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Old 29th October 2009, 06:00 AM   #1
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Default Unconventional cabinet oppositethink

Well it's like this... I went to the Kerville folk festival this year. (It was an amazing experience, tons of people sitting around playing music at a campfires) I noticed a fundamental issue that plagues acoustic guitars. The low end is pretty much non existent, especially in the bass variety. Acoustic guitars are generally built around the idea of vibrating a sound board via a bridge. Analytical photography indicates that sound propagates from the center of the bridge and the sound hole, and of course the strings themselves. So the body is just a cabinet. In good speaker design the goal is usually to be neutral but I'd like to do the opposite and really reinforce the lower tones. I have been looking at various designs and suppose that a vented or ported cabinet would be best for this. But I am wide open to suggestions. For now I am thinking of the whole front surface of the cabinet (a very thin piece of wood or other material) as the driver, with the bridge- the "magnet"- in the center. I have looked at several neat calculators for determining volume etc. but they require values (for Q etc.) from speaker manufacturers that I cannot figure out how to evaluate. Then where is optimal placement for a port? This challenge is somewhat counter-intuitive for speaker design as I think that using resonant frequencies to actually boost the lower tones is necessary. On a bass The "E" is the lowest string . It is about 41 hertz. "A" is next at about 55 HZ. "D" is about 73.5 HZ and highest is "G" at 98 HZ. In conventional designs, the top two strings and on up are reproduced quite well and are easily heard as you all know, but 40 to 60HZ really disappear. I'd really appreciate any ideas to tackle this. I thought it might be fun to tackle the other end of the sound chain! So I guess it comes down to how to calculate cabinet dimensions and size/placement for port. Same old questions. though with some wrinkles; should the back of the cabinet be thin too to act as a kind of passive radiator? Whatever it takes to boost the bass.Later of course it will be cabinet shape. (rectangular guitars are maybe only cool for Bo Diddley!)
Thanks!
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Old 29th October 2009, 02:37 PM   #2
danielm is offline danielm  United States
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I've been on staff at KFF for years... Very cool gig!
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Old 29th October 2009, 04:34 PM   #3
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by Lensmonkey View Post
..I'd really appreciate any ideas to tackle this. I thought it might be fun to tackle the other end of the sound chain! So I guess it comes down to how to calculate cabinet dimensions and size/placement for port..
Hi, This has been done with great success by Georg Bolin the inventor of the 'Tonbord' or _Tone table_ if translated word by word.

Vintage Guitars, SWEDEN - 1970s Georg Bolin Tonbord

Quote:
..Same old questions. though with some wrinkles; should the back of the cabinet be thin too to act as a kind of passive radiator?..
I think the rear of the cabinet should be made as stiff (non resonant) as possible and the thin front baffle surface should be designed with wood stiffeners like the front assembly of the 'Bolin Tonbord' + maybe include an small F hole resonator too. A Bass Shaker could easily be used to act as the baffle driver.

b
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Old 30th October 2009, 02:18 AM   #4
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Lens,

I think you are actually describing the basic design of any acoustic guitar or bass? The bridge and the front face of the instrument in effect become the speaker and the sound hole the port.

Is your intent to build "a better bass" so to speak?

I think that the problem with bass frequencies in an acoustic instrument comes down to physics that cannot be overcome with simple design changes. It really comes down to the wavelengths of the lower frequencies and the amount of energy it takes to compress a sufficient volume of air at those frequencies to be audible.

Hence the whole reason for amplification. A 12" woofer with 50 watts of power will be able to compress the air and create the sound needed. A simple string with a resonant piece of wood cannot. I would also think that to "reinforce" one frequency you do so at the expense of another.
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Old 30th October 2009, 09:19 PM   #5
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Thanks for your interest and replies. In response, basically yes, that is what conventional guitar design does, and it works well enough for the higher frequencies, just not the low ones. "Physics that cannot be overcome with simple design choices", You are so right, you can't beat physics! The current design is old; at least 1850 for classical guitar, (and arguably older for similar types of stringed instruments.) All of the books and research on tone I've seen so far, "Engineering the Guitar" comes to mind, show (with super neat-o photography) just what we intuited: the bridge vibrates the front of the instrument like a speaker, and out the sound hole. All of the designs I have seen simply try to get the most out of the same old design mostly with better materials, and careful craftsmanship. This has not yet produced a decently portable, guitar sized instrument with a good low end. So for a challenge, let's keep the idea of the guitar, but not be wed to it's traditional construction, and maybe we will find a simple design solution for more robust bass. We have math, tools and knowledge of which the early luthiers had no idea; let's rock it.
Guitars mostly vibrate the area of wood around the bridge, this functional "speaker" is pretty dense and rigid, and doesn't allow for much in the way of longer wavelengths to propagate, as coldcathode notes.
OK, soset aside structural concerns for a minute and think if the front of the instrument could be very thin and light, say like speaker material. (Remember that in this case, we are not out to create a perfect reproduction of an input tone, but a wholesome bass; it shouldn't be obscenely loud at only certain frequencies, but resonances, and overtones are the character of an instrument's timber. I guess I am thinking of something like a solid piece of very thin hardwood, being freely driven from the center, as a voice coil does.
The volume of the instrument can be what a fellow could comfortably set on his lap. If the whole surface of the cabinet is a kind of flat cone, my instinct suggests a port, but where, and what size? Right now I just don't know what formulas to employ to try to make this geometry the best for 40 to 60 HZ.
coldcathode makes a point about how a watts move a speaker which moves the air. That is kind of what got me thinking about this whole thing. Physics and energy in--> energy out. The current bass guitar seems inefficient. The fingers put a bunch of energy into the strings. that energy needs to be transferred to the air. There several parts to that problem, but right now I'd like to ask for your knowledge and experience on how to approach the best cabinet design. Thanks!
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Old 30th October 2009, 11:21 PM   #6
Ron E is offline Ron E  United States
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The place I would start, is by making the sound hole smaller, maybe put a cardboard insert in it (or something simple like that) to try it out. That will tune the cavity to a lower frequency. Maybe look at a guitar shop and play a few instruments and try to evaluate what makes one sound bassier. Just guessing, interesting problem...
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Old 30th October 2009, 11:51 PM   #7
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Thanks Ron. What I have in mind is to start from the ground up and create a box first, essentially a cabinet. I haven't decided yet what the face of it should be made of. Something like cardboard, or very thin wood, or even plastic. This would be vibrated at a given lower , frequency from the center with something maybe a voice coil or whatever I can find to work at some low frequencies. But I need a starting point for dimensions, and port placement/size. Volume should be at around 1.17 square feet. the width at around 6 inches. Pretty open from there, guidelines are the same as a sub; usable size and good lows, without the constraint of a flat response.
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