An acoustic array can be used for two purposes:
- Beam form the sound to increase listening attention in a certain direction. In this way, the sound can follow the motion of a steerable camera for instance. In contrast to a directional microphone mounted on the camera, the advantage is that one array can serve many cameras at the same time.
- Estimate the angle of arrival (AoA) of distinct sound sources. This can provide an early warning where to put the attention, and cameras can pro-actively be directed in this direction.
Savannah Sensor Combo
We are developing a sensor combination to be used to monitor critical hot spots such as waterholes and gates in the border. It consists of a microphone array and a pan tilt zoom (PTZ) camera at least, and preferable also an PTZ IR camera and a radar to support for night vision and through foliage vision. This can be operated in automatic mode, where it scans the area for sound and movement, and alerts an operator for anomalies, or it can be operated in manual mode to provide 'super forces' to the operator.
A particular use of microphones is for shot detection, in the case a poacher comes to a situation where a shot is fired. The microphone array can immediately detect this event and pinpoint the direction, and even the position for a larger array, of the poacher. The figure below gives an example of a sensor network placed around an imaginary hotspot, for instance a waterhole. Many shots from three different locations were fired, and the picture shows the estimated location of the poacher as well as the bullet path. The accuracy is within a few meters, and the target can be found by searching along a line.
cIn this field test, a rifle with supersonic bullets was fired 10 times at three different positions. The movie below illustrates how the supersonic bullet leaves a shockwave (similar to the bow wave from a boat) as well as the muzzle blast.
Both this acoustic phenomenon can be used to locate the shooter. All 30 shots in the field trial aimed at the same target (a rhino at the waterhole). Our algorithms successfully finds the exact position of the shooter in all cases, and also the bullet trajectory with high accuracy. The theory is described in
Shooter localization in wireless microphone networks. D. LIndgren, O. Wilsson, F. Gustafsson and H. Habberstad. EURASIP Journal of Advances in Signal Processing, 2010.