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

Abstract

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.

Ultrasonic Needle Hydrophone Calibration in Air by Parabolic Off-Axis Mirror Focused Beam Using Three-Transducer Reciprocity

“…An aperture mask (4 mm diameter, 3 mm away before the focal spot, refer Fig. 4) was used to reduce the effect of sidelobes in the beam [11]. The presence of the aperture also solves the beam location issue.…”

“…Furthermore, the size of the sensing element of the receiver, according to [3], should be less than twice the central frequency wavelength of the probe under test, making such a microphone suitable up to 218 kHz, whereas the required size at 1 MHz would be 0.7 mm. Needle hydrophones are intended for immersion use in water, but offer an attractive alternative for use in air, as has been demonstrated previously [10,11]. However, needle hydrophones are not characterized for use in air as calibration is typically done in water for frequencies above 1 MHz, but air-coupled ultrasound is usually below 1 MHz [12].…”

Miniature Ferroelectret Microphone Design and Performance Evaluation Using Laser Excitation

“…The evaluation of the size of the sensitive area size can be performed using a focused transducer, scanning over the probe's surface [42], [43]. However, the focused probe's beam should be 50 μm diameter or smaller, in order to scan the 0.5 mm diameter area.…”

Compact Laser Driver for Ultrasonic Arbitrary Position and Width Pulse Sequences Generation

“…Alternatively, a 75 W laser at a 100 m spot diameter can produce 10 6 W/cm 2 , which is close to the power densities that provide sufficient acoustic output (assuming 100% absorption). Many application can benefit from a small spot size: i) a wide acoustic beam is required in SAFT imaging [65]- [68]; ii) if the material investigated is thin, then the acoustic beam can be considered already focused so high lateral resolution is attained [70]; iii) photoacoustic imaging requires small laser beam size [53]- [63]. However, laser beam spot size cannot be reduced indefinitely, and is usually limited by the emitting aperture size (10 m x 200 m is common) and the optical system magnification/reduction.…”