Core Performance Evaluation Metrics for Acoustic Cameras-Part Ⅱ

1.1 Imaging Frequency Band Range

Imaging bandwidth refers to the acoustic imager can be normal imaging signal frequency range, unit Hz. array imaging bandwidth and microphone bandwidth is not the same indicator, most of the current MEMS microphone frequency response in the 20Hz ~ 80kHz broadband range. Array due to the limitations of the array aperture, imaging bandwidth range is difficult to do so wide as the response band of the microphone, most of the low-frequency band 2kHz ~ 10kHz there will be a significant imaging deviation.

Imaging bandwidth is not the wider the better, too wide a bandwidth will bring about a decrease in frequency resolution, resulting in a decrease in imaging accuracy and sensitivity of the device. Therefore, we mainly consider whether the imaging frequency range can cover the needs of monitoring application scenarios. Take partial discharge detection as an example, according to the requirements of the State Grid Corporation's corporate standard Q/GDW11061-2013 "Partial Discharge Ultrasonic Detector Technical Specification", for the non-contact ultrasonic detector, the detection band is generally in the range of 20kHz~60 kHz.

1.2 Imaging Dynamic Range

Imaging dynamic range is the difference between the upper and lower limits of the signal detected by the acoustic imager, which can be commonly understood as the difference between the smallest sound and the largest sound that the acoustic imager can "hear", in dB. If you have no idea of the source level, you can refer to the following figure:

Taking into account the actual power inspection application scenarios are long-distance non-contact detection, detection of most of the target is a weak discharge sound signal (assuming the existence of a very serious discharge sound signal, may have been detected), so the dynamic range of the imaging is not the greater the better, the acoustic imaging equipment for the detection of weak signals is often more important.

1.3 Localization Error

The localization error is the distance deviation between the actual sound source center point and the sound pressure maximum in the acoustic imaging area in the direction perpendicular to the axial direction of the acoustic array. Sound source imaging actually calculates the spatial angle of the sound pressure maximum in space relative to the array, so this position is related to the positioning angle error and distance, and the positioning error increases with distance.

1.4 Minimum Imaging Sound Pressure Level

The minimum imaging sound pressure level is the sound pressure level of the smallest acoustic signal that can be recognized and form a stable sound source image in the coaxial direction with the microphone array. The test method is generally 1 m from the source, the nominal frequency range of 1/3 octave center frequency, the imager can form a stable sound source imaging and positioning error in line with the requirements, the source 0.5 m at the sound pressure level for the lowest imaging sound pressure level.

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Minimum imaging sound pressure level is the core indicator of acoustic imaging equipment, the index of the good or bad will directly affect the detection performance of acoustic imaging equipment, such as some products customers feel more sensitive, some products customers feel that the measurement is farther away, in fact, are because of the product's minimum imaging sound pressure level is lower.

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However, taking into account the feasibility of measurement, the current description of the lowest imaging SPL refers to the imaging performance of the equipment at different single-frequency points. The performance at different frequency points varies from device to device.

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1.5 Multi-target Detection Performance

Multi-target detection performance describes some key performance of acoustic imaging equipment when detecting 2 and more than 2 targets, which mainly contains two sub-indicators: lateral spatial resolution and multi-target acoustic pressure difference.

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?Horizontal spatial resolution?

Lateral spatial resolution refers to the minimum distance between the sound centers of two sound sources that can be distinguished in the direction perpendicular to the axis of the acoustic imaging equipment. The smaller the index, the better, the smaller the lateral spatial resolution, indicating that the acoustic imaging equipment can simultaneously distinguish two closer sound sources the stronger the ability.

?Multi-target sound pressure difference?

Multi-target sound pressure difference is the maximum sound pressure level difference between two sound sources that can be accurately localized by the acoustic imaging device at the same time. In the operating frequency range, the larger this indicator, the better, the larger the multi-target sound pressure difference value, indicating that the acoustic imaging device can be simultaneously detected between the two targets of the sound pressure difference between the range of the larger.

1.6 Detection Distance

Detection distance is the farthest distance that an acoustic imager can directly detect the target signal, and it is also one of the performance indicators that customers are most concerned about. But in fact, there are many factors affecting the detection distance, and it is difficult for us to give the exact detection distance of a device without setting any conditions. The following factors will affect the detection distance of the equipment:

01

Array shape: the shape and size of the array will affect the resolution and directivity of the received signal in different directions in space.

02

Number of Arrays: The number of arrays in the array affects the spatial gain of the received signal and thus the detection distance.

03

Material and structure: Different design materials and structures of acoustic arrays have a great impact on the channel consistency and reverberation suppression ability, which also affects the detection distance.

04

Received signal frequency: low-frequency attenuation is slow and the propagation distance is long; high-frequency directivity is good and the attenuation is fast.

05

Environmental noise level: the current application environment of the noise sound pressure level, the greater the noise will lead to detection distance is greatly reduced.

In short, the detection distance is mainly determined by the detection sensitivity of the device itself and the signal-to-noise ratio of the received signal. Most of the current detection distance of the majority of equipment will be described as 0 ~ 100m, but do not mention the test conditions, a single mention of this indicator does not make much sense.

Conclusion

Partial discharge is an important cause of insulation breakdown of high-voltage electrical equipment, but also an important indicator of insulation degradation is the number one public enemy of grid safety. National regulations require substation inspection should focus on checking the equipment with or without corona, discharge, joints with or without overheating phenomenon, and the detection instrument should have discharge type identification function or give the possibility of various types of partial discharge.

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The principle of acoustic imager detecting partial discharge is to use the beam formation theory of sound source localization technology and AI environmental sound target recognition technology to detect, measure and locate the sound field signals generated by the equipment failure, visualize the presentation by means of color contour mapping, and spatially integrate the acoustic field maps with the video of the visible light, intuitively locking the position of the defective point of the sound source, and realizing the similarity to the thermal imager for the The detection of the temperature of the object is realized similar to the detection of the temperature of the object by the thermal imager.