Near Field Operation - a Further Discussion

Fig. 1 and Fig. 2 are showing the longitudinal structure (xz cross section) of the sound field in the near range in water. The probe is a B2S-N (2 MHz, 24 mm crystal diam.) and is located at z=0. This is a relatively narrow band probe with a -6 dB bandwidth of 40% approx. All measurements are made in transmit receive mode, i.e. a small hydrophon of 1 mm effective diam. is acting as the receiver for the sound wave transmitted by the probe into water. The measured dB steps of these free field conditionshave to be multiplied by a factor of two when calculating the impulse echo response for a small reflector. Fig. 1 is an isobaric chart with 3 dB steps between different colours (black, blue, red and green). The modulation in the near field is approx. 3 dB free field, i.e. 6 dB impulse echo. You clearly can see a deep minimum at N/2 (N = near field length). Fig. 2 is an isometric projection with the signal amplitude on the vertical axis with 1.25 dB per division. Fig. 3 shows a lateral cross section of the sound beam near to the probe surface. From all 3 figures you can see that we can expect maximum flaw indication off axis depending on the distance from the probe. Only for distances above 0.6 N you will observe a single maximum on axis. Therefore, the probe should be adjusted in a distance above 0.6 N if possible.

G. Splitt Krautkramer, Huerth

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