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How to Test Ultrasonic Transducers?

Manufacturers often provide time and frequency domain plots for each ultrasonic transducer. The signals below were generated by a spiked pulser. The waveform image shows the test response signal in the time domain (amplitude versus time). The spectrum image on the right shows the same signal in the frequency domain (amplitude versus frequency). The signal path is usually a reflection from the back wall (fused silica) with the reflection in the far field of the ultrasonic transducer.

Ultrasonic transducer Echo Wave and Spectrum

 

Other tests may include the following:

Electrical Impedance Plots – provide important information about the design and construction of a transducer and can allow users to obtain electrically similar transducers from multiple sources.

Beam Alignment Measurements – provide data on the degree of alignment between the sound beam axis and the transducer housing. This information is particularly useful in applications that require a high degree of certainty regarding beam positioning with respect to a mechanical reference surface.

Beam Profiles – provide valuable information about transducer sound field characteristics. Transverse beam profiles are created by scanning the transducer across a target (usually either a steel ball or rod) at a given distance from the transducer face and are used to determine focal spot size and beam symmetry. Axial beam profiles are created by recording the pulse-echo amplitude of the sound field as a function of distance from the transducer face and provide data on depth of field and focal length.

As noted in the ASTM E1065 Standard Guide for Evaluating Characteristics of ultrasonic transducer, the acoustic and electrical characteristics which can be described from the data, are obtained from specific procedures that are listed below:

Frequency Response – The frequency response may be obtained from one of two procedures: shock excitation and sinusoidal burst.

Relative Pulse-Echo Sensitivity – The relative pulse-echo sensitivity may be obtained from the frequency response data by using a sinusoidal burst procedure. The value is obtained from the relationship of the amplitude of the voltage applied to the transducer and the amplitude of the pulse-echo signal received from a specified target.

Time Response – The time response provides a means for describing the radio frequency (RF) response of the waveform. A shock excitation, pulse-echo procedure is used to obtain the response. The time or waveform responses are recorded from specific targets that are chosen for the type of ultrasonic transducer under evaluation, for example, immersion, contact straight beam, or contact angle beam.

Frequency Response – The frequency response of the above ultrasonic transducer has a peak at 15 MHz and operates over a broad range of frequencies. Its bandwidth (4.1 to 6.15 MHz) is measured at the -6 dB points, or 70% of the peak frequency. The useable bandwidth of broadband transducers, especially in frequency analysis measurements, is often quoted at the -20 dB points. Ultrasonic transducer sensitivity and bandwidth (more of one means less of the other) are chosen based on inspection needs.

Complex Electrical Impedance – The complex electrical impedance may be obtained with commercial impedance measuring instrumentation, and these measurements may provide the magnitude and phase of the impedance of the search unit over the operating frequency range of the unit. These measurements are generally made under laboratory conditions with minimum cable lengths or external accessories and in accordance with specifications given by the instrument manufacturer. The value of the magnitude of the complex electrical impedance may also be obtained using values recorded from the sinusoidal burst.

Sound Field Measurements – The objective of these measurements is to establish parameters such as the on-axis and transverse sound beam profiles for immersion, and flat and curved transducers. These measurements are often achieved by scanning the sound field with a hydrophone transducer to map the sound field in three dimensional space. An alternative approach to sound field measurements is a measure of the transducer’s radiating surface motion using laser interferometry.