The extensional velocity and internal friction are measured in a series of partially melted alloys having different volume fractions of fluid and different dihedral angles. The acoustic properties change discontinuously when melt forms: the velocity decreases while Q-1 usually, but not invariably, increases. The present results, in conjunction with previous studies, form a broadly consistent picture of acoustic properties in partial melts. The dihedral angle has a major effect; the extensional velocity decreases much more rapidly with increasing melt content when the liquid wets the grain boundaries. The data set an approximate upper limit to the volume of fluid which can be present in the low-velocity zone (LVZ) of the upper mantle and be consistent with seismic data, assuming that the LVZ is due to partial melting and the seismic frequencies are much less than the peak frequency for fluid motion. If the dihedral angle of the melt in the LVZ is zero, only several tenths of 1 vol % of fluid can be present, and less than 5 vol % if the dihedral angle is 60¿.