  The analytical theories derived here for the acoustic load impedance measured
by a quartz crystal microbalance (QCM), due to the presence of layers of
different types (rigid, elastic and viscous) immersed in a fluid, display
generic properties, such as "vanishing mass" and positive frequency shifts,
which have been observed in QCM experiments with soft-matter systems. These
phenomena seem to contradict the well-known Sauerbrey relation at the heart of
many QCM measurements, but here we show that they arise as a natural
consequence of hydrodynamics. We compare our one-dimensional immersed plate
theory with three-dimensional simulations of rigid and flexible
sub-micron-sized suspended spheres, and with experimental results for adsorbed
micron-sized colloids which yield a "negative acoustic mass". The parallel
behaviour unveiled indicates that the QCM response is highly sensitive to
hydrodynamics, even for adsorbed colloids. Our conclusions call for a revision
of existing theories based on adhesion forces and elastic stiffness at contact,
which should in most cases include hydrodynamics.
