Title:
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The determination of acoustic output power by means of the radiation pressure balance
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The total power in an ultrasonic field is frequently assessed
by the measurement of the radiation pressure, although the nature
of this phenomenon continues to be the subject of some
controversy. This thesis presents a theoretical treatment of
radiation pressure together with a discussion of the design and
evaluation of a radiation pressure balance.
The different techniques used for the determination of
ultrasonic fields are discussed in Part I. In particular, the
various radiation pressure balances described in the current
literature are reviewed.
Several apparently contradictory definitions for radiation
pressure are in current use, and a number of authors have
commented on the confusion that exists within the literature. In
Part II of this thesis, a unified and simple treatment of
radiation pressure is developed, and, for the one-dimensional
case, the time-averaged force per unit area on the surface of an
obstacle placed in the field is shown to be exactly related (i.e.
to all orders of magnitude) to the time-averaged momentum flux of
the wave, or equivalently the time-averaged pressure in the field
expressed in Lagrangian coordinates. Unlike all previous
treatments, which are based on expansions to second order, the
result derived here is valid to all orders of approximation. As it
is usual to relate the radiation pressure to other field
quantities, particularly the energy density, a relation between
these two parameters is also derived.
In Part III of the thesis, the design and evaluation of a
radiation pressure balance is described, with particular emphasis
on a realistic assessment of the sources of uncertainty. The
various factors considered in the design of the microbalance
system and target are discussed, and this is followed by an
evaluation of the effects of phenomena such as surface tension,
streaming, convection currents and target stability.
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