Self-Test of Air-Coupled Probe for Sensitivity Map Production Using Parabolic Reflector.

The spatial distribution of a transduction efficiency over the air-coupled probe surface was proposed as a convenient tool for the probe integrity inspection. In this research, a parabolic reflector is used for passive focusing of the acoustic wave on the surface of inspected probe. Therefore, no additional transducer is required for inspection: probe is used for self-inspection. This approach allows to avoid the expensive focused transducer and replaces it by the same-type air-coupled transducers as probe under test. Moreover, the use of the parabolic mirror for focusing is frequency-independent; therefore, such approach allows to inspect a wide range of the probes. Spread spectrum signals were used for excitation to improve the SNR and bandwidth coverage. The results of the experimental measurements of air-coupled transducer sensitivity map with natural and artificial defects implemented have been presented. Comparison with previously proposed techniques is given. It was found that defects presence distorts the focused beam, creating large sidelobes. Therefore, sensitivity map obtained with the proposed technique is lower quality than with previously proposed techniques. Beam profile measurements using a miniature microphone have been presented. Aperture-limiting mask has been proposed to reduce the sidelobes arising due to defects presence and resulting measurements quality has been improved.

Air-Coupled Ultrasonic Probe Integrity Test Using a Focused Transducer with Similar Frequency and Limited Aperture for Contrast Enhancement.

Air-coupled ultrasonic probes require a special design approach and handling due to the significant mismatch to the air. Outer matching layers have to be soft so can be easily damaged and excitation voltages might cause the degradation of electrodes or bonding between the layers. Integrity inspection is desired during design, manufacturing, and exploitation. Spatial distribution of a transduction efficiency over piezoelement surface is proposed as a convenient means for the air-coupled probe integrity inspection. Focused transducer of similar center frequency is used to scan the surface of the inspected probe. However, such approach creates a challenge, i.e., area of the scanning beam is much smaller than the total receiving area of the inspected probe, therefore, contrast and imaging resolution are significantly degraded. Masking aperture made from cardboard and felt, placed at the focal point was proposed as solution. Far-range sidelobes were suppressed down to the noise floor (-50 dB) and the near-range sidelobes were reduced down to -17 dB. The proposed modification allows to use a similar frequency focused transducer. Probe integrity inspection can be carried out at significantly enhanced contrast and lateral resolution. Natural and artificial defects can be detected by the use of the proposed method.