  An Isogeometric Boundary Element Method (IgA-BEM) is considered for the
numerical solution of Helmholtz problems on 3D bounded or unbounded domains,
admitting a smooth conformal multi-patch representation of their finite
boundary surface. The discretization space is formed by $C^0$ inter-patch
continuous basis functions whose restriction to a patch simplifies to the span
of tensor product B-splines composed with the given patch parameterization. For
both regular and singular integration, the proposed model utilizes a numerical
procedure defined on the support of each trial B-spline function, which makes
possible a function--by--function implementation of the matrix assembly phase.
Spline quasi-interpolation is the common ingredient of all the considered
quadrature rules; in the singular case it is combined with a B-spline recursion
over the spline degree and with a singularity extraction technique, extended to
the multi-patch setting for the first time. A threshold selection strategy is
proposed to automatically distinguish between nearly singular and regular
integrals. Numerical examples on relevant benchmarks show that the expected
convergence orders are achieved with uniform discretization and a small number
of uniformly spaced quadrature nodes.
