Performance comparison of microphone and reflector array structures for real-time and outdoor photoacoustic chemical sensing

Standoff chemical sensing using QCL and photoacoustic techniques recently have been demonstrated at a distance of more than 40 feet. [1]. To operate such a system outdoor, the system needs to be capable of rejecting other acoustic sources in the field. In this work, we experimentally compare two different implementations of acoustic beam forming array systems. One structure uses 4 microphones placing at the 4 foci of 4 parabolic reflectors. The other structure uses an array of 16 (4x4) microphones placing at the focal point of one parabolic reflector. Two acoustic sources were used to represent the signal (at 3kHz) and noise (at 4kHz). By adjusting delays to make sure signals from each phone were added in phase, we can enhance the signal and at the same time suppress the noise source. The measured results are processed in Fourier domain. The signal and noise spectra of the 4 microphone/4 reflector have a combined SNR of 11.48 dB, while the results of 16-microphones/1-reflector have a combined SNR of 17.82 dB. The worse SNR result of the 4-phone/4-reflector system may not simply due to the other system has more microphones. It also because of the average effect of the full reflector area, which can blur the phase of each element and the noise, cannot be cancelled more exactly.