Air-coupled ultrasonic is a non-contact technique which has become increasingly common for non-destructive testing, as more and more advanced materials cannot be contaminated during certain manufacturing processes by the various couplants used in regular ultrasonic testing
The air-coupled ultrasonic technique has in addition shown to be very efficient and fast for the testing of large areas, where the application of plate waves can cover long distances, and where the absence of water columns allows for high scan velocities.
The main reason for the use of a couplant lies in the large differences in impedances between air and any solid material: The impedance for air is in the order of 100 Rayl, whereas liquids and solids have impedances in the MegaRayl range. Couplants are used to reduce this impedance mismatch. Without a couplant, i.e. with air as the natural couplant, the impedance mismatch causes the following typical high reflection losses:Specimen : 60 - 90 dB
Transducers: 90 dB
Total Reflection Losses: 180 dB.
Another restriction to be considered for the application of air-coupled ultrasound is the sound attenuation in air.The following graph was calculated based on values found in: P.E. Krasnushkin, Phys Rev 190-193, 1944.
There are several techniques available today to overcome above mentioned obstacles, and to make air-coupled ultrasound an efficient and reliable method for the non-destructive testing industry. These techniques involve the following routes:
(a) High sound pressures can compensate for the high reflection losses.This can be obtained by using high voltage transmitters and by applying tone bursts to resonant transducers.
(b) High sensitivity of the receiving transducer, combined with low-noise preamplifiers. The newest tools in this category are the digital filters, which can significantly increase the signal to noise ratio of weak signals.
(c) Impedance matching layers for the coupling of the transducers to air. New materials have reduced these impedance mismatch losses, leading to a higher sound pressure in air and to a higher sensitivity.
(d) Optimizing the frequency for the specific application.
It is a characteristic of air-coupled ultrasound, that plate waves can travel longer distances, without being dampened by a couplant.This allows to use several transducer configurations with interesting results.
Some results and additional information can be found in the following publications:
The air-coupled ultrasonic technique has become a reliable and indispensable method in nondestructive testing. The availability of various frequencies and digital filters allows an optimization between high resolution for composites and honeycombs (e.g. 0.5 to 1 MHz) and high penetration for foam, pultruded materials, rubber, tires, and wood (20 to 200 kHz).