  Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to
measure spatially heterogeneous mechanical behavior in soft solids, with high
sensitivity to deformation and both spatial and temporal resolution. In this
paper, we discuss the mathematical approach behind the quantification of the
deformation rate from MSDWS data and provide guidelines for optimizing the
selection of experimental parameters in measurements. After validating the
method in extensional tests on an elastomer, we provide an example of the
potentiality of MSDWS by measuring the spatial distribution of the deformation
rate during shear debonding of adhesive tapes. We quantitatively characterize
the deformation rate distribution related to shearing and peeling under
loading. A highly heterogeneous deformation rate distribution is observed, and
time-dependent measurements reveal an increase in deformation localization
hundreds of seconds before full debonding. This behavior, previously predicted
by theory and simulation, is here demonstrated experimentally for the first
time.
