Acoustic methods

Acoustic methods (also known as sonic testing, vibration testing, mechanical impedance testing) depend on exciting vibrations in a specimen by a local impact and then measuring some properties of the vibrations, eg resonant frequency, decay time, etc. The most common acoustic method is the modern equivalent of the railway wheel tapper, using a controlled-energy impact instead of a hammer, and a transducer detector instead of ears. Electronic or digital data analysis and recording may also be used.

There are several other important applications of acoustic testing to specific products, the most important being the testing of cast-iron castings, where it has been shown that this can be related directly to the resonant frequency of the casting. Hard components all have their own resonant frequencies, and these are dependent upon the dimensions and stiffness of components. Stiffness is, in turn, dependent upon the microstructure, hardness and presence of defects. Changes in any of these dependencies will cause changes in resonant frequencies, and the components having differences from others can be detected by these changes.

Resonant inspection can be used to detect cracks, voids, delaminations, lack of bonding and changes in hardness, as well as dimensional outliers and components with one part of the production process missing or wrongly performed (coating, grooves machined, fins present etc). In addition, a ‘fingerprint’ of a high-value component can be taken, and subsequent inspections can compare the newly acquired fingerprint with the previously recorded one as a condition monitoring technique. The technology cannot ‘detect’ or identify specific defects, but can detect changes in components relative to the reference components.

As such, therefore, resonant inspection is a technology which encompasses more than pure NDT, including techniques that can be applied to engineering components to automate the inspection processes.  Bonded joints can also be examined by a similar technique using local changes in vibration decay.
Vibration measurements are also used in plant condition monitoring.

Acoustic ranging

An acoustic ranging apparatus directs a burst of acoustic energy towards an acoustically reflective target object. The time it takes for an echo to return to the ranging apparatus is measured to determine the distance between the ranging apparatus and the object. The echo is received, amplified and compared with a threshold voltage, which decays in time in a manner which simulates the attenuation of acoustic energy as a function of distance travelled in the medium. The attenuation function is substantially simulated over the distance range that the ranging apparatus is intended to accurately measure by means of a simple, low-cost first order linear circuit. The ranging apparatus responds only to echoes having a magnitude which exceeds the threshold voltage. Spurious or false echoes, such as those due to reflection of acoustic energy in the side lobes produced by an acoustic energy transducer in the ranging apparatus, are ignored.