Bridge Experiments
A couple months ago I went to the trouble of making 8 non-adjustable bridges for iii #13 out of 8 different materials (9 including the original adjustable ebony bridge), all machined exactly the same and fit in the same way. I created a pivoting helper to raise the strings so that I could switch bridges under full tension and only have to spend 1 minute tuning and adjusting for intonation while I tested them.
Setup
After taking each material’s density and mass, I tested each bridge in 4 different configurations:
Full length fit (4.5”)
Wide feet (2” wide each)
Medium Feet (1 5/8” wide each)
Small Feet (1 1/4” wide each)
For each iteration I took its mass, measured the A0 (main air mode), MT1 (main top mode) and MB1 (Main back mode) in audacity. I also made recordings with my Zoom H1N and took notes on tonal changes and impressions of each bridge. As a last effort I took 2 bridges and made them as hollow as I could to drop the mass as much as possible in the small feet configuration. The materials I tested were:
0. Original Adjustable Ebony 1. Flat sawn (FS) Spruce. 2. Quarter sawn (QS) Spruce. 3. African Mahogany. 4. FS Sugar Maple 5. QS Rock Maple. 6. FS Red Maple. 7. Walnut. 8. Rosewood
I did not measure the stiffness of the bridges or natural resonances though I would at least track stiffness if I were to do this again… and I may…
General Take Aways
As far as the different feet configurations, as the bridge feet were reduced (and mass reduced, and also likely stiffness reduced) the sound from the instrument got more clear and airy. The correlates to more treble, more polish, more space, more clarity and to some extent even perhaps more tubbiness. The largest audible difference was going from full-fit bridge to the 2 feet (or creating a 1/2” gap), from there the change was more subtle within each material. This could be a useful tip if one has a full fit bridge and wants more clarity, or if one has a two foot bridge and wants more mids and saturation.
I did the test with each bridge in succession, so the immediate comparison was between materials at each configuration more-so than the different configurations for each material. I took copious notes about tone and how well it hit my ideal. I don’t know how it would read to someone who was not there, so I will not post those specifics, only paraphrase in a couple paragraphs.
I did note that the impact on A0, MT1 and MB1 were more-or-less consistent within each bridge material, but did vary between different materials— ie, bridges of similar mass had similar impacts on those resonances. The change in mass from full-foot to small feet was generally about 10% within each type of wood, but the difference in mass in all iterations of all species was 500%.
One thing I think impacted the softer species was having the strings being in direct contact with the soft material and not a harder material like ebony. I think this especially impacted the spruce results as spruce can be rather spongy. I can see myself outfitting these bridges all with ebony caps to normalize that factor and running a test again.
Here’s my generalized take-away on bridge density and tone:
Less dense woods (spruces and mahogany)- great for sweetness, more sparkle punch on the high notes, not much mid-range growl, but added air in the lower register (slight tubbiness). Good clarity without being too bright sounding.Strumming, folky
Medium Density woods (Maples and walnut)- adding body to overall tone, mids and bass get darker and punchier with some loss of sparkle on the high end and somewhat less clarity overall. Bluegrass.
Denser woods (Rosewood ebony)- retain punchy mids but gain some clarity and brightness, added brightness pushes the highs toward shrillness and somewhat thinner. Kind of generic as far as genre, the added brightness could probably be adjusted with string, pick, and technique choices more-so than the others.
The case of the medium density woods makes me wonder the most about adding the ebony cap. I think an ebony cap on the spruces may induce a level of brightness that turns the sweet highs into more harsh and shrill territory. I could see an argument for a maple cap on an ebony bridge, kind of a reverse banjo bridge. I may experiment with this more down the road.
Here’s some photos and data. I may be able to add some audio but have been having difficulty getting it into a shareable format.
