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How it works

Acoustography* is a full-field, area inspection technique similar to radiography. 

At the heart of the acoustography method is a novel two-dimensional (2-D), super high-resolution proprietary ultrasound area detector called AO sensor that converts ultrasound directly into a visual image, which can then be electronically captured using a digital camera for computer storage and image enhancement.

For Through-Transmission Ultrasonic (AO-TTU) inspection, the test specimen is ultrasonically illuminated using a sound source from one side.  The ultrasound beam is differentially attenuated as it propagates through the test specimen, casting an ultrasonic “shadow” that is instantly converted into a visual image by the AO sensor. 

TTU acoustography inspection methods are performed similarly to conventional UT, employing a water tank system.

Acoustography is also being adapted for single-sided (pulse-echo) inspection (AO-RMU) testing without an immersion tank.

*Nondestructive Testing Handbook, 2nd Edition: vol.9, Special Nondestructive Testing Methods, Columbus, OH, American Society for Nondestructive Testing (1995), p278-284.


Image formation Principle

The geometrical resolution capability of the acoustography ultrasonic image can be assessed using classical diffraction theory. 

An opaque target (e.g. defect) is recognizable as long as the distance, L, between the target and the plane of detection (i.e. AO sensor) is less than R, the Fresnel or Near Field Diffraction length, which is given by (**):

R=D2/ l


D= target diameter

l = ultrasound wavelength

     in the test material

l=c/f, where c is the ultrasound velocity in the test material and fis the ultrasound frequency of the illuminating ultrasound plane waves.

The Acoustic Contrast Ration of the geometrically-detected defect depends on:

ACR= Ad / Am


Ad= Defect Attenuation

Am=Test material Attenuation

**Hecht, "Optics," 2nd Ed., Pub. Addison Wesley, 1987, pp 396.


How defect becomes visible


A 2D x-ray like ultrasound detector, called Acousto-Optic Sensor (AO sensor), is used to detect and instantly convert the the ultrasound image of the defect into a corresponding optical (visual) image.

The AO sensor has a super high, x-ray film like, resolution because ultrasound is sensed by a continuous layer of a proprietary "mesophase" material; sensing molecules of the layer are on the order of liquid crystal material with sensing molecules ~20 Angstroms.  Therefore, the geometrically detected defect is converted into a visual image with high resolution. 

The defect becomes optically visible because the ultrasonically-exposed region (ultrasound image) becomes birefringent, which induces a brightness change under polarized light.  The degree of brightness change depends on the ultrasonic intensity.  Therefore, the ultrasonic image, which is a distribution of ultrasonic intensity, gets converted into a corresponding optical image with brightness variations corresponding to ultrasonic variations.