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Beamforming with the ARES 100 Node

Beamforming enables the system to focus on specific areas or sources of noise, delivering more precise monitoring and identification of noise sources, useful for noise reduction efforts and compliance with noise regulations. Deploying 3D accelerometers, rather than microphone arrays, in the beamformer provides improved system performance and environmental protection, with reductions in array size, cost, and unwanted sidelobes. ARES beamformer array apertures occupy just 1 cm for a single sensor, or 13 cm for 2 sensors, and can distinguish sources in frequencies from 50 to 2 kHz with excellent angular resolution, in real-time.

A typical installation of two ARES 100 Nodes is shown positioned near railroad tracks. Two nodes are configured as a two-sensor (8 channel) AVS (acoustic vector sensor) beamformer.

Both APS (acoustic pressure sensor, i.e., microphones) and AVS devices can be employed in acoustic beamformers. A summary of AVS beamforming is presented in [1]. For airborne applications, few AVS beamformers exist due to high cost. A wide variety of microphone arrangements for various APS arrays are presently available. Many designs have arrangements of microphones that help to attenuate the sidelobes of the array, which are responsible for ghost images. In general, it can be stated that the array aperture, or the spatial breadth of the array, is inversely proportional to the lowest measurable frequency. Typical low frequency limits are about 250 Hz for a microphone array having breadth of 35 cm, or about 100 Hz if the array size increases to a meter. The number of microphones in these arrays varies from a few to over 1000, depending on the shape of the beam pattern and degree of sidelobe rejection. The measurement aperture of all microphone-based APS array systems is much larger than the <1 cm (for one), or 13 cm (for two) accelerometer-based AVS composed of a MEMS microphone and triaxial MEMS accelerometer.

In addition to reduced aperture, there are other advantages of AVS compared to APS when used for beamforming. For APS systems, at least four sensors are required to focus the array in 3D, while a single AVS sensor represents four measurements at essentially the same point in space. Secondly, in an APS array, phase delay information is used to determine direction via beamforming, which depends on array geometry and frequency. An AVS has inherent directionality based on 3D sensing, and is frequency independent. Direct measurement of the direction-of-arrival (DOA) information is present in the velocity field structure, and so azimuth and elevation measurements are independent. The enhanced phase diversity across coincident triaxial sensors improves measurement robustness to noise in AVS-based systems, which is possible for APS designs only by adding more sensors. However, because of the 3D nature of an AVS, acoustic beamforming can be performed using a single sensor.

The AVS beam patterns presented in Figure 1 indicate a lack of sidelobes for either a single or dual-sensor AVS. For the dual-sensor, the sensor separation distance is 13 cm, such that the ARES nodes are mounted side-by-side. Only the 1900 Hz (l/2 = 9 cm) pattern is predicted to have a small ghost image, but this is imperceptible from the figure. As for APS arrays, adding more AVS sensors will narrow the beampattern, though for monitoring applications this may not be necessary.

Figure 1. Single and dual AVS sensor beam patterns at a range of frequencies supported by the accelerometer-based ARES 100.

[1] M. Hawkes and A. Nehorai. “Acoustic Vector-Sensor beamforming and Capon direction estimation”, IEEE Transactions on Signal Processing. 46 (9), September 1998

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