Building Acoustics

Sound transmission in buildings. Flanking transmission. 331

problem of sound incidence, two other deviations from the assumptions. The mineral
wool did not fill the entire length of the slit, only approximately half of it. Also, the tape
used for sealing was not properly specified in the written report. For our calculations, the
first problem was solved by assuming a lower flow resistivity, approximately 1/6 of the
one for the mineral wool used, whereas the second problem was eliminated by using a
tape surface weight of 0.28 kg/m^2 measured on a similar brand of tape.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Closed

Min. wool + tape (M)

Min. wool + tape (P)

Min. wool (M)

Min. wool (P)

Open (M)

Open (P)

Figure 9.6 Sound reduction index of test wall of area 2.8 m^2 with slit of area 1240 mm x 20 mm and length 180
mm. Measuring situations as depicted in Figure 9.5. M – measurements in diffuse field. P – predicted for normal
incidence.

As shown in Figure 9.6, we arrive at a reasonable fit between measured and
predicted results using the assumptions mentioned above. The uppermost curve gives the
reduction index in the case of the slit when completely closed, this reduction index being
the one for a combined wall, a lower part being a 140 mm concrete slab and the upper
part a 180 thick lightweight construction. Based on these data and the predicted reduction
index for the slit, we may use Equation (9.2) to find the resulting reduction index for the
cases a) to c) depicted in Figure 9.5. The largest discrepancy between measured and
predicted results is found for case c), which may be attributed to resonance phenomena
more predominant at normal incidence than when “smeared out” at diffuse incidence.