At certain frequencies, the signal produces acoustic resonance in the liquid inside the container. The result is a series of resonance peaks in a spectrum. These are detected by a second piezoelectric transducer that works as a receiver. Information about the contents is derived from an analysis of the spectrum of resonance peaks. A single sweep measurement can be used to derive liquid properties that include sound speed, sound absorption, frequency dependence of sound absorption, and liquid density.
Sound speed is determined from the spacing of the peaks.
Sound attenuation at a given frequency is determined by the width of the resonance peak. Density is determined from measurements of peak width over a wide band of frequencies followed by extrapolating the peak width to zero frequency, giving the value of the acoustic impedance ratio of the wall to the material. Viscosity is determined from the interference of sound waves that go around the circumference of the container and appear superimposed on top of the liquid spectrum. In SFAI, deriving so much information from the one measurement is important, particularly in the area of chemical diagnostics. Many chemical compounds can have the same sound speed, for example, but none is identical across all four parameters.