Digital signal processing applied to ultrasonic absorption measurements

The signal pathway in a short pulse (< 10 ns) transmission system is described for two circular thick piezoelectric transducers aligned coaxially on either side of a small (30 ml) liquid filled test cell. Discrete models are used to describe transducer insertion and transient radiation coupling. Approximate inverse models are developed and used to deconvolve transducer and coupling effects so that acoustic absorption in the medium between the transducers can be estimated. The methods are applied to a prototype near plane wave absorption spectrometer for liquid materials. The instrument is controlled by a small computer that also carries out the signal corrections and other processing. The system is used to estimate absorption versus frequency (α, αλ, or α/ f 2) over bandwidths approaching 60 MHz. It has many advantages over conventional methods in that measurement is rapid (< 100 ms) and only a single pair of transducers is required. Preliminary results show excellent agreement with spot frequency absorption measurements. The apparatus has potential applications to absorption studies in non-stationary chemical systems.