and what’s it good for?

By Jeff Shepard

Acoustic cameras consist of microphone arrays used to locate and characterize sounds. There are various microphone array structures to support specific analysis needs. Some acoustic cameras also have an embedded visual camera to supply an image over which the acoustic localization information can be presented. Acoustic camera application examples range from analyzing noise inside automobile cabins, aircraft, and trains, to quantifying the noise signature of wind turbines and monitoring industrial environments for anomalies and potential machine faults.

An acoustic camera consists of a microphone array, a sound processing section, and a display. Microphone arrays can consist of dozens or hundreds of microphones. The sound processing section acquires the incoming sound information from the microphones simultaneously or with precise relative time delays. As the sound travels from the source, it arrives at the various microphones at di erent times and with di erent intensities based on the relative locations of the microphones.

Beamforming is one method used for sound localization. It works by adding delays to the microphone signals and adding the signals to amplify the sound coming from a specific direction while minimizing or canceling the sound coming from other directions to essentially “point” the array in a specific direction. The calculated sound intensity information can be displayed on a power map.

Two techniques for sound localization are time di erence of arrival (TDOA) and angle of arrival (AoA). They can be combined using a generalized crosscorrelation (GCC) algorithm. GCC is relatively simple to implement and has low computational requirements. The tradeo is that many microphones are needed to achieve accurate localization results. Using a more complex algorithm can reduce the required number of microphones but will require a more capable (and more expensive) computational section with a faster processor and more memory.

Going beyond simple sound localization, sound intensity is quantified in dB and can be measured using acoustic probes. Some acoustic cameras include sound intensity and sound particle or pressure sensing measurement capabilities. Acoustic holography is another acoustical measurement technique and is used