Tech Talk
Effective Waveguides from Lightweight Materials
Erik Lundin | Senior Mechanical Engineer
Introduction
Highly effective and precise waveguides, which direct high frequency sound exactly where we want it in the room, have become something of a staple of JBL speakers. Their intricate shapes have a distinctive look, and every curve in it serves a purpose. While the waveguides are definitely a feature which improves the function of our speakers, they are complicated and simply not practical everywhere. For the new JBL Stage 2 line of speakers, the ability to manufacture enough of them, from a light and durable material, was a challenge worth taking on.
History
JBL has been using waveguides which combine some of the loading and impedance transform abilities of a horn with shapes that deliver the sound to the right place for a while now, and has successfully deployed patented High Definition Imaging waveguides in several lines of speakers. Refinements are always part of the product development process, but the underlying math that defines the basic shape forms a sort of visually identifiable heritage.
Early Stages
From the mechanical perspective, a set of calculated cross sections of the waveguide are delivered by the acoustic engineer on the project. These cross sections make up one quarter of the horizontally and vertically symmetrical waveguide.
Product Development
In order to ensure that the product will behave the way we want, we first start with designing a sort of testable stand-in for the waveguide. The acoustic surface will closely match what we expect from the final product, but there will be very few other features. This 3D model can then be used for various simulations, such as curvature analysis, and Finite Element Analysis to determine that the part will not deform or break. In our case, we knew the part would be supported only at the exit of the waveguide, while the weight of the tweeter would be suspended off the back. While the tweeters are light, speakers can experience quite a bit of force during shipping.
The image here shows where stresses in the part would concentrate, in an obviously very exaggerated manner so that we can get the detail we need. Armed with this knowledge, we can add new features to the 3D model, such as ribs to increase rigidity, and mounting features placed in such a way that the loads are distributed properly.
We can re-do the simulation again until we are satisfied that the part will perform as expected.
Design for Manufacturing
Knowing what we need the product to do, and look like, at the end, allows us to start into the process of making sure the part can be manufactured. Having a solid grounding in this process from the very start is key to making sure there aren’t any stumbling blocks at this stage, but nothing can replace the expertise and training of the engineers who handle the actual making of the steel mold in which the plastic part will be made. We work closely with them to ensure that the chosen plastic can flow to every part of the final part, and that the part is shaped such that it can release from the mold easily when it’s done. If you’ve ever noticed that a lot of ribs and mounting bosses are angled inward along their length, that’s why!
