Back Explorations Part I
Intro
<<<<In my quest to understand the back's contribution to ff-holed mandolin tone I set up an experiment adding braces to the back of a familiar mandolin in order to measure the changes in tone and resonances to see what direction, if any could help lead me to my ideal tone. This was done in December 2023 and written up initially in May 2024.>>>>
I've recently been curious why we don't brace backs in the carved plate mandolin world. Surely our tradition is just over 100 years old so relatively young compared to violin tradition, but also less developed than guitars as far as categories, dimensions and musical style. There have been plenty of great mandolins built without back braces, but it seems to have been a neglected concept in the carved plate mandolin world and I'm curious if it was tried and abandoned or if perhaps this is a vein worth picking at for a little bit.
The mandolin on which I am testing is one I built in 2006, #13. Being an early build from an entirely self taught builder, it is a little rough, but it stays together, has nice tone, fits the parameters of typical quality mandolins. Also, it would serve no other purpose in this world if not for me to poke and prod at. The top is Sitka spruce and back/sides/neck are hard maple, likely rock or sugar maple. It has an ebony fretboard, bridge and headplate. Grover tuners and a stamped tailpiece.
Experiment
I will run four variations of braces, and vary height and shape within those. I will track the weight of the braces, the weight of the overall instrument, the main air mode (A0), main top resonance (MT1) and main back resonance (MB1). The top and back resonances correspond to the "trampoline mode" also called (0,1), ring mode or mode 5 in violins, and have a circular nodal "ring" which basically follows the recurve.
The braces will be fit to the outside of the instrument (I will eventually make the back removeable and try again on the inside) and glued with hot glue, in the hot setting. This creates a steady attachment that does not creep but it easily undone on the shellac finish. There may be fine points in the difference between hot glue and a traditional wood glue, but I think the difference will be much less than the mass/stiffness of the braces being attached.
I will record each variation in audacity to do a fast fourier transform (FFT) to identify the resonances. I will also record each variation with a zoom H1N recorder and play a variety of techniques associated with mandolin- open chords, chop chords, single note melody, double stops, tremlo for an anecotal assessment.
Bracing will have 4 variations:
1- 1 longitudinal brace along the centerline
2- 1 cross brace perpendicular to the centerline at the top of the arch
3- X-brace, with the cross at the top of the arch
4- double long brace (mirror of top bracing)
Within each of these I will have these variations
- square profile 1/2" tall
- square profile 1/4" tall
- triangle profile, 1/4" tall, tapered at ends
Results:
Notes on the table:
- The external bracing did not fit into my deflection jig except for the cross brace. In retrospect this would have been interesting information and I will incorportate it into the interior version when I do it.
- I forgot to get the mass on 1/4" long brace
- A0 did not change very much except for the last trial, which I did do a week later. Between changes in the humidity (which I was unfortunately not tracking) and mass changes it is difficult to say what is causing this.
Notes on Tone:
Initial Back Modes identified:
Back Mono pole: ~508
Cross dipole: 621
Cross Triple: 802
Initial playing notes: very nice, even tempered, warm tone with a bit of pop around. A little thin on the highs, not much punch for chops, but good meaty tone. Decent on volume and projection, great on tone. Would be an excellent recording mandolin.
Long Brace - 1/2”
Notes: Noticeable power in bass and 1st position notes. More clarity/separation between notes. Bass is tubbier but in a good way, some more pop and bubble on the D and A strings. Overall seems louder, punchier, but also cleaner. Definite more toward what I want!
Weight: 985.6
Long Brace 1/4”
weight: whoops!
Notes: Warmer, lost some punch in the bass, definitely moved back toward it’s normal tone, but still improved
Long Brace1/4” - Tapered triangle
Ends tapered from about 1 1/2” out. Cross section triangle as much as possible.
Weight: 981.1
Notes: Gained some bubble pop back, a little bit more clear. More punchy than the square version, but not as much as the 1/2”.
Back to original:
Surprised at how nice this mandolin sounds when back to normal. Feeling more pop and bounce than initially remember. Could be because I am looking for it. Definitely not as punchy or clear as with the 1/2” brace.
Cross Brace 1/2”
Weight:983.1
deflection: .020
Notes: more clarity and projection, not as much as long brace. Highs seem stronger, more supported
Cross Brace 1/4”
weight: 979.4
Deflection: .022
Notes: Warmer, less focused, less punch. Muddier somewhat, less clarity. Hard to tell vs normal.
Cross 1/4” triangle/tapered
Weight: 978.2
Deflection: .022
Notes: Some clarity returned, high notes still less strong, thinner. Some pop came back as well. Also hard to tell from original.
Back to original:
Warmth, saturation, more mellow
X Brace 3/8”
Weight: 985.1
Notes: Definite bass improvement, improved clarity. Much like the long brace at 1/2”. More focused, stronger chop. E string around 12th fret sounds phenomenal
X Brace Tapered
Weight: 980.0
Notes: about the same, not very impressive in any direction.
Back to original:
more bubble pop, warmth, responsiveness improved. I like this better than either X orientation.
**The following done 1 week later
Double long Brace- full ~3/4” tall
Weight: 1000.7
Notes: Fuller bass as with single long, seems choked in the upper mids, not sure this is an improvement over single long.
Double Long - tapered/triangle (TT)
Weight: 994.1
Notes: 7th fret D string (A440) really pops, general improvement of clarity and mid-high notes and poppines. Lower register still booms, seems up there with the single long as far as me liking it and why.
Double Long - 1/4” TT
Weight: 983.4
Notes: Similar pop at frets 6 and 7 on the d string (A/Ab). Really cool, but wolf and not consistent elsewhere. Improved poppiness, maybe my favorite iteration. Balance seems better and higher notes seem to ring out much better too.
Final Thoughts
From these results it seems that the more stiff the back (higher the MB 1 and 2 ) the more I like it. Specifically the longitudinal brace patterns seemed to pull MB 1 and 2 both up and I seem to like those better for added clarity, low end support and stronger highs (generally).