Combined microphone array and lock-in amplifier operations for outdoor photo-acoustic sensing

Mid-infrared (MIR) standoff photoacoustic (PA) sensing of explosive chemicals and nerve gas stimulants at calibrated concentration have been demonstrated in door. When they are operated out door, array beam forming technique has to be employed to reject ambient noise and enhance signal. Lock-in amplifier usually needs to be used to achieve weak signal detection in a noisy environment. If we can combine these two techniques we will be able to reject both spatial and temporal noise and achieve a great signal to noise ratio (SNR) performance. From the best of our knowledge no literature has described how to combine these two techniques. In this work we demonstrated combined array and lock in amplifier operation in outdoor environment. A simplified system includes a signal generator, a speaker source, a lock in amplifier, 4 spy-phones with 4 parabolic reflectors to collect the acoustic signal, a National-Instrument NI6259 data acquisition system with both A to D (ADC) and D to A converters (DAC), and a PC. To combine these two techniques, each of the array collected signals was digitized by the ADC. Their path delays were adjusted in the computer to synchronize the phase. By using a PC controlled ADC the processing time is very long (similar to 1s). To synchronize them without using costly high-speed customer made hardware, we delayed the reference signal by send it through the same ADC-PC-DAC path as the array signals. By doing so, a good lock-in operation with stable phase was obtained.