Two limitations on the animal-hide gelatin and graphite powder tissue equivalent (TE) materials are that they cannot be produced consistently with speeds of sound less than 1570 m/s at room temperature (22.degree. C) and that irreparable damage can result if the materials are raised to temperatures above 32.5.degree. C. An acceptable substitute polysaccharide gel (agar) has a high melting point (78.degree. C) and can be made to exhibit speeds of sound in the range 1498 m/s to over 1600 m/s at 22.degree. C. Thus TE material made with agar is environmentally stable and can be manufactured to exhibit the important speed of sound, 1540 m/s.