Acoustic differentiation is largely based on quantitative A-scan echography (Ossoinig 1974). In order to obtain useful, reliable and comparable results, standardized A-scan instrumentation is set at a defined, reproducible, rather high system sensitivity (‘tissue sensitivity’). Citrated blood models have been used since 1966 as the only standard for setting an A-scan instrument at tissue sensitivity. The use of these biological tissue models, at preparation of the cell suspensions and the procedure of finding tissue sensitivity have been described in detail (Ossoinig 1974). The time-consuming preparation of such tissue models, their lack of durability and the limited accuracy inherent in any biological system such as this were considerable disadvantages. For these reasons calibration of standardized A-scan instrumentation remained difficult and restricted to some major centers.