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Old 1st August 2014, 07:22 AM   #1
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Default horn loaded guitar

hi there guys!

first of all, i'd like to explain that at this stage, the following discussion will be completely HYPOTHETICAL. while i do plan to make this guitar at some stage, when, i'm not sure. i would go to a luthier forum, but i want to design it from the perspective of the enclosure designer/audio engineer/physicist/driver designer/speaker enthusiast! so with that in mind, let your imagination go wild! but not too wild, stay on topic pretty please


i have been thinking about building an acoustic guitar using SPEAKER theory instead of just guitar builder/Luthier theory. by this i mean i want to think of the face of the guitar as the diaphragm/piston, the strings as the motor, etc to create something that is a little different. it doesn't need to sound anything like a traditional guitar by the end of it. it will be MY design with MY sound.

just for your reference, this is the "anatomy" of the basic acoustic guitar. just so we can all get our nomenclature correct

Click the image to open in full size.

just a bit of history, I've so far had a play with trying increase the efficiency of the "diaphragm". my main method was playing with different materials as diaphragm and surround. my favorite so far is 0.5mm fine plywood with carbon fiber bracing and 0.25mm silicon sheet for the surround, but i also tried fiber glass diaphragm and rubber surround. the rear enclosure was quite small, only around 0.5 liters.

this gave it quite a percussive sound because while the diaphragm was quite sensitive, the surround was quite dampening and absorbed the sound quite quickly. this was not a good or bad thing, as it just coloured the sound, which is can be a good thing. it was about as loud as a regular guitar, but a fraction of the size.


anyway, what i propose is a rear loaded, horn loaded guitar. my reason is for ultimate efficiency, as there is not alot that would be a problem or sacrifice because it is a musical instrument. it doesn't need to be designed like a hifi system. distortion is a-o-k.

to simplify/complicate things for maximum efficiency, i propose a diaphragm for each string, with a horn for each diaphragm. this way, each diaphragm can be maximized for two main things:
*efficiency
*resonance

i think for this reason, some super thin 2-3 layer carbon fiber, for the diaphragm may be best. but i may find that each string may need a slightly different material. who knows till i try it!!

there is a pass band, and for each string this is the ACCEPTABLE pass band.
e - 82-246
A - 110-275
D - 146-365
G - 196-490
B - 247-617.5
E - 330-825
while the guitar is capable of going beyond the above pass bands, this should be good enough, especially for efficiency. the narrower the pass band, the higher the potential efficiency and DB output. they are still quite wide in my opinion, but are a good starting point for designing.

so what i want from you guys is, how would i go about designing the diaphragm to best suit a horn driver for a given pass band? i suppose i try t make the diaphragm resonance the lowest frequency it will create? i.e the low e string's pass band is 82hz - 246hz so i should make the diaphragm resonance around 82hz? when modeling this in hornresp, i assume that only the sd is the important thing? i ask that because for obvious reasons i cant input the t/s.

now that i have had quite a bit of experience with fiberglass fabrication, i feel comfortable taking on a project like this.


as i said, this is all hypothetical atm, but any thoughts guys, i would love it!! maybe if i get enough info, i may build it!
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Old 1st August 2014, 07:45 AM   #2
freddi is offline freddi  United States
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Adele - Rolling in the deep - Cover (Stroh Violin) - YouTube
http://en.wikipedia.org/wiki/Stroh_violin

Click the image to open in full size.

not a horn but thought I'd put a plug in for Karlson anyhow
http://img86.imageshack.us/img86/9372/kbassvs6.jpg

Last edited by freddi; 1st August 2014 at 07:51 AM.
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Old 1st August 2014, 08:02 AM   #3
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similar to what i was thinking! but i was thinking one horn per string but clearly there is a positive to doing it. i assume the negative is that it doesnt sound like a traditional violin and that it is more expensive to produce.
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Old 1st August 2014, 08:26 AM   #4
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totally off topic but that youtube clip has the violin sounding remarkably like Darryl Way's mic'd up violin in the original curved air recordings.....so it can't be that far off.
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Old 1st August 2014, 01:07 PM   #5
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In essence, this can not be done.

Not without making said "guitar" too large to carry, or be playable.

The dimensions of the "horn" required are simply too large.

Even if you managed to make a horn somehow, there is no compression diaphragm available to drive the horn. The closest available method would revolve around the original acoustic gramophones, that used a needle attached to a diaphragm that was placed at the throat of a horn. But again, even a gramophone sized horn is not particularly small, and would be actually large to reach what you spec'd for low E.

That's why they have piezo pickups in the bridge and lightweight portable amps these days?

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Old 1st August 2014, 01:38 PM   #6
xrk971 is offline xrk971  United States
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Default Make a Cornu guitar!

Quote:
there is no compression diaphragm available to drive the horn
The bridge could be attached to a small diaphragm situated in a chamber much like a compression driver connected to a horn. The vibrations are small in amplitude but the force is substantial owning to the compression the strings under tension apply. A regular guitar or violin in fact uses the bridge and sound board to effect amplification via cavity resonance. Having a horn that could provide gain to amplify the lower notes would be a challenge though. One was is to coil the horn up much like the Cornu.

So, I think a neat experiment would be to take a Cornu BLH speaker and replace the driver with a custom bridge-coupled diaphragm. One would attach the neck/fretboard to the face of the Cornu. The Cornu is compact, lightweight, and has double length horns to help smooth the response. One could make a double layer Cornu and have 4 different length horns. I think a good diaphragm for this experiment would be to take an existing driver with a stiff suspension and mount the bridge to the cone. The outputs of the voicecoil would double as the pickup out for an amp! With a little photoshopping you can see what I mean...

Click the image to open in full size.

The Cornu speaker has perhaps one of the largest horn expansion ratios of any BLH speaker (about 25:1) which gives it about +10 to +12dB of gain.

This would actually look kind of cool and be a total conversation piece.

The double layer Cornu looks like this before the guitar adaptation (which by the way appears to be foam core channel construction and for a Lowther - and whoever did this never posted or discussed it on the Cornu thread):
Click the image to open in full size.

Given that the acceptable passband is 82Hz for the lowest notes, this is a very reasonably sized Cornu speaker of approximately 20 in x 20 in x 3 in deep. If you made it double layer then it could be 6 in deep to give it the holdability and feel of a regular guitar body.
Attached Images
File Type: png Cornu-guitar-sketch1.png (221.0 KB, 421 views)

Last edited by xrk971; 1st August 2014 at 01:48 PM.
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Old 1st August 2014, 10:49 PM   #7
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Quote:
Originally Posted by xrk971 View Post
The bridge could be attached to a small diaphragm situated in a chamber much like a compression driver connected to a horn. The vibrations are small in amplitude but the force is substantial owning to the compression the strings under tension apply. A regular guitar or violin in fact uses the bridge and sound board to effect amplification via cavity resonance. Having a horn that could provide gain to amplify the lower notes would be a challenge though. One was is to coil the horn up much like the Cornu.

So, I think a neat experiment would be to take a Cornu BLH speaker and replace the driver with a custom bridge-coupled diaphragm. One would attach the neck/fretboard to the face of the Cornu. The Cornu is compact, lightweight, and has double length horns to help smooth the response. One could make a double layer Cornu and have 4 different length horns. I think a good diaphragm for this experiment would be to take an existing driver with a stiff suspension and mount the bridge to the cone. The outputs of the voicecoil would double as the pickup out for an amp! With a little photoshopping you can see what I mean...

Click the image to open in full size.

The Cornu speaker has perhaps one of the largest horn expansion ratios of any BLH speaker (about 25:1) which gives it about +10 to +12dB of gain.

This would actually look kind of cool and be a total conversation piece.

The double layer Cornu looks like this before the guitar adaptation (which by the way appears to be foam core channel construction and for a Lowther - and whoever did this never posted or discussed it on the Cornu thread):
Click the image to open in full size.

Given that the acceptable passband is 82Hz for the lowest notes, this is a very reasonably sized Cornu speaker of approximately 20 in x 20 in x 3 in deep. If you made it double layer then it could be 6 in deep to give it the holdability and feel of a regular guitar body.
LOVE IT! good starting point. 20inchs x 20 inchs is smaller then some of the guitars i've played, especially when it comes to acoustic basses, and only 3 inchs wider then my acoustic guitar! also, being a spiral inside, there is no reason it couldnt be a round shape instead of square, yes? or am i missing something? i take it the only reason they use a square design is for practicality? so that it can stand by itself and perhaps have a bit more strength?

well as i said, i am getting quite nifty with fiberglass, so i would most likely build the backing out of fiberglass, and have a nice timber facade on the front that that fiberglass horn spiral bolts to on the back. all i would do is get a big slab of wax, carve it to shape, and laminate fiberglass directly onto the wax. then i simply remove the wax or melt it out if its being difficult. so i could get all sorts of interesting shapes.

my knowledge of horns is limited (thus why i'm on here, i've never actually built or designed one) is there an advantage to narrowing the pass band per horn or should i simply have one large horn to guide all wave lengths? from an efficiency point of view? other then complexity of construction, are there any other problems with doing it like that?

Last edited by ubza1234; 1st August 2014 at 10:54 PM. Reason: more detail/praise lols
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Old 1st August 2014, 11:51 PM   #8
xrk971 is offline xrk971  United States
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It can be made round but you will lose a little bit of the mouth expansion at the end which is where most of it is. One could oversize the horn to say a 26 or 27 in scale and trim the corner to round it and end up with effectively a 20 or 22 inch one.

Here is the plan for the channel to give you an idea. This is the full size 75cm version.

Click the image to open in full size.

You could construct it with foam core for the channels which is the preferred route for ease of assembly. Then if you like fiberglass add a layer over the foam core like a surfboard. Cap with a nice spruce or mahogany top like a real guitar. The top piece will need to be bicker and strong to hold she freeboard and string tensioning device.

The diaphragm is where the hard work is. How to design one that efficiently couples the at ring vibration energy to drive the horns.

Can you post photos of your custom guitar with diaphragms that have carbon fiber bracing? I am thinking the design you have may be the way to go as the carbon fiber bracing is used to control the stiffness of the suspension and the silicone surround is just for the seal. I think a 5 in pro audio midrange driver with a fairly stuff suspension can be used - additional carbon fiber struts can be glued to the back of the cone and attached to the main board to adjust the stiffness.

Last edited by xrk971; 2nd August 2014 at 12:01 AM.
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Old 2nd August 2014, 12:58 AM   #9
Quard is online now Quard  United States
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well, a dobro seems about halfway there,
maybe front-loaded horn,
with sealed glove reaching in from the outside to play,
like they use at the CDC?
Dobro
CDC | Public Health Preparedness Report - Public Health Laboratories (scroll to bottom)


or maybe a banjo with a back-loaded horn,
if you can accept the sonic compromises of a tightly-constricted, oblique throat?
might take the weight off your hands if it reaches the floor--
but I'd be more concerned about the psychological effects of playing the beast regularly
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Old 2nd August 2014, 01:42 AM   #10
freddi is offline freddi  United States
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it would be fun to try such a thing whether the outcome is great or not - my 1926 Orthophonic Victrola can sound great. There would need to be a workable diaphragm under the bridge to drive the throat. (or cheat with a fullrange speaker, magnetic pickup and concealed class D amp)

for non-horn, this technique from 1951 works well enough - just make sure to brace the upper bout region well

Click the image to open in full size.
Click the image to open in full size.

Last edited by freddi; 2nd August 2014 at 01:45 AM.
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