The discretisation of the integral operators in the application of the boundary element method to Helmholtz problems
S. M. Kirkup and D.J.Henwood

Report M12, Department of Mathematics, Statistics and OR, Brighton Polytechnic (1987).

This report was later converted to chapter 3 of Stephen Kirkup's PhD which is available on-line.

Some of the concepts covered in this report were later published in the following paper:

Fortran codes for computing the discrete Helmholtz integral operators (1998)

and in chapter 3 of the book

Book: The Boundary Element Method in Acoustics (1998). On-line (2002-2004)