As noted in the ASTM E1065 Standard Guide for Evaluating Characteristics
of Ultrasonic Transducers, 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
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 transducer under evaluation, for
example, immersion, contact straight beam, or contact angle beam.
Typical time and frequency domain plots provided
by transducer manufacturers
Frequency Response--The frequency response of the above
transducer has a peak at 5 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. 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