Improvements in or relating to the measurement of acoustic impedance

Measurement of the complex acoustic impedance of a sample 1 of material comprises immersing the sample in a fluid medium 7, producing in the medium an acoustic field e.g. by an annular transducer 8, measuring at a surface of the sample using a pressure transducer 2 a pressure Po at the medium due to the acoustic field, measuring the displacement Xo of the medium at the surface by means of a laser interferometer arrangement 10-16, producing electrical signals proportional to Po and Xo respectively and processing the signals to solve a predetermined equation. The fluid in which the measurement is carried out should be a liquid such as oil or water. The acoustic field is produced by the annular transducer 8 and may be maintained at a required level by an automatic gain feedback loop 19-23. <IMAGE>