Loudspeaker Resonator Guitar . Model Z8.1

Usually, an acoustic guitar souds because strings transmit their vibrations to an acoustic wooden table.

There is another way: vibrations can be transmitted and amplified to a metallic cone (Dobro guitar) or to any box (a can or a cigar box guitar for example).

In this project (inspired by Markett in his post "https://slide.forumactif.com/t8425p25-dobro-avec-cone-en-papier-idee-fumante-ou-petard-mouille") , I used the capability of a paper cone of a loudspeaker to vibrate instead of a wooden acoustic table.

The vibrations of the strings are collected on the bridge, the bridge is fixed on a spider (like in a dobro guitar) , the and the vibrations are transmitted to the paper cone.

The spider avoids the pressure of the strings to destroy the loudspeaker, and transmit the vibrations, but , in the same time, amortizes them. Because of that, you may not hope to have the sound produced with a 10,000$ acoustic guitar!

On the other hand, you will have a unique guitar in your hands, and that is priceless.

Supplies

Materials

1- An old broken guitar with its bridge, destined to burn in the fireplace

2- 2x 50cm of squared metallic tubes for the renforcement of the box . Alumnium is lighter. (I used tent bars)

3- A large loudspeaker (Mine was oval: it is easier to fit it into the soundbox ) If The loudspeaker works, you can use it as a microphone.

4- A plastic sheet greater than your loudspeaker (mine was the cover of a school folder)

5- 30cm of a PVC tube (minimum diameter = 5cm)

6- epoxy and wooden glue, screws , nuts and bolts

7- DVDs (for mosaic), Acrilyc and varnish paint, for decoration

Tools

No special tools: large table, rules, square rules, pens, pencils, sand paper, screw driver, saws, soldering iron etc, etc..

Step 1: Repair the Old Guitar,

The structure of the future guitar must be strong,

When the hole for the loudspeaker will be done, the guitar will be still more weaker. So first of all, you have to repair the old guitar (mine was full of holes, and the acoustic table was unglued: it was awfull...).

Step 2: Drill the Loudspeaker Hole

In my case, I found an oval loudspeaker. That means I could mount it INSIDE the soundbox. From my point of view, I think it's more beautiful. But if I had a round loudspeaker, I would have had to put it ABOVE the sound box. It does not matter. It's just a matter of aesthetics. Some dimensions will change later, but nothing changes in the principle. In any case, the hole must be less wide than the loudspeaker, so that it remains wood on the soundboard to screw the loudspeaker on it.

Verify that the loudspeaker is not too deep and can fit in the soundbox.

Make a paper template for that: draw the interior shape of the loudspeaker, the location of the fixing holes of the loudspeaker, and don't forget to draw the vertical and horizontal axes of the loudspeaker. There must be a line along the along the bridge and a line down the middle of the neck. At the intersection will be the center of the speaker. Do not forget to draw these lines, because when the hole is made, you risk to forget where they were!

The distance between the start saddle of the neck and the 12th fret must be equal with the distance between the 12th fret and the bridge (just in the middle of the loudspeaker) of the sound box .

Then make a hole with a drilling machine, take your best jigsaw (little handsaw in my case) and your courage with both hands , and saw the loudspeaker shape!

Step 3: Reinforcing the Soundbox

Now you have a big hole in the sound box and the guitar is weak and will break if the strings are stretched. (tension = around 45Kgs with nylon strings and around 75Kg with metal string). It needs to be strengthened.

I took two square aluminum bars (in my case about 1cmx2cmx40cm) from an old tent and stitched them together along the length of the sound box making sure that once in place I could still place the loud speaker. That's why I made an extra thickness at the ends.

Then I glued everything inside the box with epoxy glue

NB: As you ca see on the picture, I glued 4 nuts M6 in the back of the loudspeaker, so that the mounting will be easier later...

Step 4: Preparing Loudspeaker to Receive Vibrations

It is impossible to give any mounting prescriptions because of the geat numbers of parameters according to the shape of the soundbox and the dimensions of the loudspeaker. The best is to refer to Markett's sheme where he describes the principle. According to your guitar , you'll have to adjust all the dimensions

I glued (with expoxy glue) a plastic threaded spacer in the center of the loudspeaker, vertically maintained with a piece of wood. The lighter is the spacer, the best it is. Do not use magnetic materials, because of the magnet of the loudspeaker wich is very close.

In my case, my spacer between the bridge and the middle of the oudspeaker is about 3cm long. You must imagine that the bridge fixed on the sider will go down (around 0,5 to 1cm) because of the string pressure and at the end, you will have to tune the hight of the bridge (see step 8)

Step 5: Preparing the String Holder

Take a PVC tube diameter= 50mm at least. Length 30cm. Cut it in the length. Heat it with a heating gun or an iron. Flat it and draw the shape you want

On the bridge side, the extremity of the string holder must be as close to the bridge as possible

The strings holes are 11mm spaced

On the other side, the string holder is heated, curved with a right angle, and screwed on of the soundbox

NB: the spider you see on the picture is a non working prototype (too rigid).

Step 6: Spider

The spider is a delicate piece that requires a lot of rigor. The spider must be elastic in order to transmit vibrations to the loudspeaker, but must be strong enough in order to stand the string pessure.

This piece is cut from a sheet of plastic (in my case a cover of a school binder). It must be stretched between the loudspeaker fixing screws. If it's too tight it won't mount, if it's not tight enough it floats and the bridge sinks into the speaker. Hmmm.... difficult equation....

Here is how I solved the problem:

Take the plastic sheet flat on the body so as to cover the speaker hole, drill a hole with a soldering iron, and put the screw in the hole. do the same for the other holes, one after the other. At the end, the sheet should be perfectly flat. Then take the 4 holes as a reference and draw 2 cm strips to the center. Make curves around the strips (no right angle ) at the place where the bridge will be supported. Cut the sheet according to the drawing. Secure the bridge (recovered from the old guitar) with 2 M3x8 screws at each end. The bridge is partially drilled in the middle in order to receive the hight adjustment screw. the saddle on the bridge must completely lie in the bridge.

Step 7: Microphone

Solder two twisted wires (length 30cm) on the loudspeaker terminals going to a female jack 6,35mm connector. Drill a hole in the sound box according to the female jack you have. That's it!

The sound sounds clear , but make Larsen noise if I play too close of the amplifier, and is not equivalent to a proffessionnal microphone.... The avantage is that it is efficient for less than one buck!

Step 8: Decoration

Decorating is a long but enjoyable process, because it is nearing the end!.

It's simple: do what you want

In my case, I made a mosaic with cut DVDs (you will find many tutorials on the Internet to find out how to cut them), and I used black and white acrylic paint, and an ultra-gloss acrylic varnish (not do not mix acrylic and polyurethane products to avoid bad surprises). Allow 1 day to allow time for the varnish to dry between two coats.

Be creative! I can hardly wait to see your creations!

Step 9: Tunings

It is a delicate step (common to all guitars).

Mount nylon strings if the base is a classical guitar. You may mount metal strings if the base is a folk guitar (with truss rod in the neck). Tight the strings to the nominal value. Do not mount metal strings on a classical guitar : you risk to get the neck in the nose! Some people mount ultra light jauge string metal on a classical guitar, but I don't recommend.

In both cases, once the string pressure is nominal, you have to tune the heigth of the strings on the bridge using files if the bridge is to hight, an ajusting the washer in the center of the loudspeaker if the bridge is too low.

The paper cone of the loudspeaker mus not have any deformation in the quite position. Adjust the tuning screw in the middle of the bridge to avoid any pressure constraint.

For a classical guitar the heigth between the string and the 12th fret must be about 4mm

For a folk guitar the height between the string and the 12th fret must be about 2mm or 2,5mm

Step 10: Possible Improvements

There is a slight vibration at the bridge level. If you find the way to suppress it, I would appriciate!

Step 11: Video

https://youtu.be/EhPufMnstIg

Sound is low. Sorry. I don't know how I can do better...

Step 12: Conclusion

It is my first instructable. I hope you appriciate. It was a pleasure to give a second life to this old, very old guitar and to share this project with you.. I'll enjoy to read your comments, and to see your creations!