# Glossary

# Glossary#

A glossary of common terms used throughout the knowledgebase.

- Absorbing boundary condition (ABC)#
Condition applied at the edges of a computational domain to eliminate (or minimise) reflections.

- Boundary element method (BEM)#
A computational method for solving linear PDEs for which Green’s functions can be calculated. BEM calculates a solution at the boundary of a domain, reducing the dimensionality of the problem.

- Courant–Friedrichs–Lewy (CFL) condition#
A necessary condition for convergence in analysing explicit time integration schemes. See Core Concepts: CFL Condition for more details.

- Explicit scheme#
An explicit time-stepping scheme is one where the solution at time step n+1 is calculated based on existing (known) values from time step n.

- Finite difference time domain (FDTD) method#
A computational method for solving time-dependent PDEs which uses a regular grid to discretise a domain. As a time-domain method, FDTD simulations cover a wide frequency range in a single simulation run.

- Finite element method (FEM)#
A computational method for solving PDEs by discretising a domain into smaller elements governed by simpler relationships, and combining the contribution of these elements to approximate the function across the whole domain.

- Homogeneous domain#
A homogeneous domain has the same material properties (e.g. density, speed of sound…) throughout the domain.

- Heterogeneous domain#
A heterogeneous (or inhomogeneous) domain has material properties (e.g. density, speed of sound…) that vary throughout the domain.

- Implicit scheme#
An implicit time-stepping scheme is one where the solution at time step n+1 is calculated using the (unknown) values at time step n+1, requiring the solution of an ODE.

- Jupyter Book#
Jupyter Book is an open source project for building beautiful, publication-quality books and documents (like this one!) from computational material.

- Ordinary differential equation (ODE)#
An equation involving derivatives in a single variable (e.g. how an acoustic field varies with time).

- Partial differential equation (PDE)#
An equation involving partial derivatives in multiple variables (e.g. how an acoustic field varies with time and multiple spatial dimensions). Most of the equations of interest in computational acoustics are PDEs.

- Perfectly matched layer (PML)#
An artificial ABC with non-physical properties which vary gradually throughout the layer in order to ensure that no energy is reflected into the interior of the domain.

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