Gas-coupled transducers for pulse-echo operation

The pulse-echo ultrasonic technique is widely used in industrial NDT for the detection of internal discontinuities, or cracks and as thickness gauge. Water immersion, gels or dry contract are normally used to couple transducers to the test specimen. However, there are practically no applications using pulse-echo technique and coupling by air or gases. This is mainly due to: 1) the reduced frequency bandwidth and center frequency of the existing air-coupled transducers (long pulse duration and long dead zone), 2) the limited transducer sensitivity and 3) the almost negligible amplitude of the back wall (or internal surfaces) echoes compared with the front surface echo. Actually all industrial applications of air-coupled ultrasound operate in through transmission or pitch-catch mode. This works shows a transducer design approach to simultaneously optimize both sensitivity and frequency bandwidth that make possible pulse-echo applications using air-coupled ultrasound. The realization of this design reveals that it is possible to build transducers with 1 MHz center frequency with a high sensitivity (up to −16 dB at 5 bar) and 6–dB bandwidth (∼50%), capable to operate both at room conditions or under pressurized gases, in this case, sensitivity increases linearly with gas pressure.