separating the elements as compared with the case with the common frame. The
frequency dependence is quite strong, approximately 18 dB per octave. The flattening of