Materials Characterization by Time Delay Spectrometry Ultrasound

A technique known as Time Delay Spectrometry (TDS), which has been used at JPL for ultrasonic tissue characterization, has promise for similar application in materials characterization as well. This technique differs from the approaches based on pulse-echo techniques which are used by other workers. Time Delay Spectrometry operates in the frequency domain directly. The transducer is excited by a rapidly swept frequency source and a tracking receiver is used to select signals arriving during a narrow time interval. In the reflection mode this time interval represents the range of the reflecting surface. In the transmission mode this time interval is adjusted to the desired acoustic delay, causing rejection of signals which follow extraneous paths. This swept frequency implementation makes coherent processing of the full analytic signal possible, which in turn allows more representative signatures to be obtained. In the reflection mode, for example, a better indication is obtained of the true strength of an interface or scatterer because the response can be made less dependent on the interference effects that so greatly alter the amplitude peaks of the conventional echo. This technique also permits an enhanced dynamic range to be obtained by applying frequency compensation directly to the transmitted signal. An added bonus is the ability to use data logging systems at rates commensurate with microprocessor operation in place of more expensive high speed transient recorders with limited memory capacity. Attenuation spectra taken on tissue specimens and on a few material samples will be presented. These data will demonstrate the ability of Time Delay Spectrometry to either minimize reverberation artifacts or to make use of the information contained in the artifact. INTRODUCTION Time Delay Spectrometry* (TDS) is an anechoic ultrasonic measurement technique which operates in a domain that is intermediate between the time domain and the frequency domain. This technique has been used in many other fields and is a potential method for directly measuring a "materials signature". Time Delay Spectrometry was originally developed for loudspeaker testing, where it allows an ordinary room to be used and yet achieves results superior to those obtained by other electronic techniques, even when they are aided by the use of an expensive anechoic chamber. ( 1 ,2) This technique was later applied to ultrasonic tissue characterization where it has been used to measure the ultrasonic attenuation coefficients, reflectivity, and velocity of biological specimens as a function of frequency. (2,3,4,5,6) Most recently it has been applied for imaging undersea sediments by ultrasonic reflection, with a resolution and discriminatiory ~uperior to that of pulse-echo techniques. \7 J