back brace size

[Blaine]

I was peeking inside my Strunal flatback and my initial thought was "these braces are huge." I'm curious, are there general dimensions for these guys? It would seem to me that if they were too big the sound would suffer. Do you luthiers out there ever shave or thin them down? Any other brace thoughts or theories?

[Bob Branstetter]

I can't say that I've ever been asked to thin the back braces on a bass. On occations where it is necessary to replace the back bracing as part of an overhaul, I follow the Charles Traeger guidelines and make the combined thickness of the soundpost brace no more than 3/4 inch total. The exact thickness being determined by a lot of factors that past experience teaches us. The purpose of the back braces is to give the back stiffness so that it vibrates approximately the same as a carved back. Too heavy does impede the sound, but at the same time, a too thin brace makes the back too floppy and can lead to structural problems. Manufactured basses do tend to be a little heavy, but considering that you have to remove the top or back to thin the braces, the cost tends to be quite expensive when compared to the amount of sound improvement gained (looking at it as a cost/benefit ratio).

[Jeff Bollbach]

Bob's on target as usual. I've done this job a bunch of times, with significant tonal results-but it would be an expensive job just to do on its own.

[Blaine]

Thanks Bob. i'm a bit confused at combined thickness of 3/4". Is that the combined thickness of the brace and back? Also, any guidlines for the upper and lower brace.

[Bob Branstetter]

Yes, the combined thickness of the brace and the back itself. I didn't mention the other braces simply because the soundpost brace (the one it sits on) is the most important. The upper most brace (at the bend in the back) tends to be about the same thickness or a little less than the soundpost brace. The other two braces (if it has total of 4) are not as wide, but up to about 1" to 1 1/4" thick. Notice that these braces are rounded on the top (like the bass bar is on the top) whereas the soundpost brace is not. These are tuned in a method similar to tuning the bass bar on the top. Luthiers have different methods of tuning them, but that is beyond the scope of this discussion. In the end, the thickness of the other braces is related to the thickness and pitch of the soundpost brace.

[Aroneng]

In general, what do you try to tune the supports to? Is matching the tap tone of the back typical or maybe 1/2 step above the back? (I have heard trying to tune the bass bar a 1/2 step above the top's tone).

[Bob Branstetter]

As I mentioned before, the subject of tuning is beyord this discussion, and there are several different methods used to tune the bars and the back as a whole. I personally do NOT use the tap tone method. I tune using Chladni pattern eigenmodes and try to match (as closely as possible) modes 2 and 5 of the back to that of the top. This method was pioneered by the Catgut Acoustical Society. If you wish to learn more info on this, I suggest you join the CAS.

[arnoldschnitzer]

I prefer tuning using the Islamic overtone series, matching to modes 3 (Phrygian) and 7 (Locrian), though the Locrian series may occasionally induce wolfiness. Tap on the center crossbar only with a doctor's reflex hammer while holding the bass' back by one corner. You must be standing on one foot for this to work effectively. If you can hear the Myxolidian mode while tapping, the braces are too thick. Lydian, too thin, etc...

[Bob Branstetter]

Originally posted by arnoldschnitzer
I prefer tuning using the Islamic overtone series, matching to modes 3 (Phrygian) and 7 (Locrian), though the Locrian series may occasionally induce wolfiness.

Arnold - you can eliminate the wolfiness by matching the mode 3 (Phrygian) with mode 196 (Locrian). And.. don't forget you have to stand on the Left foot for it to work correctly in the northern hemisphere.

[Jeff Bollbach]

Somebodys been sniffing hide glue.