Civil Engineering Reference
In-Depth Information
Bulk wave in air
and frame
Rayleigh wave
received signal
(at 25cm from excitation)
input force
0
2
4
6
8
10
12
14
16
time (ms)
U
z
(r)
of the surface of
Figure 8.2
The point force
F(t)
and the vertical velocity
the frame at
r
25 cm (Allard
et al
. 2002). Reprinted with permission from Allard,
J. F., Jansens, G., Vermeir, G. & Lauriks, W. Frame-borne surface wave in air-saturated
porous media.
J. Acoust. Soc. Amer.
111
, 690 - 696. Copyright 2002. Acoustical Society of
America.
=
Measurements have been performed with different sources, line sources and circular
sources having a radius around 5 mm. The first measurement of the velocity and the
damping of a Rayleigh wave over an open cell foam is described in Allard
et al
. (2002).
The experimental set-up is represented in Figure 8.3.
The thickness of the layer is
l
=
10 cm. The amplitude and phase of the normal
velocity is measured at several distances from the source with a laser vibrometer and a
scanning mirror. A small patch of retro-reflecting tape is glued to the foam to obtain a
good signal. A sine burst with 10 - 20 periods is used as excitation and the detected signal
is cross-correlated with the excitation in order to obtain the time of flight. The results of
the measurements are shown in Figure 8.4. These measurements give a phase velocity
of 68 m/s and a damping of 24 m
−
1
.
scanning
mirror
Polytec
vibrometer
LDS
shaker
Foam block
Figure 8.3
Experimental set-up for the observation of surface waves over open-cell
foams (Allard
et al
. 2002). Reprinted with permission from Allard, J. F., Jansens, G.,
Vermeir, G. & Lauriks, W. Frame-borne surface wave in air-saturated porous media.
J.
Acoust. Soc. Amer.
111
, 690 - 696. Copyright 2002. Acoustical Society of America.
