Figure 1: Test section configured with acoustic boxes |
Figure 2: Internal view of test section with acoustic treatment installed |
The acoustic treatment system consists of 20 inch deep boxes constructed from
medium density fiberboard, stuffed with fiberglass and sealed with acoustic foam
that are attached to the tunnel test section. The flow surface is created using
nearly acoustically transparent sheets of heavy gauge screen. Figure 1 presents a
schematic of the test section with acoustic treatment boxes attached.
The system has proven to be quite successful when recently used in the measurement
of unsteady surface pressure on an airfoil in turbulence. An internal view of the
tunnel test section with treatment installed is shown in figure 2.
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| Figure 3: Pressure sound level at 30m/s with and without acoustic treatment
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| Acoustic measurements
were also performed to validate the improvement in acoustic quality at various flow speeds.
The acoustic measurements of Larssen and Devenport (1999) made with the test section in
solid wall configuration are compared with similar measurements in the acoustic treatment
configuration. The pressure sound levels of both measurements are presented in figure 3
for a flow speed of 30m/s. Considerable attenuation of the ambient sound field occurs between
10 and 500Hz with as much as 50dB of attenuation occurring between 10 and 20Hz. The spectra
converge and crossover at 500Hz with the acoustically treated spectrum rising up to 10dB above
the solid wall spectrum. The increase in spectral level above 500Hz with the acoustic treatment
in place is likely the result of flow interaction with the screen. Currently, plans are
underway to improve the flow surface quality, perhaps with pre-stretched Kevlar.
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