structure having the leak, undampedultrasonic piezoceramic transducershaving a strong radial response areparticularly sensitive to leakage noise.In addition, instruments for mostacoustic emission test systems havebandpass filters, which tend to have aslight peaking effect at the edges of thebandpass frequency range when drivenwith random frequency (noise) signals. Ifthe bandpass is set for the radialresonance of the sensor, a very highsensitivity can be obtained for leakagesignals. The contact sensor is frequentlymuch more sensitive to leakage noisethan are air coupled sensors. Therefore,leaks that may be classified as passivebecause the air coupled sensor lacks thesensitivity required for their detectionmay prove to be violently active with acontact sensor.Coupling Ultrasound Sensors toStructures during Leak TestingWith acoustic emission systems, contactsensors coupled indirectly to structureswith ultrasonic waveguides will not havethe sensitivity of sensors ultrasonicallycoupled directly to the subject structure.Ultrasonic coupling means that the sensorface is coated with an oil, short fibergrease, resinous material or an adhesiveand pressed into intimate contact oraffixed to the structure for the purpose ofeliminating an air interface between thesensor face and subject structure.However, some coupling fluids mayreact with the test material, causingcorrosion or erosion acoustic emission(noise) that may remotely resembleintermittent leakage noise. The fluidcausing the corrosion or erosion noisemay be either the couplant or the fluidcontained by the structure. Such noise isusually detectable by only the mostsensitive vibration sensinginstrumentation.Airborne Acoustic Signals for LeakTestingWhen structures cannot be monitored bydirect coupling of sensors to theirsurfaces, air coupled or water coupledmicrophones can be used to detectultrasonic emissions generated by leakage.Air coupled sensors are convenient but aremost usable on active, external leaks. Thesensitivity and directivity of remotemicrophonic detectors can be enhancedby addition of parabolic reflectors.Leaks at high velocity generate broadbandwidth amplitudes that center atfrequencies of about 40 kHz (40 000vibration cycles per second). Structuresimmersed in (or filled with) liquids arealso observed to generate about 40 kHzpeak signal amplitudes at the onset ofhigh velocity, low volume leaks.Ultrasound leak detectors are oftendesigned to respond to this 30 to 50 kHzsignal frequency range and signals atother random frequencies are suppressed.This reduces interference from machineryor other ambient noise sources.Artificial Sound Sources for LeakTesting in Large ContainersLarge containers can be leak testedwithout internal pressurization by placingan ultrasound generator in the fluid insidethe container. An ultrasound detector isthen moved about the outside of thecontainer until a sudden increase in theultrasonic signal amplitude is observed.
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