J.H. Janssen 335 
dBre 
1 pw 
90 
ee 1 =0.005 
7 = 0.015 
70 
60 1 pw 
reverberant room 
1 2 4 8 iS SB ses} 12s) 20) Seo) | 2 4 8 1691925163) 
pf Hz kHz 
Fig. 17.10. The same plate used for Fig. 17.8 was excited by half-octave bands of noise. The air- 
borne sound power (level Lp) radiated into the reverberant room was measured for two values of the 
loss factor 7 of the plate, while the effective value of the exciting force was 1 newton. A factor of 3 
increase in 7 (obtained by an additional damping layer with negligible mass) corresponds to a decrease 
in Lp cf 5 db, approximately. Tneory—heavy line, based on Eq. (30); actually measured 7 -values are 
inserted—agrees very well with experiment. 
50 
40 
30 
20 
——____—__ = power level (Lp) 
independent of frequency. The discrepancy is not understood; probably the plate 
was too small. It is clear, however, that increasing 7 does not reduce appre- 
ciably the radiated power, contrary to constant force excitation. 
In practice it is sometimes not quite clear which path is followed by the noise 
Originating in a known source. It is, then, very useful to have at one's disposal 
a relation between the average velocity level of a radiating plate and the sound 
pressure level at a certain distance. It might be derived from Eq. (32) and par- 
ticularly Eq. (33) as soon as the relation between the "average" velocity over 
the plate and the excitation-point velocity were known. 
It turns out that the factor 0.45 in Eq. (32) becomes 1.0 and the factor 0.43 
in Eq. (33) becomes 0.64 if v2¢ equals the value of the time-effective velocity 
squared and averaged over the plate. Formulas were given for the power radi- 
ated by a plate excited by a point force or by a line force. In general, for all 
practical purposes, for the same average v&; over the plate, line excitation ra- 
diates more sound power than point excitation. It would be interesting to know 
if area excitation forms an even more efficient method to radiate sound; area 
excitation occurs, for example, when the shell is excited by air-borne noise. 
In this respect Fig. 17.12 is interesting. Area excitation and point excitation 
are compared for the same value of v2 averaged. It is outside the scope of this 
Paper to treat further the radiation of sound. 
