In the 1950s, the U.S. Navy and British navy began to investigate passive sonar, in which an array of sensors detects noises from a target sub­marine. Prior to this, navies used active sonar, whereby a submarine emitted a signal that reflected off a target subma­rine. Unfortunately, this let the target know it was monitored. Passive sonar systems allow the submarine to become stealthy by "listening" to noise emitted from a target submarine and by utilizing a series of sensors - equally spaced on a cable - to receive signals from a target passively. Powerful sonar algorithms convert the signals (pressure fields) into actionable information.
Passive sonar flow technology is the newest flow measurement technology since vortex, Coriolis and ultrasonic flow­meters were introduced in the 1960s and 1970s. Engineered for a wide range of multiphase and single-phase flows, the technology is especially well­suited for erosive and corrosive slurry flows, such as in the mineral processing industry, since the flowmeter installs and measures from outside the process pipe and does not contact the process media. The ability of the sonar technol­ogy to passively measure flows from the outside the pipe enables unprecedented measurement performance and reli­ability in highly aggressive slurries and fluids, which is designed to virtually eliminate maintenance costs associ­ated and process downtime experienced with traditional slurry flow technolo­gies such as electromagnetic flow and ultrasonic meters.