AIAA-2005-565-450Acoustic Testing of the dielectric barrier(4)

推进器的声波特性

C. Sensor and output The acoustic emissions in the first set of tests were measured with a generic dynamic microphone placed in an aluminum box to eliminate electrical noise. This microphone was connected to an oscilloscope which in turn was connected to a data acquisition system (DAQ) which recorded the waveform averaged over a number of cycles. The microphone output voltage (peak-to-peak) was determined from the recorded waveforms. The raw waveform and its calculated fast Fourier transform were recorded by Labview. For the second set of experiments an ElectroVoice ND468 supercardioid microphone, with a frequency response (3 DB) of 22 kHz, was used. The output of the microphone was fed to a pre- Figure 4: Polar plot showing the orientation of amplifier then to an HP 3052B oscilloscope connected to a the actuator with respect to angle and induced laptop that recorded the raw data in Excel. To eliminate velocity. electro-magnetic noise post-processing filtering of high frequency noise (MHz and higher) was used and then the peakto-peak voltage of the remaining waveform determined. D. Test parameters The parameters varied in the first set of tests were the forcing voltage, dielectric material size, and forcing voltage waveform. Forcing frequency was held constant at 5 kHz because it is near the optimum point for our stepup transformer, and the signal was ensemble averaged over 64 traces to clean up the signal. The tests using the second experimental set-up held the voltage constant at 5 kV while varying the frequency from 5 kHz to 8 kHz. In this case ten thousand sample points were used for a single sweep of two to three cycles. Both tests included taking measurements at different angles; five degree increments for the first set, and ten degree increments for the second set of tests.III.ResultsA free field, a region surrounding the source where the sound pattern emulates that of an open space15 with no walls, was needed to avoid reflection from any hard surfaces. Realizing the outdoors is the best free field available because there are no hard surfaces near by except for the ground, a reference data set was taken to compare to indoor experiments. Figure 5 shows the fast Fourier transform (FFT) plot from this data. The distinctive properties of this plot are that there is a first harmonic at the same frequency as the forcing frequency and a second harmonic close to the strength of the fundamental (or forcing frequency). The third and fourth harmonics are more than an order of magnitude lower than the second harmonic. Testing was then conducted indoors while taking precautions to ensure that the absorbing surfaces of the booth were indeed eliminating acoustic reflections from a near-by wall. This was done by completing two set of tests where the plasma actuator rests on the stand described previously, and response was measured at angles 0 to 180 degrees in 5 degree increments. The sound-deadening material was removed on the side nearest one wall and then replaced. The resulting FFT plots are shown in figure 6. With the material removed, the second harmonic is stronger than the fundamental, indicating some additional signal Figure 5: Frequency spectrum in a free field most likely coming from reflection. With the material replaced, (outdoors) for first set of acoustic measurements. the FFT plot is similar to the free field plot of figure 6, indicating some attenuation of reflected energy. 4 American Institute of Aeronautics and Astronautics