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I want my next build- steel string jumbo, to have a lot of sustain but I'm not sure about how to purposefully build this in. I imagine that a low neck angle would improve sustain but would compromise the volume. Do really stiff sides help? Does an active back add sustain? What else?
I'd be really interested in people ideas on this or to be pointed in the right direction.

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Good sustain is a complex issue. The neck is very important to sustain, though I don't really feel neck angle is a part of it. The neck needs to be stiff so as not to wick off the vibrational energy of the strings. Adding mass to the headstock can increase sustain. Of course the top has much to do with sustain as well. A bracing design that is well balanced in how it reacts to the string vibrations in both directions of movement will help sustain. Properly placed mass at the bridge can help sustain by increasing the flywheel effect. Flexibility at the edge of the top where it is joined to the rims can help sustain. There is no magic formula, one must find their way based on trial and error.

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Brian

You never know what you are capable of until you actually try.

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In my experience a heavier neck really helps with sustain. Get the darkest heaviest piece of mahogany you can find, and build it as thick as is comfortable. Also an almost dead straight fretboard with very little relief seems to increase sustain (and clarity). A well done fret job will help too. In general, building a bit heavier overall will increase sustain. A longer scale length will make a difference as well. And yeah what Brian said. Good stuff there.

Have you read the Somogyi books? I like his concept of "type 1" and "type 2" sustain.

Type 1 is impedance based. A more massive, stiffer top is harder to get moving, so less energy is bleeding out of the strings into it. Solid bodies are the extreme of this. The strings only die out because of their own internal damping (and a bit of neck flexibility). Nylons have high internal damping, so they never have much type 1 sustain. A jumbo steel string probably will have quite a bit simply by virtue of the large soundboard being hard to make lightweight.

Type 2 is flywheel based, storing vibrational energy in the back for later return to the top. Reduces peak volume, but gives more sustain, and generally sounds better. And it's not dependent on the top being difficult to move, so you can have your cake and eat it too. This is where low damping back woods are important, so it will continue to vibrate long enough to provide useful sustain. Make the back reasonably lightweight, and aim for 2-3 semitone difference between the main top and main back frequencies (in the fully coupled closed box tap, not free plates). Padauk would probably be good for a jumbo, given its low damping and relatively light weight.

Massive sides probably help too, by reducing energy loss into the player's body. But I haven't been disappointed with the sustain on my thin sided guitars, so I'm not sure it's really that important. Not sure if side stiffness really matters for sustain either, but I do use full height side braces, FWIW.

I haven't built enough yet to properly verify this, but I wouldn't do a low neck angle on a jumbo that's designed for standard tuning. Reason being, the large size and high mass tend toward low resonant frequencies. So to get them back up, you need to add stiffness, and that means more than is strictly necessary for survival. In which case you can crank up the neck angle without over-torquing the top. May reduce your type 1 sustain, but not type 2.

What style of soundboard design are you doing? Flat? Domed? Cylindrical? Thin plate? Thick plate? Somewhere inbetween? Standard X bracing where the plate is left to fend for itself behind the bridge, or a pattern that includes bridge support?

Thanks very much for your informative replies. I have a few questions:



How do you place mass properly at the bridge?



I was thinking of using black walnut for the neck as it fits my wood palette design, what you think of that?
How do the frets affect sustain?



Very niece answer, thanks, Dennis. I have one of the Somogyi books, I'll check it out tonight.
To excite the main top and back frequencies, where do you tap them and how to distinguish between them and the internal air resonance?
I am going to use Padauk, so that is good news. Actually I'm using Padauk for back and sides, fingerboard and bridge, bridge plate and binding too.
I'm doing a domed top, 25 foot radius. I was thinking about 3mm top thickness, thinned at the edges. And standard x-bracing. I'm using Sitka Spruce top. Black walnut neck with Padauk centre strip.

Thanks again, looking forward to more help!

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We are all in the gutter but some of us are looking at guitars.

'Sustain' can be thought of as the length of time that the sound of the guitar stays above the threshold of hearing. There are two ways to get it. One, which Somogyi calls 'type 1', is to establish a large impedance mismatch between the strings and the soundboard, so that it takes a long time for the energy to 'leak' out of the stings and into the top. So long as the top is light enough and flexible enough to produce an audible sound you'll get long sustain. Anything that adds mass or stiffness is likely to increase this sort of sustain, and the closer to the bridge the more effective it will be. The main trick is to keep the balance of stiffness and mass such that you get the frequency response you want. You can get this sort of sustain by adding a bunch of mass to the bridge (say, with depleted uranium bridge pins and an African Blackwood bridge and plate)), but that tends to cut into the treble too much. Alternatively, you could use a very stiff top, and 'tapered' bracing that's tall in the middle and slopes off toward the edges. A top like this with a light bridge can be pretty 'treble balanced', so this is not the place to use that light Indian rosewood.

The other type of sustain is trickier: it involves making a very light top that is extremely efficient at turning string forces into sound. The sound has a fast attack and decay, but it gets so loud that it actually stays above the hearing threshold for a long time. I think of this as 'banjo' sustain (the other kind is 'Les Paul' sustain), but it's the way Flamenco guitars work too. Dreads with scalloped bracing are also moving in that direction: they tend to use a heavy bridge, in part IMO, to help control 'wolf' notes that can be an issue with a light flexible top. So, for that matter, do some of the 'modern' Classicals with light 'sandwich' tops and heavy backs. The back doesn't so much 'store' energy in those instruments, IMO, as it does simply help keep it in the top.

I don't think neck angle makes a lot of difference, except that a high neck angle that requires a tall bridge puts a lot of stress on the top. That calls for solid construction, which gets massive and stiff in a hurry, so you end up with 'Les Paul' sustain.

On most acoustic guitars the neck is enough stiffer and heavier than the top that it should not be draining energy off too fast, except, possibly, at some strong resonant pitches. So far I haven't seen much evidence of this in the measurements I've made, but it's possible.

I'll end up with the usual caveat: it's all more complicated than it 'ought' to be. For example; since your hearing is much more sensitive around 3000 Hz than it is at the open string fundamental pitches (below 360 Hz, say), anything that enhances the high overtones could add to sustain, even if the fundamentals don't hold up for very long. It's possible, for this reason, that adding mass at the bridge could cut sustain, because it enhances sounds that are harder to hear at the cost of those that are easy.

What I would do:

Walnut or sapele neck, probably with a non adjustable steel bar reinforcement, and an ebony fretboard.
Fairly light and thin top with tapered bracing.
Moderately thick back but thin sides.
Bridge on the heavy side.

The neck draining energy is an interesting question. It seems to me that it does, though I'd like to see some testing. I always recall a particular physics lab, in which we learned that it was primarily the flexing of the support that stopped a pendulum from swinging, not friction in the air or in the string.

The easy way is to tap on the bridge (I just use the pad of my index finger) and record it in Audacity (free audio program), and look at the frequency spectrum. There should be 3 fairly obvious peaks for the air, top, and back.

The difficult way is to use your ears. I'm still learning, but lightly tapping with a fingernail on the top and back seems to be a good way to activate them individually without the air resonance overwhelming everything. But it's still hard to identify the main plate frequencies unless you already know they're 2-3 semitones apart, and then can tap back and forth between the top and back and listen for that interval. Plus the frequencies may shift a bit in the fully coupled mode, and even after identifying them, it's hard to hear anything past the air resonance.


Padauk back and bridge sounds good. 3mm sitka top is too heavy and mellow for my taste, but probably good if you're looking for a classic sort of dry jumbo sound.

If it was mine, it would be redwood from Dave Maize. Super stiff and light and low damping. But together with padauk, maybe excessively zingy for some. Also it would be a very red guitar But for the sustain goal, maybe a heavier ebony bridge would be good with the lightweight redwood, and then you could use an ebony fingerboard to match, and maybe Indian rosewood binding and narrow center wedge on the back to tone it down a bit.

Joe, Rick Turner use to write about sustain here all the time. Try to find some discussion on his use of carbon fiber, the Antarctica guitar, Henry Kaiser, etc. It's likely to come up in those topics.

EDIT: Found this video. Didn't watch it, but do remember from seeing it in the past that Rick brings some of these things up.

1. Stabilizing the relationship between the nut and the bridge increases sustain. I wouldn't presume to tell anyone how to do this on an acoustic guitar but the physics of the problem are simple to understand. The nut and bridge are fulcrums. Unstable fulcrums will dampen vibration between them.

2. Increasing moment of inertia also increases sustain. Even if the fulcrums are completely stable relative to each other and the retention of energy between them is optimal, energy will be lost outside the fulcrums as a direct result of low moment of inertia....low mass. Anything you do to add weight on each end is going to aid this aspect but only up to a point. Once the mass reaches a certain point the limited energy of a vibrating guitar string will no longer appreciably move it...but you can't really ever get to that point on an acoustic guitar...nor would you want to, I imagine.

Sustain is the continuance of the fundamental tone. Having far less energy, the harmonics and other tone influencing factors fade while the fundamental remains. You might be on the road to creating a tone generator with all the musical allure of an oscilloscope. Obviously, the trick is to have an attractive mix of retention and loss of energy in between the nut and bridge. Retention for sustain...and loss so the wood gets a chance to influence the tone and create some volume.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I'm a total amateur but I believe that Somagyi is correct in building these supports at the head blocks into his guitars and I have followed suit. I've only finished one but I am very happy with the sustain on my first. Happier than I thought I would be actually.

What made me think this was correct is that it solidifies the soundboard under the fretboard all the way to the sound hole. In theory this should make the lower bout the primary area of vibration. If that is true, then control of that vibration would be a bit more simplified as it is more centrally located. Maybe I'm way off, but this is how I am building.

Solid linings also help, reversed kerfed lining help as well. The idea is to great a bigger impedance mismatch between solid sides and the top. The bigger the impedance mismatch the more the vibrations will be reflected back into the the top, Vibrating the sides eats energy and does not really deliver sound.

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