Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Acoustic coupling
We put the new prototype shock mount through extensive test flights on a Cessna 208B using
accelerometers to measure vibration at various locations: on the top plate, the centre steel plate, and
the base plate.
The results presented in Table 1 show that a significant level of vibration remained on the gravimeter
chassis itself, which is mounted to the top plate, despite the high level of isolation of vibrations
originating from the aircraft floor. The increase in vibration magnitude from the steel plate to the top
plate is just over 30% at 1,600 rpm, but as high as 130% at 1,800 rpm.
Table 1. The magnitude of accelerations on the top plate, steel plate, and base plate showing the
high level of isolation achieved with the new shock mount design after the first stage of isolation.
There still remained a significant level of noise on the top plate above the second stage of
isolation. The line number in the table refers to the pass along a repeat line; the letter in
parenthesis refers to the flight direction north (N) or south (S).
Speed
Duration RMS
Line (knots) RPM Location (s) (m/s 2 )
1(S) 100 1,600 Top plate 439 0.086
2(N) 100 1,600 Steel plate 339 0.065
3(N) 100 1,600 Base plate 324 0.476
4(S) 100 1,600 Base plate 519 0.485
5(N) 130 1,800 Top plate 499 0.179
6(N) 130 1,800 Steel plate 200 0.076
7(S) 130 1,800 Steel plate 359 0.067
8(S) 130 1,800 Base plate 559 0.677
We produced a large number of spectrograms from the raw acceleration files to study the magnitude
and location of vibrations throughout the gravimeter installation. An example of one of these
spectrograms is shown in Figure 4, which we collected on the base plate on Pass 8(S). A large
amount of energy can be seen at the fundamental propeller frequency at 87.8 Hz and at its third
harmonic at 263.3 Hz. We observed that several of the spectral lines collected on the gravimeter
chassis matched peaks calculated to originate from propeller blade passes (not propeller shaft
rotation). We concluded that these vibrations could only be transmitted to the gravimeter by way of
acoustic coupling through the aircraft cabin air.
We then carried out several more flights to test the theory of acoustic coupling. For the initial test
flights, we used two types of acoustic isolation foam in various configurations (Figure 5). The foam
was fastened to the sides of the gravimeter, with coverage ranging from the top and the four sides
completely enclosed, to having just two sides enclosed.
Acoustic foam is known to have a very low heat conductance, thus it was not the preferred long-term
solution to reduce acoustic noise. We only used it during the tests to help determine if acoustic
coupling was a serious problem.
After the initial flights with the foam, we suspected that the two sides of the gravimeter which
contained a rectangular access panel were more susceptible to acoustic coupling than the other two
sides. These two opposing panels were constructed from thin aluminium which acted like drum
membranes, vibrating in harmony with each other. As proof of this, tapping on either of the side panels
instantly set up sympathetic vibrations on the opposing panel.
We designed a new set of access panels to act as acoustic dampers. These new panels were much
thicker than the original panels, but the two panels had a 1 mm difference in thickness to prevent
sympathetic vibrations from building up at a single resonant frequency.
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