Sensor Tips

resulting from the surface being measured having di erent relative depths at di erent points. If the space is large, those errors can be small, but in small spaces, the error can be significant.

When implemented with a double-layer channel system of microphones, 2D arrays can implement near-field and far-field measurements using intensity mapping. In addition, with the necessary software, acoustic pressure can be mapped while particle velocity/ acoustic intensity measurements are being mapped. Handheld 2D arrays are available for troubleshooting and portable applications, while standalone arrays can provide higher precision laboratory and engineeringgrade measurements. Various array structures are available that are suited for di erent applications:

Ring arrays are suitable for beamforming and are used indoors and outdoors for far-field and nearfield measurements.

Star arrays are also suitable for beamforming and are mostly used for far-field measurements.

Fibonacci arrays are suitable for holography or beamforming and can be used for near-field and far-field measurements with the same array. Based on a Fibonacci spiral pattern of the microphones, these arrays can provide a wider dynamic range than other structures.

Paddle arrays are good lowfrequency, near-field measurement tools (Figure 1).

3D acoustic measurements

3D acoustic cameras consider surface nonlinearities and correct errors in measuring the distance between the microphone and the surface being measured. These cameras use a 3D model of the surface or space being analyzed. If the camera encounters a sound from a source that is not included in the model, errors can arise, like mapping the sound to a random location, or the sound may be eliminated from the measurements.

3D acoustic cameras are especially suited for analyzing enclosed spaces like room or vehicle interiors. These cameras consist of a sphere of microphones, with each microphone pointing outward perpendicularly to the surface of the sphere, that can provide omnidirectional sound measurements (Figure 2). These cameras often employ beamforming with the measurements mapped into 3D point clouds or a 3D computer-aided drawing (CAD) model of the environment being measured.

Point-and-shoot acoustic imagers

A handheld acoustic imager has been developed for applications like identifying leaks in compressed air, gas, steam, and vacuum systems and detecting and localizing partial discharge conditions in insulators, transformers, switch gears, or high voltage (HV) powerlines. The 64 microphones in the acoustic array operate from 2 to 100 kHz with a detection range of up to 120 meters. The array has a field of view of 63° ± 5° and takes images at 25 frames per second. The integrated digital camera has the same field of view plus a 3x digital zoom capability with a resolution of 5 megapixels. In addition to displaying still images, the system can take videos up to 5 minutes long (Figure 3). To minimize background noise interference, the imager automatically compensates for background noise and has multiple bandwidths that are selectable via manual inputs or with usermade presets.

Sound scanners

Devices called sound scanners are available that can simulate up to 480 microphone positions using 5 microphones on a boom that’s rotated in a circle (Figure 4). The scanner has been developed specifically for use in field measurements since it’s lightweight and compact, making it easy to transport. It’s intended for use in building acoustics and environmental noise measurements. The 1.32-meter diameter of the sensor structure means that precision visualization of lowfrequency sounds is supported.

Two arrays can be better than one

A new method has been proposed that uses two microphone arrays with only two microphones in each array to form “left” and “right” channels. TDOA was used to convert the channels into angles using simple GCC without additional algorithms (Figure 5). A prototype of this two-channel approach produced a position error of about 2.3 cm and an angle error of about 0.74°using only four microphones in an indoor environment.

Summary

Acoustic cameras are a well-established technology and include both 2D and 3D imaging systems. They use algorithms based on techniques like beamforming, sound intensity measurements, and acoustic holography. The addition of AI is enabling the development of high-performance acoustic cameras at a lower cost. In addition, techniques are being developed that enable fewer microphones to be used to produce high-accuracy measurements, further reducing the cost of employing acoustic cameras. DW

This engineer just set up several ultrasonic sensors for a new machine line. Despite the varying ranges he had to set, he used a single software application. He set the distances. He adjusted gain. He filtered out anomalies. And those settings will remain for future replacement sensors.