Transducer Beam Profiling

Beam profiling is the means to measure where the sound energy goes and how it gets their. The classic method is Schlieren which shows a cross-sectional image of the sound traveling in transparent material. Although Schlieren is a powerful tool it also has a powerful price tag and so is not available to most of those in ultrasonic testing.

The classical means of beam profiling for those of normal means is the ball on a stick method. The ball is bonded or soldered to a stick and the ball end of the stick is pointed at the transducer. This method works well but is usually used to only obtain the maximum amplitude at each point in a cross-section normal to the beam. That is no time or beam curvature is obtained. The ball itself has reverberations and creeping waves that distort latter time information so bandwidth as a function of time is hard to get. The actual sample taken by the ball is a function of the water path distance so that the profile in the near field is different than at the focal plane.

An alternative is to use conical shaped targets with the tip of the cone pointed at the transducer. The radius of the tip can be made as small as 62.5 micrometers which allows very fine sampling of sound beam. The problem of changes in the effective sample being a function of the water path is less but can be neglected for all practical cases. Fabricating the conical targets is expensive and the small tip is hard to find in a large tank.

The image at the top of the page is a profile in the near field of a 10 Mhz, 12.5 mm diameter, 32.5 mm focal length transducer. The contours are 10% of the peak value and the blue values are negative while the green and yellow are positive. This image was made with a piano wire 125 micrometers in diameter that has been made spherical with a file. The method of manufacture is illustrated in Fig. 1. The wire is held in a bit of plastic and ground flat so that the top of the wire is coplanar with the plastic surface. It is not necessary to have precise alignment of the wire because errors of 30 degrees can be tolerated without effect on the profiling result.

Beam profiling is a form of near field imaging because in principle the tip is acting as a small aperture. Resolution less than a wavelength is not possible because the sampled wave has to travel all the way back to the transducer and undergoes diffraction.

The target is easy to find because it is in the same plane as the disk surface. Sitting in a cavity created by the removed modelling clay. Diameters smaller than 125 micrometers are probably possible.

The images above of a 10 MHz transducer were made with the 125 micrometer wire and illustrate the more conventional display of amplitude only. Note that these images provide entirely different views of the same data. The only change is the computer color lookup table. The field of view is 25 x 75 mm and the images have been decimated to reduce their size with some loss of resolution.
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