Loudspeaker Resonator Guitar - Model Z8.2 (=Model Z8.1 Improvement)

In my previous instructable (https://www.instructables.com/Loudspeaker-Resonator-Guitar-Z8.1/), I showed you the Z8.1 model guitar. The resonator of this guitar is a speaker cone instead of being a wooden acoustic table, as it is usually . This guitar was mounted with nylon strings.

This Z8.1 guitar sounded well, but some vibrations bothered me especially on the A string, so I thought of some improvements.

On the other hand, I wanted to try to mount this guitar with metal strings, in order to have more sonic power, but I knew that by doing this, I would increase the problems of vibrations.

On the Z8.2 model, in order to slove this problem, I completely changed the designs of the spider (wich becomes more rigid), the bridge (wich becomes softer), and the saddle (wich becomes more tunable) .... and the tail piece (because the bridge becomes larger)

Now there are no vibrations anymore, and the sound is clear . I m really very satisfied on the result.

Supplies

Materials:

Spider

In my case, I used a glass fiber sheet from a self-service tray. dimensions 300mmx200mmx3mm, Just Perfect.

Bridge

2 x Aluminium sheets 110x50x2mm (Layer 1 and 2)

3 x Aluminium sheets 110x15x1,6mm (layers 3.1 and 3.2 and 5)

2 x Aluminium sheets 110x20x1,6 (Layers 4.1 and 4.2)

1 x hinge 70x15mm H=6mm

1 x spring : made from a metal sheet from a sardine can folded in 3 strips (15mm large). Another possible way is to reuse 3 clothespin spring

3 x M4 nuts (to be glued inside the bridge) 3 x M4x15 screws(to hold Layer 5)

4 x TFM4x15 screws + 4 x Nylstop nuts (to maintain the bridge on the spider)

Saddle

1 x T 20x20 mm Aluminium profile ,Thickness length= 110mm Thickness=1,6mm

Tailpiece

It was possible for me to reuse the Z8.1 tailpiece with a little modification (making it 15mm shorter)

Tools :

Static drill press machine, square, rule, M4 tap, metal files, (scroll saw if possible)

WD40 or oil, Epoxy glue, acetone or alcohol, one drop of nail varnish,

Step 1: Nylon or Metal Strings on a Classical Guitar? ... That the Question...

(Hamlett, act III, scene 1)

When 6 nylon strings are stretched, they produce a force of about 45 Kg

When 6 "Ultra light" 8-40 metal strings are stretched, they produce a force around 40 kg

When 6 "light" 10-50 metal strings are stretched, they produce a force around 60 Kg

When 6 "medium" 13-56 metal strings are stretched, they produce a force around 80 Kg

And I've seen that some string manufacturers (eg DallDario, Martin...) sell 12-47 "silk and steel" strings compatible with classical guitars. They should produce a force of around 50 - 55Kgs...(*). like strings between "ultra light" and "light"strings.

On the internet it is possible to read everything and its opposite about mounting metal string on a classical guitar: some have destroyed their guitar in one day, others have mounted metal strings for years with no problem...

I hesitated...I hesitated more ...I hesitated more again ...and at the end of the end, I phoned to the devil, and he told me that I could try "ultra light" or "special classic light " metal strings but not "Light" and "medium" strings because they are too strong... He told me to buy Martin strings - MA130S Silk and steel 11,5-47 . Theses metal strings are officially designed and sold for classical guitars. (There are others brands availiable) ... I'm weak... I gave in...

I'll begin my tests with nylon strings , and finish with metal strings... So ... help me God in order do not get the neck in my face!.

(*) rough assessment according to https://www.mcdonaldstrings.com/stringxxiii.html

Step 2: Forces on the Saddle and the Spider

When 6 nylon strings are streched, they produce a force around 45 Kg between the neck saddle and the tailpiece . If there is no tailpiece, the force is taken by the bridge, and the bridge risks to unglue. If there is a tail piece, this part takes most of the forces. That is why I choosed this option in Z8.1 and Z8.2 models .

In the Z8.1 model, the 6 nylon strings are oriented to the tail piece with very low angle (about 1 degree , so that the pressure applied on the bridge was around 45kg x Sin(1 degree) = 0.8Kg (rough approximation). This low value can explicate a large part of the undesirable vibrations because the strings "float" on the bridge.

When 6 "light" metal strings" are stretched, they produce a force of approximately 55 Kg between the neck saddle and the tailpiece. In the Z8.2 model, the strings will face the tailpiece at a larger angle (about 5 degrees), so the pressure applied to the bridge will be around 55 kg x Sin (5 degrees) = 4.8 kg ( rough approximation). This value suppresses unwanted vibrations, but on the other hand, it also increases the pressure on the Spider (4 legs = 4.8Kgs/4 = 1.2Kg /leg).

Conclusion: the Z8.2 spider must become stronger (and consequently more rigid) than the Z8.1 model spider in order to stand the presure, and the bridge must become softer.

Step 3: The Project

In Z8.2,

the principle of the spider changes : it becomes rigid. (In Z8.1, the spider was a simple soft plastic sheet)
the principle of the bridge changes : the assembly has two parts. The first part (= the base) is rigid and screwed on the spider. The second part (= bridge + saddle) can move thanks to a (hinge + spring) system above the rigid spider. This part transmitts the vibrations of the saddle to the middle of the loudspeaker .
the spider and the bridge are stronger in order to permit metal strings to be mounted
the strings are crushed near the top of the saddle, so the vibrations are eliminated. (Layer 5)
the saddle can easily be removed in order to facilitate the adjustment of the central washer and the height of the saddle

Step 4: The Spider

The Spider

In the Z8.1 model , the spider had to be soft, because the principle was that the bridge and the spider moved together .

In the Z8.2 model,

the spider must be done in a hard material because it must not vibrate. Any hard wood, hard plastic sheet , aluminium sheet may be good. I suppose that two LPs glued together may be good too. In my case, I found a super glass fiber sheet from a self-service tray. Perfect. Thickness 3mm.
the spider may have the same shape than in Z8.1 model, but I choosed to reinforce the sides
It is fixed with the 4 screws maintening the loudspeaker (like in Z8.1 model) .
The deepness of the spider must be adjusted because the bridge is higher in Z8.2 rather this of Z8.1

The spider is made with a fiber glass sheet 3mm . Use a drill press and a scroll saw in order to drill the holes

Like in Z8.1 model, take the loudspeaker mounting hole as reference marks. Draw the center, the holes to hold the bridge. etc etc.. ,

The shape of this new spider changes: the four legs are reinforced with a ring , so that the piece is now completely rigid.

Drill a hole D= 10mm in the center (to let the central washer free)

Drill 4 holes D=4mm for maintaining the bridge

Drill The big holes with a scroll saw or plenty of little holes + handsaw + file. (That's what I did)

Paint the spider in black

Step 5: The Bridge + Saddle

The Bridge

In the Z8.1 model, the bridge was simple. Just an adaptation of the brige of a classical guitar

In the Z8.2 model , the bridge is much more complicated.

From bottom to top

The bottom layer (1) is fixed on the bridge . There is a big hole (10mm) in the center, where the washer going to the center of the loudspeaker can freely go through .
Then there is a hinge and a spring bet tween the layer 2 , so that the layer 2 can move (because of the hinge) and can vibrate (because of the spring). This part transmit the vibrations from the saddle (T aluminium profile) to the central washer (going to the center of the loudspeaker)
Over the layer 2 is a rail ( layers 3.1 , 3.1 , 4.1 , 4.2) able to stand a T aluminium profile , which is itself the (removable) saddle.
Above the layer 4.2 is the layer 5, screwed into the bridge body with 3 M4 insert nuts. This layer has 6 stripes intended to guide the 6 strings from the top of the saddle (in order to reduce vibrations) to the tailpiece.

The bridgeis the hardest part to manufacture in this project. it needs rigor and precision. Do not hurry on each step, adjust correctely the saddle , and let the glue drying,

Layer 1 and 2: No special problem.

Layer 3.1&3.2&L4.1&4.2 : Glue layers 3.1 and 3.2 to layer 2. Put WD40 between L3.1 and L3.2. Insert the T-profile oiled with plenty of WD40. Let dry overnight. The next day remove the T profile, clean the assembly with alcohol or acetone, remove the excess glue, and glue L4.1 and L4.2 over L3.1 and L3.2. Let dry another night. Clean again, remove the excess glue, and if the T profile does not fit, sand it, put WD40, remove the excess glue again, etc, etc... It must fit and it slip moderately

Trace the location of the strings on L5 (11mm spacing for a classical guitar). Drill 3 holes diameter 3 between the strings, as shown on the drawing. Fix L5 with an adhesive tape on L4.2. Counterdrill the 3 holes from L5 into layers 2, 3.2, and 4.2.

Turn the part, and counter-drill the 3 holes diameter 3 to a diameter of 7.5mm to a depth of 3mm intended to receive 3 M4 nuts. Attention, it is necessary to do this operation with a sensitive drill, because the holes must not lead! Fill the holes with glue and drive the 3 M4 nuts into the holes with a hammer.

Leave to dry overnight, counter-drill the nuts full of glue to diameter 3, and tap M4 starting on the saddle side.

Layer 5: Counter-drill the holes from diameter 3 to diameter 4 . Shorten the sides. With a small triangular file, file 6 short notches spaced 11 mm apart. These notches will guide and crush the strings later, to prevent them from vibrating above the saddle.

Glue the hinge (taken on a kitchen furniture) between L1 and L2. Use a 6mm spacer to make sure L1 and L2 are perfectly centered and parallel.

Spring: At first I used 3 clothespin springs, but it was not so easy to mount. Then I used sheet metal from a sardine can lid bent into 3 15mm strips (like a "Z"). This should work well too with a can of mackerel or tuna ((Ha! Ha! Ha!))

Clean the folded sardine tin , and glue this spring with epoxy on layer 1 only.

The saddle

must be able to slip slightly into the bridge. The saddle is removable because

it is easier to screw or unscrew the tuning screw of the central washer
it is easier to tune the height of the bridge at the final tuning. If the tuning is not correct , it is possible and easy to shape another T profile without any other concequence.

The top of the saddle may be shaped as you want, but do not touch at its height at the moment! (it 'll be filed on tuning step)

Step 6: The Tailpiece

As the bridge is larger than in Z8.1 model, I needed to shorten (15mm) the tail piece I buit before. It should be possible to reuse the Z8.1 tailpiece

Step 7: The Assembly

Screw the bridge over the spider, screw the spider between the loudspeaker and the acoustic table, and tune the loudspeaker and the saddle accordding to the 3 following constraints:

Constraint 1:

the top of the bridge (without the saddle) must be around at the same height as the fingerboard. Respecting this constraint mean that the heigth of the saddle is hight enough to be tuned. I can't give you a dimensional prescription, as it depends on the loudspeaker and the materials you used. Just respect the constraint above. You may need to add or subtract millimeters from the height of the spider or the base of the bridge. In my case, as wanted to mount metal strings (it is the worst case) , in order to satisty to this constraint, I needed to increase the distance between the sound table and the spider with four washers (thickness 3 mm) under the 4 screws wich maintain the loudspeaker. Otherwise, the saddle is not high enough, and it is impossible to file and tune the saddle at the correct heith value

Constraint 2:

mount the strings and tune the guitar in E,A,D,G,B,E. At this moment, you have the nominal pressure on the bridge.
adjust the central screw so that the loudspeaker cone is in the nominal position (the loudspeaker must not sink)

Constraint 3:

File the top of the saddle so that the string height is around 4mm (for nylon strings) or 2mm (for steel strings) above the 12th fret.

Do iterations and intermediate checks, and finally:

The top of the fingerboard and the bridge (without the saddle) should be at the same level.
The speaker should be in the unconstrained position with the strings nominally taut.
The saddle should be about 4+4mm (for nylon strings) or 2+2mm (for steel strings) higher than the 12th fret (the 12th fret is located exactly in the middle of the string).

When all is well, with these 3 constraints,

Put the saddle out
put a drop of nail polish on the central screw,
replace the saddle,
crush the strings on the saddle (Layer 5) with the 3 screws M4.

Step 8: Final Assembly

Final assembly of the guitar with silk and steel strings.

Step 9: The Videos

Video 1: Model Z8.1 with nylon strings - Diatonic range + MLF_Le_Temps_Des_Marguerites : https://youtu.be/0n40UanDccM

Video 2 : Model Z8.2 with nylon strings - Diatonic range + MLF_Le_Temps_Des_Marguerites : https://youtu.be/sPaO2hqvAJk

Video 3: Model Z8.2 with nylon strings _Lettre a Elise :

https://youtu.be/2jw-wYxUOew

Video 4 Model Z8.2 with "silk ans steel" strings -diatonic range + MLF Le_Temps_des_marguerites

https://youtu.be/bLxJu5o-KZw

Video 5: Model Z8.2 with "silk ans steel"strings : GM_Rue des fossés saint Jacques

https://youtu.be/JC7cpdWvsy8

Step 10: Possible Improvements, Final Thougths

The speaker I used had a paper cone. If you can find a plastic cone, I think it would be better for outdoor use, because of the humidity.

The guitar is heavy. Much of the weight is due to the weight of the loudspeaker magnet. If you don't use the loudspeaker as a microphone, the best is to remove the magnet to make the guitar lighter.

The loudspeaker used as a microphone works: that's a fact. However, its principle is to capture acoustic vibrations. This is why this microphone is very sensitive to feedback (Larsen effect) . If you play guitar with this option, you will have to play away from the amplifier...