A Bent-Top Parlor Guitar

I wanted a guitar that played like an electric, but one that I didn’t have to plug into an amplifier to hear.

What I enjoy about the electric guitar are the bendable notes, the sustain, the greater sensitivity to vibrato and tremolo and the physical ease of playing. All of these playing features come from the use of extra light gauge strings on electrics. These require less tension to pull them up to pitch, and less energy to excite them. This works well on a solid body guitar where the string’s vibrations are picked up and amplified electronically. On an acoustic guitar however, sound production depends on the vibrating string inducing strong enough vibrations in the body of the guitar to cause pressure waves in the air around the guitar. This requires more energy and an extra-light string simply cannot absorb and release as much energy as a heavier, higher tension string.

Designing to manage stiffness and weight

A large part of acoustic design in stringed instruments is concerned with balancing the stiffness to weight ratio of the vibrating parts. Higher stiffness is needed to to resist the bending / destroying forces caused by holding a string under tension. Lower weight is needed to to allow the vibrating parts to be excited with a minimum input of energy. For this reason we look for materials for the vibrating parts that have a high stiffness to weight ratio. The problem can also be addressed through the design of the body structure: creating a structure that requires less material to yield the same resistance to bending.

An acoustic guitar designed for extra light strings would have to have a body that could be excited with less energy, that meant making the body, particularly the top, as light as possible. The first way to reduce weight would be make a smaller body. This would mean a quieter instrument and one with less fullness in the bass range. That was fine with me, I’m used to playing a solid body guitar without amplification and most anything would be an improvement over that.

Reducing the weight of the body could also be achieved by finding a way to make the structure of the body stiffer which would allow the components of the body to be lighter and thus more responsive.

Normal Flat-Tops

Let’s take a quick look at the main way the the top of “flat top” acoustic guitars succumb to the pressures of the strings. The top of most acoustic guitars are made from a thin, flat sheet of wood, with the strings anchored at the bridge, which is glued to the top. With the strings under tension the bridge rotates forward, causing the guitar top to pull up behind it and to drop in front of it.

The Bent Top

To resist this bending distortion in the top, wood braces are added in strategic places to the inside of the front and back of the body. In addition, most guitars also have a small amount of dome built into the top. This doming is imperceptible at a casual glance but is enough to reduce the distortion in the top. When I was making my last steel string guitar my friend and mentor Harry Fleishman, pointed out that simply bending the top on a single axis along the grain of the wood, was equally effective and easier to do than doming it. This is the key to the design of this parlor guitar. I took Harry’s idea and amplified it, radically bending the top along the grain would make a structure that was very stiff and resistant to bridge distortion. This structure could be built with a thinner top and lighter bracing and it would, I hoped, be significantly more responsive and capable of being driven by ultra light strings

The dome on conventional “flat top” guitars is usually made as part of a sphere with a large radius of between 15 and 50 feet. This is slightly tricky to engineer requiring bulky jigs. For my top I didn’t want a segment of a circle, I wanted a curve with a tighter bend in the center, straightening out towards the flanks. More of a parabola. This would make the centerline of the top, where most of the bridge distortion occurs, very stiff, while the flatter areas in the flanks would vibrate more freely at lower frequencies. The deflection of the top from center to edges is 35mm

Luckily it is quite easy to achieve this type of parabolic curve, it is the curve that naturally occurs when you take a piece of evenly thicknessed wood and bend it from its extremes.

Bent Back Too

Visually the bent top would create some interesting lines where it joined the sides. To complement these lines I decided to also bend the back on an axis 90° to the longitudinal bend in the top

The Wedge

When I had the top and the sides joined, I was able to hold the guitar body in my lap and realized that, though the sides were quite short, the actual distance of the bridge from the back to the top at the bridge was enough that I had to reach uncomfortably with my right arm to play. Also I wasn’t able to see the fretboard. So I tapered the sides quite severely towards the neck. This made a much more comfortable arrangement and also allows for easier access to the higher frets.

(Body thickness along the center line: 66 mm at the neck, 120 mm at the bridge, 105 mm at the endpin).

Sound Holes

The placement of the sound hole between the bridge and the fingerboard on conventional guitars, seems designed to promote top distortion from the bridge. Moving the sound hole off to the side would significantly increase stiffness of the top and would allow the top to be built lighter than usual.

I also added a second sound hole in the side of the body. This sound hole presents a fuller tone to the player. My choice of location for this second sound hole, in the lower bout of the guitar wasn’t great as it tends to get covered by my sweater when I’m playing.

Construction

I used hand tools for all the joinery and I used as few jigs as possible. The sides were free bent to a line on a piece of paper. For a one-off experimental project this is simply a lot quicker than making a lot of machining jigs and fittings that may never be used again. To my mind jigs and power tools only make sense if you are going into production. Asymmetries and small slips of the hand tools here and there add to the charm of the finished instrument.

Since I had no innovations to make on the neck, I ordered a pre-made one. For under $100, fretted and finished, it was quicker and cheaper than anything I could have made from scratch.

To help visualize the the way the bent top and back would look, I made a simple model that I carried around with me for a couple of weeks while I thought about the design

Forms for shaping the top and back were made from masonite screwed to plywood and wedged to make the desired curves.

The top was lightly moistened and hot sand bags (actually couscous bags) were laid on top

the top bent readily, in fact it over-bent

The back didn’t bend so easily, preassure had to be added as well as heat. I ended up bending parts of it freehand on a hotplate
This would all have been much easier if the grain was oriented at 90 degrees to that of the top.

Te braces were laminated directly onto the top and back

The bracing was a lot lighter than on conventional guitars. For thge top I used a standard X-brace, The back is also X-braced and has extra very light struts

The top was temporarily taped to the sides ….

… and an off-set line was marked

With the top glued on the neck mortice was cut.

Notice the temporary target bridge glued to the top. In order to be able to clamp the bridge during glueing, It was necessary to fit and glue it before the back was on. Normally clamps can be reached in through the sound hole, but that wasnt an option with this design

Drilling the recesses for the furniture bolts.
Notice the two temporary stabilizing-supports running between the upper and lower blocks . This was the only mold I used for the body. The supports were removed just before closing the box.

Marking the off-set from the back

Trimming the sides

Cross straps were added to the sides with the thought of limiting any cracks that might form. I did include an electric pick-up. I probably wont use it much but, without a conventional sound hole for access, fitting one later would be difficult.

For the bridge a 2mm piece of rosewood was bent to the shape of the top

A piece of ebony was chalk fitted to the rosewood and the bridge was fully shaped before …..

….being chalk fittend to the top.
The extra work of fitting the braces and the bridge to the curved surface of the top could well be the reason that not many guitars like this have been seen before.

As far as I can see the main purpose of the bridge plate is to stop the ball ends of the strings pulling up through the top. I made this one out of ebony. To reduce weight, it is a lot smaller than usual.

The back, ready to glue on. Clamping the back and front to the sides for glueing was difficult.
Instead of clamping, I simply used a lot of blue tape to hold them in place

Binding the edges of the body was difficult because the binding had to bend in two directions at once. I tried various options before settling on the no-binding solution. Aesthetically it works with the “modern” lines. For durability: I think its unlikely that I’ll wear the edges off in my lifetime

Assessment

The guitar sounded fabulous with 10 – 47 guage phosphor-bronze strings. With such a small body it doesn’t have a hugh low end, but the middle and upper registers are clear and clean. However the strings didn’t bend as easily as I wanted them to, so I switched to 10 – 46 guage nickel-wound electric guitar strings, These play just the way I want. These light strings are a little twangy or banjo like, but that’s a sound to have fun with. The guitar is not very loud but it’s really intended for living room – or parlor- use, so that’s not a problem. It has a great sustain and is as sensitive to tremolo and vibrato as I had hoped. Altogether I’m really enjoying playing it.

This body design was new territory for me. I went as light as I dared with the top graduations (2.7 mm in the center, 2.0mm in the flanks), and with the braces. Having assembled the body and felt how stiff it is, I now think that I could have gone thinner in the center.

The extra curves in the body do present some challenges for assembly, particularly with fitting the braces and the bridge, but I think that the results amply repay the extra trouble.

I’m already planning my follow up guitar: A 000/OM sized body with the same bent top and back. It will have a cut away and a 14 fret neck. This one will wear light to medium acoustic strings and I predict that it will be a great strummer.

Parallel Play

When I was halfway through building this guitar I heard that there is a maker in Colorado, Alan Hanson, who has been making guitars with a similar design for years. Reportedly his traveling guitar stole the show at the open stage sound demonstrations at this year’s Guild of American Luthiers. I resisted looking at his work until I’d finished working through the various problems myself, but now I recommend his series of YouTube videos if you would like to dig deeper into this type of guitar. He has some sensible innovations that I will adopt when I make my next, improved version. You can see the full range of his work and order your own example through his website .