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#006: A Loudspeaker Driver - CAD

When working with Finite Element Analysis (FEA), CAD (Computer Aided Design) is key. Below I show a loudspeaker driver; an application which I have simulated a multitude of times, for several different companies.

This driver I constructed myself in a CAD software. It is not based on any one existing driver, but is simply a concoction from memory. In upcoming posts we will see how well this driver (or a derivate thereof) perfoms, but for now the essentials of constructing such CAD is to be discussed.

CAD terminology If CAD is a new topic for you, there will probably be a steep learning curve when embarking on drawing yourself. We will have a brief look at some terminology. Below is shown part of the sketch for the spider geometry.

The spider has a certain desired geometry; it is "wavey". If some of the dimensional parameters, i.e. the 4's and other numbers, are changed, then some lines may cross; so having an initial shape of an object results in some intrinsic contraints, that are fullfilled as long as the geometry keeps it's desired topology. There are also some explicitely defined constraints, that e.g. dictate that two arcs should stay concentric; these are sometimes called relations. With the parameters and relations you can contrain the degrees of freedom, until the desired geometry is obtained. This can be quite difficult at times, since some parts have many degrees of freedom starting out.

Have a look at the spider below:

A spider is probably contained in its own file; this would be called a part. All the parts are then typically placed in an assembly. In this assembly the parts are placed and positioned via mates, e.g. parts can be coinciding or offset by a distance.

CAD for Finite Element Modelling (FEM)

When preparing cad for finite element modelling, it can be a good idea to get rid of any overlaps and/or separations in the assembly. There are way to deal with CAD which has not be cleaned up well, but we will ignore those for now. Between some parts there may have to be glue-parts inserted to ensure smooth interface. Also, the tolerance with which the CAD file is imported in the FEM software

Finally, once in while, you will encounter faulty geometry, which simply will not import correctly into the FEM software, even though no apparent errors are seen in the CAD software. Sometimes you will never find out what exactly is the problem, and so you may have to redraw that part, but one clue can sometimes be to check if the part in question was originally made in one software package, and then at a later point subsequently imported in a different CAD software.

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