Building Acoustics

Sound transmission 255

respectively. The measured results in Figure 6.28 are reproduced from Hansen’s data but
the calculated results are performed using Equation (6.111), setting a fixed upper limit
for the incidence angle of 78°. The differences between these data and Hansen’s own
calculation are negligible when applied to the large measurement area.
Other results does not fit equally well with the calculated ones as in this example.
Hansen does point out that when high accuracy is demanded one has to rely on measured
results, which may be unnecessary to point out. A typical feature that often is observed in
measurement results of corrugated panels is “dips” in the curve not attributed to
coincidence phenomena. This may be seen in the measured result around 4000 Hz. Two
explanations are suggested: acoustic wave standing wave resonances between the ribs
and panel vibration resonances. By panel resonances we shall not understand the ones
due to the eigenmodes of the entire panel but to the sub-panels of the cladding, i.e.
vibration modes of particular sections of the profile. A later series of measurements
combined with predictions using FEM analysis indeed have substantiated the latter
explanation (see Lam and Windle (1995 a, b)).

Figure 6.28 Sound reduction index of corrugated panel of weight 4.6 kg/m^2 and critical frequencies (fc1, fc2)
equal (378, 30400) Hz. Measurement data reproduced from Hansen (1993). Predicted data from Equation
(6.111).

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Measured

Predicted

Mass law

24.5 mm