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While detractors call it "just another tailpiece", the inventor refers to the TurboTail as a "free soundboard device." Free of what? Free of the need for conventional bracing.
I think of the TurboTail as a small, externally mounted fixture designed to counter the tremendous pull of the strings on fixed-bridge guitars. Traditionally, these forces have been managed by undergirding the soundboard with bracing.
I maintain that the TurboTail is very much an educational tool for guitar makers, offering insight into long-held beliefs regarding the soundboard's role in resisting string tension. It has allowed me to begin to safely explore what a soundboard might sound like without all that wood attached beneath it.
The TurboTail
Burning Questions
According to its inventor:TurboGuitar website where you can also find several YouTube product demonstration videos.
As an externally-mounted fixture, the TurboTail is secured to the tailblock using two small screws. Two small holes are drilled through the top of the bridge, where two miniature cables enter the body and are retained by the bridgeplate. visit the manufacturer's site .
Jump to the tests, located toward the end of this article, by clicking the link, below:Test Results
I am sharing my experiences and understanding of the TurboTail with anyone who is interested. I would think that adventurous acoustic guitar makers might be curious enough after reading my findings to want to put the fixture to the test for themselves.
If you want to purchase a TurboTail, please contact the manufacturer. I have paid for all the ones I have put to use.
Forces at Work
I go much deeper into this issue in my article » The Four Casualties of Steel String Tension . I have included a few paragraphs, below, for context.
For guitars with fixed bridges, into or onto which the strings are anchored, the torque of multiple steel strings across the crest of the saddle is constantly working to rotate the bridge forward, in the direction of the headstock.
The builder must ensure a sufficient bond exists between the bridge and the soundboard to prevent the strings from tearing the bridge loose. The soundboard must have sufficient integrity to resist being torn off the guitar.
It must maintain a bond with the bracing beneath it, and the bracing must be sufficient to minimize the effects of the forward rotation of the bridge. Additionally, a balance must be achieved between structural integrity and the freedom to translate string vibration into quality sound output.
Builders consistently successfully accomplish all of these feats, and we have all enjoyed the results. Yet our bridges still rotate forward, some more than others, and (eventually) create a depression (however slight) in the soundboard in front of the bridge and a rise, or "belly", (however slight) behind it. The resulting "S" curve in the soundboard, viewable from the side, began life as a flat (or even slightly convex) plane.
This wavy top (that often exaggerates in appearance, over time) was not a deliberate design thought up by some brilliant, artistic luthier long, long ago. Instead, what we live with is called damage , a distortion of what started out as a flat plane.
The so-called "modern acoustic guitar" is constructed by intentionally re-creating a design that permits soundboard deformation. Yes, there is a certain range of "sound" associated with a top so constructed, and yes, most of the guitar world has no interest in altering much of anything - unless they thought of it, first. But that is beside the point.
Rather than attempt to reduce or eliminate the role of the soundboard as a mitigator of collapse, many appear to prefer to think of (and even promote) such deformation as a feature. I find that curious, if not telling.
By countering the pull of the strings, and the resultant forward rotation of the bridge, the TurboTail has the potential to eliminate the deformation of the soundboard without removing the torque at the saddle. I find that motivational.
Challenging Convention
Question : Is it really possible to support the pull of the strings on the acoustic guitar without having to glue a bunch of sticks to the backside of the sound generator (the soundboard), but still produce a sweet-sounding instrument? In other words, by adding a TurboTail to my design, can I brace much more lightly, or even completely replace the bracing, and still make an effective guitar?Answer : I believe so, and this article explains how and why I came to that conclusion.
Consider my loose analogy of equating conventional steel string acoustic guitar soundboard bracing with wooden stringers on a set of stairs.
Braces and Stair Stringers
A braceless soundboard (or a soundboard having dramatically reduced bracing) on a guitar fitted with a TurboTail could then be equated with a set of stringerless stairs.
The TurboTail and Stringerless Stairs
In the event you are unfamiliar, stair treads may be adequately supported on stairs both with and without stringers. Stringers are just more common, practical, cost-effective, etc. Likewise, I have experimented and discovered that soundboards may be supported on acoustic guitars both with and without braces, though that does not appear to be widely known.
Everyone knows that stairs aren't musical instruments. When we make our way from one floor, up or down, to another, we aren't stepping on "sound" treads. And yet, we are referring to the front plate of the acoustic guitar, the effectual "deck" to which the bridge is glued and onto which the strings are attached (strings pulling in excess of of 160+ lbs of tension) as a "sound" board.
We could just as truthfully refer to the front plate as a "string tension resistance" board. Or a "guitar implosion prevention" board. Or a "bridge support" board.
Or even a "flexible body stiffening" board. When the strings are plucked, that "structurally sound" board does indeed make sound. And yes, it does so with all those braces glued on underneath it.
And yes, that is how guitars have been built for the last 100+ years. And yes, we all have favorite guitars that are built this way.
The sound of my acoustic guitar is not just the sound emanating from the wooden front plate. It is more a combination of that plate, the significant bracing that undergirds it, the tension the whole thing is put under when the strings are tightened, and maybe even a little of that deformation, too.
What Did I Want to Learn?
What might a flat, wooden soundboard "sound" like if it didn't need all those braces added just to keep the guitar from imploding?
When I asked the question, I quickly realized that very few people were interested in finding out. We don't build guitars like that, so there weren't any examples available for me to go listen to. I wanted to hear it for myself, and the TurboTail was the perfect fixture to start experimenting with!
As expected, a fan-braced guitar strung with steel, while sounding pretty sweet initially (and surprising more than one listener), will begin to suffer deformation. This will quickly render the guitar nonviable for most players as a daily driver.
To be fair, the so-called Classical guitar was not designed to manage such high string tension. However, if you will take the steps necessary to support the neck from curling forward, and prevent the bridge from rotating forward, you may find yourself with a rather cost-effective, responsive steel-string acoustic guitar!
I believe the inventor of the TurboTail had effectively demonstrated that point, as he had already posted a few videos of conversion guitars prior to contacting me. Upon learning of the TurboTail, I wanted to take things a step farther, and construct a guitar from the ground up whose soundboard would need to rely on the TurboTail for support.
But I also thought it would be wise to personally recreate a couple of Nylon-to-Steel conversion guitars, and record a few test measurements. This would provide me with firsthand knowledge, as well a benchmark against which I could compare my progress.
Being familiar with the well-established commercial approach to countering bridge rotation, that of the JLD Bridge Doctor, I wanted to conduct head-to-head experiments using both of these fixtures, the TurboTail and the Bridge Doctor, and document the test results. I had some suspicions that I wished to either prove or disprove, once and for all. In the end, regardless of the outcome, I knew I would gain additional knowledge and experience.
I purchased two identical Nylon-string guitars that were fitted with adjustable truss rods for my experiments. I installed and tested both the Bridge Doctor and the TurboTail in both guitars. What an education! If you wish to jump ahead, I documented all of this in a later section of this article subtitled » The Tests .
SPOILER : The tests confirmed my hypothesis, and I moved forward by designing and constructing the first steel string acoustic guitar built specifically to rely on the TurboTail, completing it toward the end of Summer, 2025. My build article is titled » The First TurboTail Guitar .
Do I think that, by using this fixture, I built the ultimate acoustic guitar? No. I think that I established the viability of the TurboTail as the component I needed to begin to explore the possibilities of reducing, if not eliminating soundboard bracing, all in my pursuit to learn what a "soundboard" could actually sound like. Read on to learn more.
Just Another Tailpiece?
A casual observer might confuse the TurboTail with a conventional tailpiece, such as may be seen on an acoustic archtop or Gypsy Jazz guitar.
D'Angelico with Tailpiece
If the definition of a tailpiece is a "piece" that mounts to the "tail" of the guitar, then the TurboTail qualifies as a tailpiece. However, referring to the TurboTail as "just another tailpiece" would be like referring to carbon fiber as "just another building material".
There is an exciting difference between a guitar fitted with a TurboTail and a guitar fitted with a conventional tailpiece. The TurboTail requires the strings to be anchored into or onto the bridge.
For that requirement to be met, the bridge must be permanently affixed to the soundboard. On such a guitar, as the strings come under tension, the torque at the saddle increases and the bridge wants to rotate forward. Only sufficient soundboard bracing prevents disaster.
The TurboTail mounts at the intersection of the tail end of the guitar and the soundboard. Two cables are secured to the bridge and attached to the TurboTail using a clever tensioning system.
Tightening the cables pulls the bridge back toward the tail block, directly countering the forward rotation of the bridge caused by the pull of the strings across the saddle. It does so while maintaining the torque of the strings across the crest of the saddle, even increasing that torque in some cases.
By contrast, a conventional tailpiece anchors an instrument's strings to its tail block, not to the bridge, and the strings press downward onto the bridge and soundboard. They do NOT rotate the bridge forward.
There is significantly lower (read: "very little, if any") torque across the saddle of a guitar fitted with a conventional tailpiece. Additionally, there is no "tensioning" adjustment on the conventional tailpiece. Tightening the strings simply raises the pitch.
Tailpieces are commonly found on mandolins and banjos, as well as the bowed stringed instruments, including acoustic upright bases, cellos, violas and violins. The bridges on these instruments typically float (remain moveable) on the surface and are held in place by the downward pressure of the strings. While less common, a guitar fitted with a tailpiece may include a fixed bridge (glued-on), as we see in this 1932 Maccaferri tenor (4-string) guitar.
1932 Selmer Maccaferri Tenor Guitar
In order to generate sufficient energy down through the saddle, into the bridge and out onto the soundboard that will result in audible tone, the strings of most acoustic instruments fitted with a tailpiece tend to crest the saddle relatively high off the surface of the soundboard, higher than you might find on your typical acoustic guitar having bridge-anchored strings.
Failure to provide the strings with adequate downward force against the saddle will result in lackluster performance. Failure to provide sufficient structural support to a soundboard of an acoustic guitar fitted with a tailpiece, a soundboard that becomes the recipient of not an insignificant amount of downward force, will result in collapse! This is NOT the case with a TurboTail-fitted soundboard.
Traditionally, to mitigate collapse of the soundboard from the downward force of the strings, the soundboard is typically either domed ("radiused") or arched, the purpose being to increase its ability to handle the load imposed by the strings. Doming is effected by bowing, bending or pressing a thinner plate into shape and fixing it in place with glued-on radiused bracing. An arched top soundboard is formed by carving the desired contour from a thicker billet of wood.
When I see the Great Stone Bridge of China, I think of of a radiused soundboard with scalloped bracing.
China's Great Stone Bridge
By contrast, the TurboTail can be installed on guitars having perfectly flat soundboards. No arching or doming of the soundboard is required.
Tailpiece vs TurboTail
So, what's the difference?
Guitar Fitted with Tailpiece
Tailpiece-anchored strings
Floating bridge
Low-torque saddle
No bridge rotation to counter
Archtop and/or well-braced soundboard required
Guitar Fitted with TurboTail
Bridge-anchored strings
Fixed bridge
High-torque saddle
Significant forward bridge rotation to deal with
No bracing requirement
The JLD Bridge Doctor
In November of 1993, Don Kendall patented his apparatus for "... reversing or preventing warpage in the top plates of guitars and similar musical instruments attributable to the tensile forces within the strings ..."
"The apparatus employs a compression block attached within the interior of an instrument's
Both versions of the JLD Bridge Doctor, the "Screw Mount" version, which requires that a hole be drilled through the bridge, and the more easily reversible "Brass Pin Mount" version (formerly, the JLD Bridge System), which replaces your bridge pins with its own brass versions, directly address the forward rotation of the bridge that is responsible for the "warpage in the top plate", or bellying, by applying a less than or equal rotating force directly to the underside of the bridge in the opposite direction.
With a wooden block secured to the underside of the bridge, leverage is used to pry the bridge back into position, accomplished by pushing the wooden dowel back against the tail block. Leverage is adjustable using the set screw housed (horizontally) within the wooden block.
JLD Bridge Doctor
With a JLD Bridge Doctor installed, string energy is still transferred to the soundboard. Mass has been added to the equation. Weighing as much as 34 grams we have, in effect, added the weight of a second ebony bridge to the soundboard.
The Brass Pin version, a "non-destructive" variation of the JLD Bridge Doctor, utilizes 6 brass pins that replace the standard bridge pins and adds 30 more grams to the overall weight, coming in at a total of 60 grams or, using my analogy, the equivalent of 2 additional bridges.
Users of the JLD Bridge Doctor offer a mix of reactions, with some expressing gratitude for the visual reduction in (if not complete reversal of) bellying, while there are those who claim little or no reduction occurred. Some users have cited a noticeable increase in bass response, with others claiming they are experiencing more pronounced midrange frequencies. Some say their guitar is louder and others think they may have killed the sound.
I included the JLD Bridge Doctor in my tests, for comparison with the TurboTail.
Bridge Doctor Installed in a Cordoba C5
Comparison: TurboTail and JLD Bridge Doctor
Unlike the JLD Bridge Doctor, which (mostly) hides away inside the guitar, the TurboTail mounts outside the instrument, on its exterior. There is a definite aesthetic to consider, though it is immediately apparent that much thought has gone into the design and appearance of this fixture, making it both functional and attractive.
The components of the TurboTail I installed weigh 44 grams, in total. The majority of the weight rests on the tail block, with less than 1 gram being added directly to the bridge.
What does the TurboTail do differently from the JLD Bridge Doctor?
The TurboTail is not specifically designed to counter the rotation of the bridge (though that is definitely a side-benefit). Instead, it it shares the pull of the strings with the tail block, effectually suspending the bridge on the soundboard.
Here is a loose analogy: Lay a big truck tire flat on a 6 ft. tall platform outdoors between two large trees, and secure the tire to the platform. I trust you can rest the platform on some hard, stable soil. Attach one end of a rope to the tire and wrap the other end of the rope to the large tree 20 ft. away, securing it at a point approximately 6 ft. off the ground.
Tighten the rope sufficiently to just start to lift the legs on the backside of the platform up off of the ground. Call this setup “My bridge under string tension.”
TurboTail versus Bridge Doctor
The JLD Bridge Doctor directly addresses the forward rotation of the bridge by applying a less than or equal rotating force in the opposite direction. Leverage is used to pry the bridge back into position, accomplished by first securing a post to the bridge and then pushing a dowel held at a right angle with that post against the tail block. Using my truck tire analogy, bolt a big block of wood to the underside of the platform on which the tire rests.
Like the keel of a ship, engineer it to hang 4 ft. below the deck of the 6 ft. platform. Position a jack between the big block of wood and the tree on the other side of the platform, the one without a rope tied to it. Expand the jack and make use of leverage to drive the back legs of the platform down to the ground.
Ta da! The principle works. Rotation has been countered. However, do you notice how both the pull of the rope (strings) on the tire (bridge) and the push of the jack (wooden dowel) on the block are forces working in the same direction?
With a JLD Bridge Doctor installed, the soundboard directly in front of the bridge is the direct recipient of these forces. A load is applied across the surface of the soundboard between the bridge and the soundhole.
I have also added weight to the equation. This will lower the resonant frequency of the guitar (demonstrated in my test results). Whether this produces pleasing or displeasing results, a change in tone will definitely occur, so plan for it.
The TurboTail is working the problem differently. Unlike the JLD Bridge Doctor, which is anchoring a lever to the underside of the bridge and literally pushing that lever away from the tail block and toward the headstock, the TurboTail is pulling the bridge toward the tail block, directly countering the pull of the strings in the direction of the headstock.
The strings are pulling the bridge forward. The TurboTail is pulling the bridge backward. No change occurs in resonant frequency.
Using my truck tire analogy to visualize this, after removing the jack and the big block of wood (the "keel") that was mounted beneath the platform, simply attach a second rope to the tire on the opposite side of the first rope and stretch it to the adjacent tree, the same one we were pushing our jack up against. Wrap it around the tree and pull that second rope nice and tight.
Watch as the back legs of the platform re-connect with the ground. One rope pulls the tire in one direction, and a second rope, attached to the opposite side of the tire, pulls the tire in the opposing direction.
As the tests will show, the JLD Bridge Doctor definitely decreases bridge rotation, but it does so at the expense of also decreasing top deflection. It affixes the soundboard in place. This may or may not prove to be a good thing, but it is how it works.
It also increases the load across the surface of the soundboard directly in front of the bridge. Additionally, the JLD Bridge Doctor dramatically lowers the resonant frequency of the soundboard, on which the TurboTail has no measurable effect. See the test results. Or conduct your own.
The TurboTail will also have a nullifying impact on bridge rotation, eliminating it altogether, if so desired. Most importantly (and quite surprisingly), the TurboTail actually increases top deflection and it accomplishes this without altering the resonant frequency of the soundboard. This isn't theory, by the way. I measured these significant factors.
TurboTail on a Steel String
Player FAQ
If your guitar's soundboard has already been braced to withstand the pull of steel strings, the TurboTail will have less opportunity to bear the load. I don't know how much difference you might experience. I installed a TurboTail on the blue guitar in the photo, above, and noticed a marked increase in sustain, but not volume. You may want to try it and discover for yourself.
If you support the neck, then yes. For my testing, I used nylon string guitars fitted with an adjustable truss rod. Where the TurboTail counters the pull of the strings on the bridge and soundboard, a truss rod assists in countering the pull of the strings on the neck. A normal tension set of nylon string pulls at 86 lbs. A set of light gauge steel strings pulls at nearly 160 lbs. The extra tension is sufficient to bow an unsupported neck forward.
At the very least, lowering the action and adjusting saddle compensation for improved intonation will definitely make for a more enjoyable steel string playing experience. If the wider neck is uncomfortable, it can be narrowed, though this also introduces an aesthetic factor.
The TurboTail excels at allowing for greater tension strings to be installed on more lightly-braced instruments (such as converting fan-braced Classical guitars to steel strings, or converting a steel string (or even a nylon string) acoustic into an acoustic bass). It puts a spotlight on how heavily structured our existing steel stringed instrument soundboards really are, braced in an effort to withstand string tension, resist bridge rotation, and prevent collapse.
I don't know. Great strides have been made in perfecting the center soundhole, X-braced soundboard - a nearly 2 centuries old design. Advocates argue, "If it ain't broke. don't fix it." My own interest lies in utilizing the TurboTail in pursuit of an alternate approach to soundboard (and guitar) design and guitar construction. Give it a try and find out for yourself.
I would think so, though I have not done this, yet. The TurboTail relies on the structural soundness of the frame of the guitar (the outside edge) to counter the rotation of the bridge. While not required, a carbon fiber rod properly fixed between the tail block and the neck block would go a long way toward ensuring structural integrity of that frame over time, especially in the presence of 307.5 lbs of tension (D'Addario EJ86 strings). The neck would likely benefit from increased stiffening, such as is provided by the Dragonplate D-Tube (for example).
For the Builders
Imagine : What if the need for an X-brace (or V-brace, or Falcate, or Kasha, or any bracing) was suddenly removed?
Adding a TurboTail effectively suspends the bridge (and, subsequently, the soundboard it is attached to) above the back. This can be visualized as a stretching of the cable the rest of the way across the ravine, as one would with a Tyrolean traverse, a Zip line, or a rope bridge.
Imagine the bridge of my acoustic guitar as a deck or platform mounted to the simple suspension bridge in the photo, below, positioned approximately 1/3 of the way across the ravine from the left of the image (beneath the couple shown in the photo - the guy wearing the blue pants, who is just one step in front of the lady wearing the dress).
In order to allow the visitors to safely traverse the bridge, and without running a post straight down to the ground below, how much structural support would be required if we were to remove the suspension cables? How much "bracing" would we need? Applying the analogy to the traditional acoustic guitar soundboard, can you envision the difference?
Capilano Suspension Bridge
By sharing the tremendous pull of the strings on the bridge with the frame of my guitar I no longer need to rely on the undergirding of the soundboard alone to handle the load. I can re-focus my attention on the soundboard's sound generating role.
By this point in the article, I trust that you are able to appreciate the difference between a conventional tailpiece-fitted guitar and the TurboTail.
Utilizing the TurboTail, any subsequent soundboard bracing - formerly a catastrophe mitigation requirement - now becomes an exploration in re-directing acoustical properties. Put another way, if I am going to glue sticks to the backside of my sound generating board, I can do so solely for the purpose of manipulating the sound. I don't have to think about withstanding string tension. Get it?
For guitar makers, I consider the TurboTail to be, firstly, an educational tool. Having control over the tensioning of the cables that secure the TurboTail to the bridge, I am free to experiment and explore the relationship between my guitar, as it currently exists, and various string materials and gauges. I can increase string tension with greater confidence.
Going farther, I can re-explore the acoustic flattop soundboard, as I do not require an arched and/or braced surface just to mitigate collapse. I can look into reducing the mass of the bridge, reducing (or removing) the bridgeplate, altering my bracing size, altering my bracing pattern, exploring new bracing patterns, even eliminating bracing altogether, as well as modifying the thickness of the soundboard.
The test results that follow provide additional information which may assist you as you draw your own conclusions.
The Tests
I have tested both the JLD Bridge Doctor and the TurboTail with two brand new, nearly identical Cordoba C5 nylon string guitars. I have reduced the height of the block of the JLD Bridge Doctor in order to fit it inside a shallow-bodied Classical guitar, reducing its weight from 34 g to 30.5 g (this step would not be necessary in a typical deeper-bodied steel string guitar). For reference, I have included measurements from two additional, unrelated guitars: an un-modified Classical guitar along with an un-modified steel string guitar.
There are seven (7) measurements I would like to capture across five (5) tests:
Top Deflection (With 1 Kg weight at the bridge, how much deflection occurs with the strings under tension?)
Top Deflection (With 1 Kg weight at the bridge, how much deflection occurs with the strings de-tensioned?)
Bridge Rotation (With an inclinometer attached to the bridge, how much rotation is measured as the the strings are tensioned and de-tensioned?)
String Height (Between top of the 12th fret and the bottom of the string)
String Height (Off the surface of the soundboard, directly in front of the bridge)
Body Frequency (Air Resonance - T(1,1)1, Open Soundhole, Soundboard coupled with the back)
Top Frequency (Soundboard Monopole - T(1,1)2, Closed Soundhole, Soundboard isolated, uncoupled from the back)
Here are the five (5) tests:
Measure all four guitars, unmodified. The balance of the tests are restricted to the two Cordoba V5s, only.
JLD Bridge Doctor installed, nylon strings.
JLD Bridge Doctor installed, steel strings (Note: I chose not to conduct this test after measuring the significant drop in top frequency associated with the JLD Bridge Doctor).
JLD Bridge Doctor removed, TurboTail installed, nylon strings.
TurboTail installed, steel strings.
1982 Guild Mark V (Nylon)
Test 1 - No mods
0.19 mm/kg
0.20 mm/kg
1.0°
3.04 mm
111.3 Hz
210.0 Hz
2024 Yamaha LL56 (Steel)
Test 1 - No mods
0.12 mm/kg
0.12 mm/kg
0.9°
1.4 mm
95.1 Hz
167.3 Hz
2024 Cordoba C5 (SN # CC12401735)
Test 1 - No mods
0.19 mm/kg
0.20 mm/kg
0.8°
3.07 mm
97.5 Hz
181.0 Hz
Test 2 - Bridge Doctor Nylon
0.18 mm/kg
0.16 mm/kg
0.5°
2.79 mm
96.2 Hz
160.2 Hz
Test 3 - Bridge Doctor Steel
-
-
-
-
-
-
Test 4 - TurboTail Nylon
0.20 mm/kg
0.20 mm/kg
0.3°
3.07 mm
97.2 Hz
182.0 Hz
Test 5 - TurboTail Steel
0.21 mm/kg
0.20 mm/kg
0.3°
3.5 mm *
97.0 Hz
180.7 Hz
* = The installation of steel strings on a Classical guitar bows the neck forward, raising the action. After adjusting the truss rod, the action height returned to the original, unmodified, nylon string measurements.
2024 Cordoba C5 (SN # CC12401735)
Test 1 - No mods
0.18 mm/kg
0.20 mm/kg
0.8°
2.99 mm
97.2 Hz
181.0 Hz
Test 2 - Bridge Doctor Nylon
0.17 mm/kg
0.16 mm/kg
0.5°
2.74 mm
95.9 Hz
159.4 Hz
Test 3 - Bridge Doctor Steel
-
-
-
-
-
-
Test 4 - TurboTail Nylon
0.19 mm/kg
0.20 mm/kg
0.2°
2.95 mm
97.1 Hz
181.2 Hz
Test 5 - TurboTail Steel
0.21 mm/kg
0.20 mm/kg
0.2°
3.45 mm *
97.0 Hz
180.9 Hz
* = The installation of steel strings on a Classical guitar bows the neck forward, raising the action. After adjusting the truss rod, the action height returned to the original, unmodified, nylon string measurements.
Test Equipment and Methods
Strings used for testing: D'Addario ProArté Nylon EJ45 (85.85 lbs tension) and D'Addario 80/20 Bronze Light EXP11/XS (157 lbs tension).
Strings used for testing: D'Addario ProArté Nylon EJ45 (85.85 lbs tension) and D'Addario 80/20 Bronze Light EXP11/XS (157 lbs tension).
Top Deflection is measured with a custom-built gantry using 80/20 Alumin(i)um components, a (modified) Digital Depth Gauge and shop-made wooden parts and secured to the guitar with a Star Tech Wrap N Strap. Weight is provided by a 1 Kg calibration weight (class M1, steel cylinder). The weight is applied, gauge is zeroed, weight is removed and measurement is recorded. Each test is conducted 3 times and averaged.
Testing Top Deflection
Bridge Rotation is measured using a Wixey Inclinometer (Angle Gauge). Strings are loosened, gauge is mounted to the bridge and zeroed, strings are tensioned to pitch and measurement is recorded. Test is conducted 3 times and averaged.
Testing Bridge Rotation
Action (String Height) at the 12th fret of the 6th string is measured using a StewMac Nut Slotting Gauge. The installation of steel strings on a Classical guitar bows the neck forward, raising the action. After adjusting the truss rod, the action height returned to the original, unmodified, nylon string measurements. Overall String Height of the 6th string off the surface of the soundboard, directly in front of the bridge, is measured once using setup blocks and feeler gauges.
Resonant Frequencies are measured using a calibrated miniDSP UMIK-2 connected to a Dell laptop and recorded with REW (Room Equalization Wizard) software. With the soundhole Open, the body resonant frequency is measured as a "coupled" reading (Air Resonance - T(1,1)1) involving both the front and back plates.
The soundhole is then Closed (blocked) in an attempt to "uncouple" the front and back plates, "isolating" the frequency of the soundboard, alone (Soundboard Monopole - T(1,1)2). All tests were conducted 3 times, then averaged.
Testing Equipment
My Assessment
The BIG reveal here was the 20+ Hz drop in the soundboard's T(1,1)2 resonant frequency when the Bridge Doctor was attached!
Further, with no modification to the soundboard or its bracing and with the TurboTail installed, no change to the soundboard's resonant frequency occurs, bridge rotation may be nullified and top deflection actually increases.
I was surprised at how much louder I perceived a former nylon string Classical guitar outfitted with a TurboTail and steel strings to be (than it was prior to being modified). Just as sophisticated measurement tools are not required to identify someone shouting versus whispering (especially in a small, enclosed space), so detecting the increase in loudness in these "conversion" guitars is unavoidable.
The cables securing the TurboTail to the bridge are adjustable in tension. The means of measuring that tension currently includes listening for the plink! when plucking one of the cables with a fingernail and, if desired, noting the frequency at which the plink! occurs.
Several days after installation I noticed an audible drop in the volume of one of the test guitars. Upon inspection, I realized the cables had loosened (which I attributed to a tightening of the manually-tied knots at each end and subsequent lengthening of the cables). Upon re-tightening the cables the guitar volume was immediately restored. Very cool! This is not an insignificant issue.
I re-measured bridge rotation and the degree of movement had decreased from 0.2° to 0.1°. EDIT : I have confirmed that bridge rotation can be brought to zero with additional tensioning. I do not want to overshadow the significant contribution of the TurboTail regarding forward bridge rotation. The ability to reduce forward bridge rotation to 0.0° is HUGE ! And no, it does not produce a deleterious effect on the output of the guitar!
I had set out to prove whether or not I could use the TurboTail as a tool to explore the sonic potential of the soundboard by not having to first undergird it with bracing just to prevent failure when subjected to the pull of the strings.
Witnessing bridge rotation effectually reduced to zero under string tension, increased top deflection and the undeniable increase in volume on guitars fitted with the TurboTail, I have not only proven that this is the right tool for me to move forward with, but I have also demonstrated that the violent back and forth rocking of the bridge cannot possibly be the primary driver of the sound, at least, not in the way in which it has been presented across the years.
Nor does a soundboard need to be "loaded" (with the visual "S" curve of deformation) to produce pleasing output!
Summary
The Misconception: Guitar makers who lived a little over a century ago did not design and construct steel stringed instruments from the ground up, having envisioned the future sound of the acoustic guitar. Instead, they began adding steel strings to existing gut stringed instruments which then had to be reinforced to prevent deformation. One could say that we have grown accustomed to hearing the results of a purposeful repair and reconstruction effort, an approach that must be respected if we wish to avert disaster.
The Alternative: The "Free Soundboard" effort from TurboGuitar changes things, permitting guitars built with soundboard-mounted bridges into/onto which steel strings are terminated to forego structural support of the soundboard (provided the frame or shell of the body is of sufficient rigidity). It is no longer necessary to first brace the soundboard to prevent collapse, then work within whatever range of frequencies remains to develop sound. Unlike a conventional tailpiece-fitted guitar, torque at the saddle is maintained; it even increases in some cases. The soundboard is free of structural encumbrances to make full use of the string energy for sound generation.
For Repairs: Resolve bridge rotation/bellying issues without compromising the sound.
For Players: Secure existing instruments against soundboard deformation. If desired, convert a Nylon string to steel for an impressively responsive guitar. Perform a little saddle compensation to correct the intonation and modify the nut for a more comfortable action. Take it even farther and modify the neck width and depth, for an even more enjoyable experience.
For Makers: Blaze new trails and explore previously uncharted soundscapes. Think: "Braces Not Required". Use tone bars to govern the sound. Let's go where no one has gone, before.
