Broadband Air-Coupled Ultrasound Emitter and Receiver Enable Simultaneous Measurement of Thickness and Speed of Sound in Solids

Abstract

Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the low air-to-solid power transmission ratios (10−3 to 10−8). Currently used resonant transducers trade bandwidth—a prerequisite for material parameter analysis—against pulse energy. Here we show that a combination of a non-resonant ultrasound emitter and a non-resonant detector enables the generation and detection of pulses that are both high in amplitude (130 dB) and bandwidth (2 µs pulse width). We further show an initial application: the detection of reflections inside of a carbon fiber reinforced plastic plate with thicknesses between 1.7 mm and 10 mm. As the sensors work contact-free, the time of flight and the period of the in-plate reflections are independent parameters. Hence, a variation of ultrasound velocity is distinguishable from a variation of plate thickness and both properties are determined simultaneously. The sensor combination is likely to find numerous industrial applications necessitating high automation capacity and opens possibilities for air-coupled, single-side ultrasonic inspection.

Air-Coupled Ultrasound Spectroscopy Air Parameters Compensation Technique

“…HE ultrasonic thickness gauging is a well-established technique [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The principle is based on measuring the delay for ultrasonic pulse traversed the material thickness.…”

“…The resonance ultrasound spectroscopy (RUS) is the solution for thin plates parameters measurement [3][4][5][6][7]. While initially applied in immersion setup, later it was expanded to electromagnetic acoustic transducers (EMAT) [7], laser ultrasound [9,10] and air coupled ultrasound [11][12][13][14]. Usually RUS-based plate parameters estimation involves two measurements: i) through-transmission between two transducers (addressed as calibration measurement) and ii) through-transmission between two transducers with test sample inserted (sample measurement).…”

Development of non‐contact foreign body imaging base on photoacoustic signal intensity measurement

“…On the other hand, the use of ultrasonic frequencies in PA measurement presents new challenges. Ultrasonic sensors generally require contact with the object surface through a coupling medium to improve the impedance between the object and the sensor 19,20 . Consequently, this measurement faces some challenges on certain objects such as small object areas, 21 uneven surfaces 22 and infections 23 .…”

“…Ultrasonic sensors generally require contact with the object surface through a coupling medium to improve the impedance between the object and the sensor. 19 , 20 Consequently, this measurement faces some challenges on certain objects such as small object areas, 21 uneven surfaces 22 and infections. 23 Some experiments have reported the use of audio frequencies to overcome this.…”

Special Issue on Acoustic Sensors and Their Applications (Vol. 1)

“…Bente et al [ 4 ] proposed broadband air-coupled ultrasound-emitting and -receiving transducers, demonstrating the combined use of a broadband thermoacoustic emitter and an optical microphone as receiver. They showed an initial application for simultaneous determination of thickness and sound velocity of a solid material.…”