"Air Coupled Ultrasound - A Millenial Review"

Author: Joe Buckley, Sonatest Plc

Date of Publication: Presented at the 15th World Conference of NDT, Roma (Italy), 15-21st October 2000.

A modified version of this paper was presented at the 39th Conference of the British Institute of Non-Destructive Testing, Buxton, 13th September 2000

Extract from "Theory of Air-coupled Ultrasound" section from full article:

When sound passes across an interface between two materials only a proportion of the sound is transmitted, the rest of the sound is reflected. The proportion of the sound that is transmitted depends on how close the acoustic impedance of the two materials matches. Water is a fairly good match for most commonly used materials – for example typically around half the sound energy is transmitted at the interface between water and a carbon laminate. After four solid- liquid interfaces (from the probe, to the couplant, to the test piece, and then back again) there is still a few percent of the original energy left so accurate measurement is possible.

Conversely if the sound has to move between the test piece and air (which has very low acoustic impedance) only around 1% of the sound energy is transmitted. Thus after four transitions very little sound energy is left – Typically the overall path loss may be 100 dB higher using air as a couplant than when water is used. The path loss is significantly higher with metals, which have a high acoustic impedance compared to plastics which are lower in impedance.

It is therefore apparent that we must work hard to minimize losses at every stage if we are to achieve acceptable signal to noise for the inspection.

Follow this link to view the full article:
http://www.joe.buckley.net/papers/WCNDT_2000.pdf