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Leak testing by this active sonic technique depends

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Leak testing by this active sonic technique depends on transmission of the ultrasonic waves through the leaking fluid to the external atmosphere, rather than on the surface waves of the enclosing structure. Acoustic Detection for Locating High Voltage Electrical Leak Sources Another use made of ultrasonic leak testing equipment is that of locating sources of radio and television electrical 185 Acoustic Leak Testing F IGURE 5. Orifice conditions necessary to permit ultrasonic leak detection at distances shown in Fig. 4, where A is the cross sectional area of approach to the orifice and B is the cross sectional area of the orifice. A A B 20 B
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interference, corona discharges and high voltage insulation breakdown and arcing. The ultrasonic energy emitted by these electrical phenomena is similar in its frequency characteristics to the sounds such phenomena create on radiofrequency interference locators or in a portable radio. Each of these electrical phenomena is associated with leakage of electricity from bare or insulated conductors and with the resultant local ionization and heating of air or surrounding fluids. Remote Directional Sensing by Ultrasound Leak Detectors Because of the directional sensing capability of ultrasound leak test units, they can also be aimed at the sources of electrical leakage conditions producing acoustic emission from a distance. For example, electric power utility inspectors use ultrasound detection units to scan overhead transmission lines and pole mounted electrical hardware to detect sources of corona or arcing that lead to interference or to indicate dysfunctional equipment or insulation. Similar acoustic detection techniques have been applied to detection of shorted cable pairs within telephone cables. Instrumentation for Detection and Conversion of Ultrasound Leak Signals Instrumentation used in various applications of ultrasound leak testing shares a basic design. The sensor detects either an acoustic wave traveling over a solid structure or a longitudinal sound wave radiated into a gas or liquid. A preamplifier with a frequency band that extends about from 20 to 300 kHz is used to amplify the received signals. (In some devices, the preamplifier is assembled with the sensor to operate at a distance from the display or loudspeaker unit.) A second amplifier receives the preamplified signal and mixes this amplified signal with the output of a tunable oscillator. Four output frequencies result from mixing: (1) the original ultrasound leak signal frequency, (2) the original oscillator signal frequency, (3) the sum of these two frequencies and (4) the difference between these two original signal frequencies. The audible difference frequency is selected and amplified to drive headphones or a loudspeaker. In this way, the inaudible ultrasound leak signals are converted to sound signals within the range of human hearing (30 Hz to 16 kHz). The originally detected ultrasound leakage signal is amplified to drive a sound intensity indicating
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  • Fall '19
  • Acoustic Emission

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