  We experimentally observe the shear and secondary compression wave inside
soft porous water-saturated melamine foams by high frame rate ultrasound
imaging. Both wave speeds are supported by the weak frame of the foam. The
first and second compression waves show opposite polarity, as predicted by Biot
theory. Our experiments have direct implications for medical imaging: Melamine
foams exhibit a similar microstructure as lung tissue. In the future, combined
shear wave and slow compression wave imaging might provide new means of
distinguishing malignant and healthy pulmonary tissue.
