Bracing

BRACING Bracing is probably one of the most important things you can do to your cabinet. No matter how good the drivers are, no matter how good the crossover blends the drivers, if the cabinet resonates, it will destroy the clarity and imaging of the speaker you have built. If the panels resonate, they will produce their own sound which will interfere with the sound being radiated by the drivers. In discussing resonance of panels in this context, what is being discussed is the excitation of the actual panel; NOT the standing wave bouncing back and forth between the wall panels.

Excitation of a panel is by three primary means:

Vibration from the vibrating air in the enclosure,
Vibration from the driver, and
Vibration from movement of other panels.

An article by James K. Iverson titled, The Theory of Loudspeaker Cabinet Resonances in the Journal of the Audio Engineering Society, April 1973, Volume 21, Number 3, studied panel resonances. Some of the results are summarized:

Above 1,200 Hz, very little panel movement is observed. Lesson: Raise the panel resonant frequency into this range so the panel won't flex,
Resonance can be raised above 1,000 Hz (where panels are less likely to flex) by spacing braces no farther than 10" apart,
From curves of cabinet pressure vs. frequency, there is very little air pressure above 500 Hz to force movement of the panels,
A hole in a panel decreases the plate stiffness which lowers the fundamental frequency. Likewise, so does suspending a mass on the panel. The upshot of this? Brace that baffle!
While absorptive materials may slightly help reduce panel excitation by absorbing some acoustic energy, it is no where near as effective as bracing,
Bracing is most effective in the longitudinal direction and can raise the resonant frequency by 2 1/2 to 3 times,

He showed results of another experiment by P. W. Tappen on bracing a metal plate clamped at all edges. In terms of raising the fundamental frequency, the dowel cross-brace was the least effective, while the longitudinal brace was the most effective. Here are Tappen's results summarized in a chart:

Bracing Method	Theoretical Resonant Frequency (Hz)	Experimental Resonant Frequency (Hz)
None	46	60
Dowel cross-brace	---	85
Widthwise	108	100
Diagonal	---	115
Longitudinal	185	160

As can be seen, the longitudinal brace is the most effective at raising the resonant frequency. Iverson also tried a 1x2 pine brace with screws at 3" on center bracing a plywood/particle board enclosure and noted that it did not appreciably raise the resonant frequency. He believed it showed that high stiffness braces were preferred. (Of course, it could also show that he should have glued the brace instead of using screws.)

At least two different articles (there may be more) from Speaker Builder/Audio Express also explore Bracing. One article is entitled, Navigating Speaker Design: Listening to Walls, by Mark Wheeler (Speaker Builder 8/99) and is the third part of a three part series entitled, Navigating Speaker Design. In it he covers some background on panel resonances and sets up a listening study whereby various commercial loudspeakers were modified with bracing and then auditioned. Following are some key observations:

Mid-panel front to back or side to side braces (cross-braces) were ineffective enough to show no observed change, or at the very least, they were the least effective,
Vertical braces were more effective than horizontal ones or even circumferential ones. He states that a number of designers report the greater effectiveness of bracing the long direction,
He notes that the type of glue can affect bracing quality with white wood glue being least effective, while two part epoxies were best,
High density (hardwood) braces outperformed low density (softwood) or particle board. Even 1/4" thin hardwood braces maintained their effectiveness,
Bracing effects were similar in orders of magnitude to driver selection,

The second article is by Jim Moriyasu from AudioExpress (Feb. 2002) entitled, Panel Damping Studies: Reducing Loudspeaker Enclosure Vibrations. In it the author builds a test box with an 8" subwoofer, with a removable square panel and then clamps panels with various forms of bracing to the box. He then measures the vibration using an accelerometer and graphs the frequency and waterfall response. He first tests blank panels of MDF, particle board, and 7-ply birch plywood. His tests show that all three are very similar in the frequency and level of resonance, though MDF shows a 1 to 2 dB lower level of the primary resonances, but admits there's no major differences between the three. He decides to use the MDF panel for all the bracing tests. He then tests how much air vibration is cut down by stuffing the enclosure. He notes a 2 to 3 dB reduction in peak levels. He goes on to test multiple panel layers, applied damping materials, constrained layer damping, and bracing (as well as woofer isolation). He tried the following braces: 1 1/8" dowel rod cross-brace, 3/4" thick x 2 1/2" wide MDF brace (placed across the width of the panel), and various shelf (window) braces featuring square windows, oval windows, and a single large oval. His results favor the 3/4" x 2 1/2" wide MDF brace placed across the panel. He used a square panel so there is no distinction between widthwise or longitudinal. The window braces were next best with the oval windows outperforming the square ones. The single oval one was not quite as good as the other window braces. The dowel rod showed mixed results - some higher modes were reduced by 10 dB, but lower modes did not change much. The third mode was actually 5 dB higher in level than no brace at all. When I have time, I will chart his results for bracing as well as the other forms of damping he checked.

From the studies above, it is apparent that the purpose of bracing is to raise the panel's natural resonant frequency up to a point where it won't flex the panel. Stuffing and applied damping compounds can then control any remaining resonances and will keep the panel from "singing." If you divide a panel in half, you double it's natural resonant frequency. Of course we are also adding stiffness, which raises the resonant frequency even more - for a total of perhaps 2 1/2 to 3 times the original frequency. If we divide the panel with bracing every 10" or so, we keep the resonance above 1,000 Hz where it's less likely to cause the panel to move. Bracing "damps" panel movement. Note that the other form of damping is adding a damping compound of some kind to the enclosure wall. This is the antithesis of bracing, since adding mass lowers the resonant frequency. With bracing we are trying to raise the frequency.

I don't want to propose that you don't add some kind of damping compound, because I do believe it's also beneficial in stopping the panel from transmitting sound. However, be aware that the panel can still resonate at a particular frequency and color the sound even though you have damped most other vibrations. A combination of bracing and damping is best, but if using damping compounds, or if you apply heavy coats of it, you may want to apply bracing even more liberally. Perhaps the spacing can be decreased to 6" or 8" instead of 10".

Other notes:

Make sure the brace is rigidly attached. Don't just screw the brace to the panel. It must be glued throughout it's length! And clamped tight! Therefore, if you use screws and glue, drill the hole in the panel slightly larger than the diameter of the screw threads so the threads won't bite into the panel itself. Of course the screw hole in the brace must be smaller than the threads. :-) Then when you put the screw through the panel and into the brace, the threads will bite into the brace and pull the brace and panel together providing very high clamping pressure. Otherwise, if the screw threads were to bite into both the panel and brace, there will always be a minuscule gap between the brace and the panel that you can't quite get rid of by tightening the screw harder, and the two pieces won't bond properly.
As mentioned before, use hardwood braces. I used oak braces in my subwoofer.
Another point to consider if using a longitudinal or widthwise brace - Don't put it in the center of the panel! Offset it slightly to the side (say somewhere between 1/3 and 1/2 the width of the panel). This will cause the two "subpanels" to resonate at different frequencies instead of "stacking" the resonant frequencies on top of each other.

I can recall one other Speaker Builder article that I've read and if I locate it I will add it's conclusions.
more to come...

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Last Updated 10/06/04