Tech Talk
Subwoofer Bracing
Erik Lundin | Senior Mechanical Engineer
Introduction
From a mechanical perspective, speaker boxes are pretty lively things. With any box, we’ll need to take natural vibrations, shock from shipping, and similar loads into account. But with a speaker, we’re attaching a giant vibrating mass to one or more walls, on purpose. Usually, this means we have to brace some of the walls to make sure things stay stiff and strong. And then there’s resonance.
Early Stages
Like with any speaker, the job of engineering an enclosure starts with a lists of requirements, and a list of what I like to call “desirements”, or nice-to-haves. I knew the size of the transducer that needed to go in the box, and was given a fairly finished industrial design to follow. The acoustic engineer had provided some requirements for internal volume, and there was some guidance on what the apparent size of the front face should be. So I drew up a box with those dimensions and adjusted its depth to achieve the required volume. This gave me a starting point for doing some math!
Resonance in Boxes
So, all physical objects have frequencies at which they become resonant to some degree. But it is particularly noticeable in hollow objects with regular shapes, and also very easy to calculate. However, there are complicating factors, like material choices. But we won’t be making an empty box, we’ll be adding bracing, which sort of turns the box into multiple boxes.
From a mechanical bracing perspective, we can almost intuitively see that the best place to brace a big flat surface is right at the middle. I’ve simulated a straight beam to illustrate the obvious here, and we can see that the amount of bowing experienced by the beam goes down dramatically when we support it right at the middle. But what this also shows, is that the amount of flex on either side of the beam is identical.
From a resonance perspective, this means that while we’ve changed our big resonator into two resonators (and increased the resonance frequency), we’ve also now got two pieces that can resonate together at the same frequency. That’s not what we want at all. Moving the brace off the center by just a bit gives us two very different sections. In a box, this is the equivalent of making two boxes which share an air volume, but whose walls have different (and preferably competing) resonant frequencies.
Additional Concerns
Obviously, it’s very important to us that the enclosure of a subwoofer is rigid, strong, and free of disadvantageous resonances. But as the cutaway above shows, there are other things to be taken into account as well. In this instance, two rather long port tubes had to traverse almost the entire depth of the cabinet. In addition to that, this is an active subwoofer, which means there are amplifier parts inside. Perhaps even more importantly, according to the people who write rules about such things, there is AC current connected to your wall outlet flowing in that thing. The amplifier is mounted to a plate set into the back of the subwoofer, but it could easily be accessed by mistake through the thin cone of the transducer on the front.
In this case, the opportunity was there to make the enclosure bracing do triple duty as a brace, a support for the long port tubes, and a protective wall for the amplifier components. I think we all like solving several problems with a single solution.
