254 Building acoustics
fixed the upper limit at 0.96, which corresponds to a maximum angle of incidence of
approximately 78°.
The first example, given in Figure 6.27, shows the sound reduction index for a
specific case where the panel has a weight of 7.5 kg/m^2 and where the critical
frequencies are 400 Hz and 4000 Hz, respectively. The results shown are calculated by a
numerical integration of the integral in Equation (6.111), where the integration is
performed for loss factors of 0.01 and 0.1 (1% and 10%). In addition, Heckl’s
approximation for the frequency range between the two critical frequencies is also
included. As the loss factor is not included in this approximation one could expect that
the best fit would be obtain for a low loss factor. The most important thing to note is,
however, the far lower results one get as compared by the mass law.
Figure 6.27 Predicted sound reduction index of corrugated panel of weight 7.5 kg/m^2 and critical frequencies
(fc1,fc2) equal (400,4000) Hz. Calculated using loss factor 1% and 10%, additional data using Heckl’s
approximation in the range fc1< f <fc2.
The second example is taken from an extensive series of measurements performed
by Hansen (1993). The series comprised 10 different types of corrugated panel, where
the panels partly had the 10 m^2 size normally used in laboratory tests, some very small;
approximately 1.5 m^2. Experiments were also conducted by additional damping of the
panels.
We shall show results from measurements on the type denoted Hi Span ‘800’ (see
Figure 6.28). This has a large measuring area, and in Hansen’s own calculations he uses
a loss factor of 0.011 (1.1%) and the critical frequencies are 378 Hz and 30 400 Hz,
63 125 250 500 1000 2000 4000Frequency(Hz)01020304050607080Soundreduction index (dB)Mass law
Predicted, 10%
Predicted, 1%
Heckl's approx.