  We analyse the ultrasound waves reflected by multiple bubbles in the
linearized time-dependent acoustic model. The generated time-dependent wave
field is estimated close to the bubbles. The motivation of this study comes
from the therapy modality using acoustic cavitation generated by injected
bubbles into the region of interest. The goal is to create enough, but not too
much, pressure in the region of interest to eradicate anomalies in that region.
  Here, we derive the dominant part of the generated acoustic field by a
cluster of bubbles taking into account the (high) contrasts of their mass
density and bulk as well as their general distribution in the given region.
  As consequences of these approximations, we highlight the following features:
1. If we use dimers (two close bubbles), or generally polymers, then we obtain
a remarkable enhancement of the whole echo in the whole time. 2. If we
distribute the bubbles every where in the region of interest,then we can derive
the effective acoustic model which turns out to be a dispersive one. We show
that, for a given desired pressure, we can tune the effective model to generate
it.
