These electrical signals are then amplified at their

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These electrical signals are then amplified at their high ultrasonic range of 36 to 44 kHz, which results in an amplitude modulated signal with a central or carrier frequency near 40 kHz. This stage corresponds to the radio frequency amplifier stage of the conventional amplitude modulated radio receiver. The amplified high frequency input signal is then mixed with a frequency derived from an internal oscillator to provide a different frequency signal in the audible frequency range. The airborne ultrasound detector or contact probe receives the high frequency ultrasonic mechanical vibrations generated by leaks and converts these signals to high frequency electrical oscillations. These electrical signals are then amplified and heterodyned. Finally, the audio frequency signal is amplified and reproduced by loudspeaker or headphones. This audible leak signal is interpreted by human listeners as the typical sounds of hissing leaks on vibrating objects. In other words, the ultrasound leak detector is only slightly different from a radio receiver, in that the original frequency range is ultrasonic (near 40 kHz) rather than in the conventional amplitude modulated broadcast frequency range (0.55 to 1.4 MHz). From these audible signals, it is possible to analyze the amplitude and characteristics of ultrasonic signals from leakage. Inspection Modules The two main test techniques for detecting passive ultrasound leaks are distinguished by their probes: (1) ultrasonic scanning to detect the sonic signal transmitted though air from the test object and (2) use of a stethoscope probe to detect ultrasound by direct contact with the surface of the test object. The ultrasonic detection scanning probe can be used as an ideal system for leak testing and signature analysis. The techniques of leak testing with an ultrasound detection scanning probe are similar to the technique used with the ultrasound detection contact (stethoscope) leak test probe. In addition, improved scanning probes have adjustable frequency controls enabling the user to tune the frequency generated by the leak. The bandpass is adjustable, not only to allow selection of the frequencies unique 191 Acoustic Leak Testing P ART 2. Instrumentation for Ultrasound Leak Testing F IGURE 9. Block diagram of an ultrasound leak detector. Amplifier Modulator Amplifier Transducer 20 to 100 kHz Meter Local oscillator 20 to 100 kHz Headphones 50 Hz to 5 kHz
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to the leakage frequencies, but also to discriminate against artifact noises. The contact probe differs from an ultrasound detection scanning probe in which the contact piezoelectric sensor (transducer) is mechanically coupled to the device suspected of having a leak. The flexibility of adjusting ultrasound detection systems for operation in various environments makes these systems very attractive for isolating the sound sources of interest to an inspector. In addition, the detected sound may be further analyzed with a spectrum analyzer to differentiate the normal machinery noises, such as sound emanating from
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  • Fall '19
  • Acoustic Emission

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