These electrical signals are thenamplified at their high ultrasonic range of36 to 44 kHz, which results in anamplitude modulated signal with a centralor carrier frequency near 40 kHz. Thisstage corresponds to the radio frequencyamplifier stage of the conventionalamplitude modulated radio receiver. Theamplified high frequency input signal isthen mixed with a frequency derived froman internal oscillator to provide adifferent frequency signal in the audiblefrequency range.The airborne ultrasound detector orcontact probe receives the high frequencyultrasonic mechanical vibrationsgenerated by leaks and converts thesesignals to high frequency electricaloscillations. These electrical signals arethen amplified and heterodyned. Finally,the audio frequency signal is amplifiedand reproduced by loudspeaker orheadphones. This audible leak signal isinterpreted by human listeners as thetypical sounds of hissing leaks onvibrating objects. In other words, theultrasound leak detector is only slightlydifferent from a radio receiver, in that theoriginal frequency range is ultrasonic(near 40 kHz) rather than in theconventional amplitude modulatedbroadcast frequency range (0.55 to1.4 MHz). From these audible signals, it ispossible to analyze the amplitude andcharacteristics of ultrasonic signals fromleakage.Inspection ModulesThe two main test techniques fordetecting passive ultrasound leaks aredistinguished by their probes:(1) ultrasonic scanning to detect the sonicsignal transmitted though air from thetest object and (2) use of a stethoscopeprobe to detect ultrasound by directcontact with the surface of the test object.The ultrasonic detection scanning probecan be used as an ideal system for leaktesting and signature analysis.The techniques of leak testing with anultrasound detection scanning probe aresimilar to the technique used with theultrasound detection contact(stethoscope) leak test probe. In addition,improved scanning probes have adjustablefrequency controls enabling the user totune the frequency generated by the leak.The bandpass is adjustable, not only toallow selection of the frequencies unique191Acoustic Leak TestingPART2. Instrumentation for Ultrasound LeakTestingFIGURE9. Block diagram of an ultrasound leak detector.AmplifierModulatorAmplifierTransducer20 to 100 kHzMeterLocal oscillator20 to 100 kHzHeadphones50 Hz to 5 kHz
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to the leakage frequencies, but also todiscriminate against artifact noises. Thecontact probe differs from an ultrasounddetection scanning probe in which thecontact piezoelectric sensor (transducer) ismechanically coupled to the devicesuspected of having a leak.The flexibility of adjusting ultrasounddetection systems for operation in variousenvironments makes these systems veryattractive for isolating the sound sourcesof interest to an inspector. In addition,the detected sound may be furtheranalyzed with a spectrum analyzer todifferentiate the normal machinerynoises, such as sound emanating from
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