Building Acoustics

Sound transmission 231

term in the expression. One easily observes that the variability of σf is much smaller than
the corresponding one for resonant radiation. The radiation index 10⋅lg(σf) seldom goes
below –5 dB.

6.3.4.4 Radiation factor for stiffened and/or perforated panels

Finally, we shall name some additional factors which are important concerning sound
radiation from plates. The first deals with the effect of studs or stiffeners. One might
argue that the stiffeners will divide the plate into a number of smaller plates, with the
effect that the total circumference comprising all partial plates becomes larger. This will
then increase the radiated power due to the resonant modes when f < fc; see Equation
(6.48). This effect is experimentally confirmed as apparent from Figure 6.17. The
radiation index is shown for the same aluminium panel used for Figure 6.16 but now the
panel is stiffened by aluminium studs attached to the panel in a centre-to-centre distance
of 400 mm. In one case, the stiffeners are running in one direction, in the other there
were crosswise stiffeners as well.

Figure 6.17 Radiation index of a 4 mm thick aluminium panel, 2.7 x 3.4 metres, measured using mechanical
point excitation. The panel is stiffened using Al-profiles in one and two (crosswise) directions. Measurements
according to Venzke et al. (1973).

The results shown are measured using mechanical point excitation. However,
Venzke et al. also present similar data using diffuse field excitation. These exhibit

63 125 250 500 1000 2000 4000 8000

Frequency (Hz)

-40

-30

-20

-10

0

10

10 lg

σ (

dB

)

Unstiffened

Stiffened - one direc.

Stiffened - two direc.