Time and Frequency Domain Measurements of Materials with High Ultrasonic Attenuation Using Time Delay Spectroscopy

Abstract

Naval Su rface Weapons Cente r Silver Spring, Marylanrl 2oqo3-5000 Time delay spectrometry (TflS) uses a swept frequency source and a tracking receiver in the place of a conventional pulse-echo system for ultrasonic measurements. This technique can directly provide a frequency domain display of amplitude and even phase. Since the signal is of longer duration than that of a pulse-echo system, a large amount of averaging is intrinsic with this method, making it ideally suited for the study of highly attenuating materials. PRINCIPLESThe operation of TDS in the frequency domain has been treated elsewhere [l ,?l and exemplary data shown on measuring attenuation [ll particularly of highly attenuating materials. With suitable processing, Tns can provide better time domain information than pulse-echo methods, with agreat signal-to-noise improvement from compressing the longrluration signal.The system in Figure l demonstrates both the time and the frequency domain operation of TDS in the single transducer [31 reflection mode. The transducer is energized by a source whose frequency is linearly swept in time. If the sounrl velocity does not vary appreciably over the frequency range swept, the reflected signals exhihit a linear frequency-vs-time trajectory that is offset from the corresponding trajectory of the transmitter by a frequency that is proportional to the time delay of propagation. If the linearly swept frequency i s subt racted from the recei ved si gna l (as by het rodyne and selecting the difference frequency), the result is a narrow hand signal with frequency components that remain practically steady throughout the sweep. Each component represents the reflection of ultrasonic energy from interfaces at a particular range. These specific range components can be selected by further filtering.The best performance of any hetrodyne scheme is obtained when the amplitudes of the signals are appropriate. The hybrid coupler provides the proper levels by attenuating (but not eliminating) the signal from 759

A closer look at ultrasonic attenuation and heating in a tissue-mimicking material

“…attenuation measurement) and the broadband calibration of transducers (e.g. hydrophones) (Gammell et al 1979, Gammell 1986, IEC 62127-2 2013. There is essentially no difference in methodology between the two processes, in that both involve a subtraction of logarithmic spectra to obtain an attenuation curve (via using two difference thicknesses of material) or a hydrophone response (via using reference and test hydrophones).…”

Towards a Quantitative Ultrasonic NDE of Thick Composites

“…Included here are the random signal systems developed by Newhouse, et ai. more than a decade ago [5] and the time-delay spectrometry systems (TDS), which were pioneered by Heyser [6] and applied to measurements in ultra-absorptive materials by Gammell [7]. Both systems operate at low peak ultrasonic power, with the peak power approximately equal to the average power.…”

“…Both are high-gain systems and immune to interference signals. Materials representing insertion losses up to 150 dB have successfully been measured with TDS systems [7]. Such systems can also extract either time-or frequency-domain data and they can be assembled from off-the-shelf electronic components.…”