Sound ranging

Sound ranging exploits the finite speed of sound in air (about 343 m/s at 20°C). By measuring the time difference of arrival (TDOA) of a sound at several spatially separated microphones, the source position can be triangulated. In its simplest form, a hyperbolic positioning system is used: each pair of microphones defines a hyperboloid of possible source locations, and the intersection of multiple hyperboloids yields the source coordinates.¹

Modern systems use digital signal processing to cross-correlate signals and estimate TDOAs with high precision. For artillery location, the sound of the gun firing (muzzle blast) and the projectile's shock wave are both used; the shock wave's characteristic N-wave shape helps distinguish it from other noises.²

Sound ranging was pioneered during World War I by Allied scientists, including the British physicist William Lawrence Bragg and the French engineer Lucien Bull. The first operational systems used arrays of microphones connected to a central recording station, where photographic film captured the arrival times. These systems could locate enemy artillery within about 50 meters at ranges up to 10 kilometers.

After the war, sound ranging continued to be refined, with improvements in microphone design and recording technology. During World War II, it was used for locating mortars and for air defense, though radar increasingly took over this role. The technique also found civilian uses, such as in studying thunder and in geophysical exploration.³

Sound ranging has been used to study atmospheric phenomena. For example, arrays of microphones can locate the source of infrasonic waves generated by severe weather, such as tornadoes and lightning. The International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) operates a global network of infrasound stations that use sound ranging principles to detect nuclear explosions and other atmospheric events.

In geophysics, sound ranging is applied to locate earthquakes and volcanic eruptions by analyzing the arrival times of seismic waves at multiple seismometers, though this is more commonly called seismic location. However, acoustic ranging is used to locate the source of volcanic explosions and to monitor volcanic activity by detecting infrasound from eruptions.⁴

Sound ranging is increasingly used in ecology to locate and track animals by their vocalizations. Arrays of microphones can triangulate the position of singing birds, calling whales, and even elephants. This non-invasive method allows researchers to study animal behavior and population density without disturbing them.

For example, in marine environments, hydrophone arrays are used to locate and track whale calls over large areas. In terrestrial habitats, microphone arrays have been used to estimate the number of singing males in a bird population and to map their territories. The technique is also used to monitor the spread of invasive species, such as the coqui frog in Hawaii.

One lesser-known application is in structural health monitoring: acoustic emission testing uses sound ranging to locate cracks or defects in materials by detecting the ultrasonic waves they emit. This is used in aerospace and civil engineering to monitor the integrity of bridges, pipelines, and aircraft components.¹

Another niche use is in sports: in tennis, the Hawk-Eye system uses multiple cameras, but some systems have used microphones to determine whether a ball landed in or out by analyzing the sound of the bounce. Additionally, sound ranging has been used in forensic acoustics to determine the position of a gunshot in a crime scene, and in wildlife conservation to locate poachers' gunshots in protected areas.²

TDOA

Time Difference of Arrival – the difference in arrival times of a signal at two sensors.

Hyperbolic positioning

A method of locating a source by intersecting hyperboloids defined by TDOA measurements.

Infrasound

Sound waves with frequencies below 20 Hz, often used in long-range detection.

N-wave

A pressure wave with a characteristic N-shaped waveform, typical of shock waves.