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19 3.3 Experimental Microphone Array Engineering 3.3.1 Mechanical Design As the result of the development described in the previous sections, a 70-microphone elliptical array was designed, with an aspect ratio of 1.7, a width of 4 ft (1.2 m), and a height of 12 ft (3.7 m). This design should provide source resolution down to 250 Hz with side lobe suppression of -12 to -14 dB. The microphone array assembly and data acquisition system were further developed by Wyle. Due to the significant size of the array aperture, it was assembled on a metal frame consist- ing of three separate sections vertically mounted together and installed on a four-wheel metal base. The sections, each approximately 4 ft by 3 ft (1.2 m by 0.9 m), could be easily dis- assembled for shipping. Fourteen PVC pipe spokes, each hold- ing 5 microphones, were mounted on the frame sections, providing the 70-microphone elliptical pattern that was designed. The identical lower and upper frame sections hold five spokes each, with five microphones mounted equidistantly on each spoke. The middle frame section holds four spokes with five microphones each. During the field tests of the array, additional microphones were mounted along the central ver- tical axis of the middle frame section, raising the total number of microphones to 73 or 77 for some tests. The assembly with additional microphones is shown in Figure 10. 3.3.2 Data Acquisition System The array was equipped with the 0.24 in. PCB Piezotronics Series 130 array microphones with 0.25 in. ICP® preampli- Figure 10. Experimental microphone array (note fiers Model 130P10 or integral ICP® preamplifiers. The seven additional microphones in the array center). microphones/preamplifiers were inserted in holes predrilled in the spokes of the array, each provided with a 3.5 in. windscreen. (IRIG). Another channel received a signal from a pair of photo- Prior to array assembly, the microphones were phase-calibrated cells installed on tripods near the microphone array to regis- in pairs using a Brüel & Kjćr Type 51AB sound intensity ter truck passbys. The photocells were Banner Engineering calibrator. Model SM31EL/SM31RL mini-beam emitter and receiver. The data acquisition system was completed using a National Another PXI channel was used for recording signals from IT's Instruments Model PXI-1044 embedded controller chassis vehicle tracking system. The system includes a Banner Engi- with 12 data acquisition cards providing analog-to-digital con- neering Model Q45BB6DLQ infrared linear position sensor version for a total of 80 data channels for signal recording. The attached to the front bumper of the truck. This photosensor measurement signals from the array microphones were indi- detects white strips painted every 5 ft (1.5 m) along the track vidually fed into the PXI channels through 50 ft (15 m) long pavement and telemeters a series of signals to a remote receiver. microphone cables. A controller onboard the PXI chassis The signals from the receiver were fed into the PXI. Truck enables synchronization of all the data acquisition cards, pro- speed was then determined by the distance between the strips viding simultaneous recording across all channels. The soft- and the time between the voltage pulses received in the signals. ware for running the system in real time and transferring data from the PXI to a laptop computer for post-processing was 3.3.3 Preliminary Testing developed by Dr. William Blake and Wyle. During the proof-of-concept testing described in Section 3.4, Prior to full-range proof-of-concept testing scheduled at one of the PXI channels was used for recording the time signal IT's facilities in Ft. Wayne, Indiana, a preliminary testing of the from an ESE Model ES-292 GPS-based time code generator experimental beamforming microphone array was performed