Any sound field is described completely by both its sound pressure and its acoustic particle velocity. For the sake of comparison, if sound pressure would be the acoustic equivalent of voltage (a potential so to speak), acoustic particle velocity is the acoustic equivalent of amperes (a flow so to speak).
Whereas microphones have been available since the beginning of the last century, acoustic particle velocity in air only became a directly measurable quantity with the invention of the Microflown sensor in 1994. As such, each Microflown sensor measures acoustic particle velocity in one direction.
Sound pressure is a scalar value, just a number. Acoustic particle velocity is a vector value.
An acoustic vector sensor (AVS) is a four channel probe that consists of a single sound pressure transducer and three orthogonally placed acoustic particle velocity sensors. Whereas the sound pressure transducer detects what sort of acoustic event is happening, the particle velocity vector value points in the direction where this acoustic event if taking place.
Yes, in essence an acoustic vector sensor (AVS) hears all around in a full spherical bubble. Whereas many sorts of sensors have only a narrow field of view (pipeline/straw) or only a 360 degrees panoramic view, an AVS has the best possible field of “view” one may think of.
4. How does an AVS and an array of sound pressure transducers differ in obtaining directional information?
In order to obtain the directional information of a sound source, at least two microphones are required, placed at a certain intermediate distance. Human beings obtain acoustic directional information using both their ears, measuring the delay in the time of arrival on the sensor. This concept of only capturing phase information has been industrialized using a larger number of sound pressure transducers. The intermediate distance between the microphones determines the optimal frequency where sound sources can be detected. A high frequency event like a sniper shot requires a relatively compact array of sound pressure transducers with a main dimension of 50 centimeters. A very low frequency event like a mortar blast however requires an array intermediate distances of around 50 meters!
Whereas any sheer sound pressure based array is optimized for the detection of a certain kind of acoustic event, AVS are simply covering the entire audio band width, making them really broad banded. AVS capture both the phase and amplitude information of the sound field.
Acoustic vector sensors (AVS) merge and exceed the acoustic detection capabilities of all existing sound pressure based acoustic detection systems that are, by design, all dedicated to detect a certain kind of acoustic event. AVS are acoustically speaking broad banded, allowing them to detect both low frequency blasts of mortars and high frequency bursts of snipers shots. Furthermore, AVS can detect all sorts of tonal sound sources like ground vehicles or helicopters. So a full and comprehensive 3D acoustic situational awareness can be achieved.
Acoustic vector sensors (AVS) weigh less than 100 grams, have the size of a dice and consume less than 100 mA. These features also allow deployment on all platforms, not only vehicles or stand alone systems, but also on small airborne platforms (UAVs) or dismounted soldiers.
Sheer sound pressure based systems are narrow banded, having only an optimal signal to noise ratio in a very small part of the audio spectrum. Most sorts of events of course have an acoustic signature that covers a wider part of the audio spectrum. Acoustic vector sensors (AVS) are acoustically speaking broad banded. The false alert rate of an AVS goes down as the presence and direction of an acoustic event being detected can be confirmed for various parts of the audio spectrum.
The lower the frequency to be detected, the larger the intermediate distance between the sound pressure transducers needs to be. In practice, especially for locating lower frequency sound sources, often distributed systems are used are used. All acoustic data are to be obtained in a central unit, requiring a collaborative mode of operation.
An acoustic vector sensor captures the complete directional information on both bearing and elevation in a single node allowing a non-collaborative mode of operation. This has several advantages in terms of its signal processing robustness, data transfer requirements and possibilities for covert operation.
When placing sound pressure transducers in a horizontal plane, information is obtained for the bearing of the sound source. When placing sound pressure transducers in several horizontal layers, also information for the elevation of the sound source can be obtained. But each sound pressure transducer is not able to discriminate between the signals that come directly from the sound source and the signals that are reflected on the ground. An acoustic vector sensor (AVS) captures in a single point in space all signals that travel the shortest way between the sound source and the AVS.
In sheer sound pressure transducer based acoustic detection systems, in each microphone the signals from all sound sources and reflections are simply added before being compared with the data from other microphones where the same sort of summing of signals takes place. Obviously this causes inaccuracies.
An acoustic vector sensors (AVS) captures in a single point in space all signals that travel the shortest way between the sound source and the AVS.
This depends both upon the scenario and the level of sophistication of signal processing algorithms applied. Scenario relevant parameters are above the loudness of the noise source itself and, to a lesser extent, terrain conditions (e.g. wide open versus hills and trees) and meteorological conditions (mostly direction of wind and the wind force).
But as a rule of thumb, as a lower estimate, what humans in their infancy can hear, acoustic vector sensors can detect. But to give a grasp of feeling, light firearms can be detected up to 2 km, heavy caliber mortars can be detected up to 40 km and commercial airliners can be detected at 10 km cruising altitude.
Advanced signal processing algorithms can be used to improve the range detection performance up to a level where for instance up to 30 km distance mortar blasts can be detected.
Yes. Acoustic vector sensors do not illuminate a target and cannot be detected electronically. They don’t disclose their presence. Passive sensors consume far less power than active sensors.
Yes, they cannot be jammed electronically. As compared to radar, this is a distinctive benefit as North Korea blinded South Korean radar before shelling the South Korean island in 2010. Furthermore AVS are resilient to spoofing, i.e. emitting electronic signals for the purpose of confusing the opponent.
Acoustic vector sensors (AVS) meet first of all the microUAV payload relevant criteria of small size, low weight and low power. As compared to land based acoustic detection, airborne acoustic detection capabilities benefit from the fact that there are no obstacles or acoustic reflections between sound source and receiver. On top of it, AVS can also be used for two other functions crucial to UAVs, namely Hear& Avoid and Automated Take Off and Landing.