Air-coupled ultrasonic interferometry method

An air-coupled ultrasonic interferometric method, using an air-coupled ultrasonic transducer as a probe to directly face the surface of a workpiece, characterized in that:

• an ultrasonic wave is reflected back and forth between the ultrasonic transducer and the surface of the workpiece, and the phase difference of a first echo reflected from the surface of the workpiece and reaching the air-coupled ultrasonic transducer is measured;
• based on the change of the ultrasonic frequency and wavelength, the measured distance is transformed into the rate of change of the acoustic phase with respect to the acoustic angular frequency, wherein the change in the acoustic angular frequency is a product obtained by multiplying 2π by the bandwidth fB of the air-coupled ultrasonic transducer, and the change in the acoustic phase is a product obtained by multiplying 2π by the difference between the sound paths at the upper and lower boundary frequencies of the bandwidth respectively divided by the wavelength,
• wherein the relationship between the rate of change of the acoustic phase with respect to the acoustic angular frequency (i.e., the ratio of the acoustic phase difference to the acoustic angular frequency difference) and the sound path is as follows:
• L=λ●(ΔΦ/2π)/Δf/f)=c●ΔΦ/Δω,
• where ΔΦ is the acoustic phase difference, and Δω is the acoustic angular frequency difference;
• wherein the rate of change of the acoustic phase with respect to the acoustic angular frequency is a constant linearly related to the measured distance, and can also be transformed into the difference between the frequencies corresponding to two adjacent phase periods, having the relationship with the sound path as follows:
• L=c/Δf=c/(f2−f1);
• and
• wherein if the acoustic angular frequency difference and the acoustic phase difference are replaced by the relationship between the highest frequency F2 (whose corresponding sound path is an integer multiple of the wavelength) and the lowest frequency F1 (whose corresponding sound path is an integer multiple of the wavelength) within the bandwidth fB of the air-coupled ultrasonic transducer, the formula of the sound path will be converted as follows:
• L=c●(N2−N1)/(F2−F1).