332 Lecture 17 
steel plate 
2.5mm 
110 n= 0.015 
90 
@ 
Oo 
o 
(e) 
———_=— impedance level (L,) 
uO 
{e) 
40 
] 2 4 8 lie 2 ©3 125 250 Sco) jl 2 4 8 le 32 es 
Sao ii Hz kHz 
Fig. 17.8. Point-impedance level as measured (average of three points as shown; half-octave-band noise) 
on a steel plate bolted to a steel doorframe in wall of reverberant room (156.3 m*, empty; reverberation 
time approximately two sec). The measured values are connected, For comparison with theory, a curve is 
shown (heavy line), computed according to Eq. (26) and using measured 7-values (derived from reverber- 
ation times of plate; 7 = 2.2/Tf,). 
A solution to the first problem can be estimated as long as f<f,,. For this 
frequency range, it may be assumed that the water presents a mass load only 
because of inefficient radiation. The magnitude of the extra vibrating mass may 
be assessed by taking the thickness of the vibrating water layer near the plate 
as \, /4. Thence, the total mass of the plate plus water becomes approximately 
equal to 
PwrB 
M (! + Aph ) 
which means a considerable increase of the impedance compared with the plate 
in vacuo, a 10% to 20% decrease of Xz, and a factor 0.6 to 0.8 for the resonant 
frequencies of metal structures (e.g., ship propellers). 
The second problem is very complicated as long as the source of structure- 
borne noise is in "rigid" contact with the mechanical structure (e.g., the foun- 
dation). Figure 17.9 shows how a tendency might be estimated if the source is 
resiliently mounted. Again the model chosen to represent a practical situation 
is very simple. It is supposed that the contact plane at the source side of a spring 
is vibrating in two directions only, viz., causing a "normal" excitation and a 
