Comparing Two Acoustic Top Brace Designs

Many luthiers and customers believe that scallop X-bracing an acoustic guitar top is the best way to brace the top. I want to question this belief. There are two objectives for top bracing:

  1. Long term top stability, (i.e., flatness or retention of the original arching).
  2. Great sound, including a full range of harmonic overtones.

The acoustic guitar top works much like a loud speaker. It has to move air at many frequencies. The fact that the top does not move the same way throughout is beyond the scope of this article. My experimentation shows that top flexibility near the rim is key to great sound.

Unfortunately, too much flexibility may contribute to top deformation. Too much flexibility is just as bad as not enough. So, beware of a good thing. Scalloped bracing has a thick cross-section near the rim, and a thin area closer to the bridge plate. In my not so humble opinion, this is wrong. This brace design causes two problems:

  1. Lack of flexibility at the rim
  2. A significant structural weakness below the bridge.

Now, I know many readers will hold firm in the belief that their X-braced 80-year-old dreadnought acoustics sound the best. However, today these guitars would have much less top distortion and zero bridge roll if built with tapered braces, and they would sound just as good or better. My top bracing design testifies to this, although I may hear some complaints when my guitars are 80 years old. May be not.

I recently repaired four non-vintage scalloped X-braced acoustic guitars. These are all beautifully built inside and out. And yet, all four had serious lower bout dome distortion and bad bridge roll. This contributed to high play action even with very minimal saddle height. Bridge roll is the angling of the bridge bottom towards the sound hole. The bridge bottom is no longer parallel to the original top surface. I contend that scalloped bracing was the cause for this. The problem is the scalloped cross-section of the braces causing unsupported area in a semicircle four inches or so around the center point of the bridge plate in the lower bout.

My solution? Tapered bracing. Tapered braces are at full thickness near the "X" and taper in a straight line towards the rim. Tapered braces have no weak spots below the bridge plate. And if the brace ends near the rim taper and flatten gradually to a 1/32 to 1/16-inch thickness, excellent sound and long-term top stability will result.

Of course, no bracing pattern except maybe ultra-stout V-bracing can resist heavy gauge strings on a guitar stored in a hot, high humidity environment. Even V-bracing will fail eventually if abused.