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It seems to my inexperienced guitar making mind that thicker backs and sides would be an advantage acoustically. My reasoning is a resonating soundboard needs more resistance to push against. The more a back bends the less power is available to drive the top. If every action has an equal and opposite reaction wouldn't a thicker, stiffer back produce the most volume?

Surely I'm not the first person to consider this.

Might be true, or truer, if you play with the back away from your body.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

I believe that stiffer sides are good but I am usually limited to a thickness that I can bend successfully. Backs can either be stiff or live. Live backs are designed to resonate with the top - I've built one with a live back using the falcate design from Gore/Gillette.

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Steve Smith
"Music is what feelings sound like"

On the side I give guitar lessons. Yesterday I was teaching this guy about varying the sound of his guitar with body contact, and other subtle variances in playing style so I'm familiar with what you're talking about to some degree but are you saying build it according to plan and use your body to control the effect?

Which did you prefer?

Which guitar was the loudest, had the most PUNCH?

How thick have you successfully bent EIR?

The guitar with the live back and the falcate bracing was the loudest by far and with the most punch too. How much the live back contributed I don't know. I've only used this guitar once in a concert to play a driving Irish rhythm and it did great-should be said it is also a dred.

I bent some rosewood the other day at 0.094" but only because I forgot to thin it down to 0.080" like I had planned. I bend on a pipe.

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Steve Smith
"Music is what feelings sound like"

Yes, a thicker, stiffer back can produce more volume, although I'm not sure your reasoning is correct about why.

Greg Smallman has also considered this, and his designs are quite different.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

It seems you get two very different 'camps' with this topic. On the one hand you have people who like thin active backs, like Ervin Somogyi and Trevor Gore. These are designed to have constructive interferance with the top, working a bit like like a natural amplifier (at least that's how I understand it) Look up some of Somogyi's videos on youtube where he gives the demonstration with the rubber band.
On the other hand you have the reflective backs, used by people like Greg Smallman and David Anthony Reid for example. These are very stiff and rigid and project the sound like you say.
From what I gather, reflective backs tend to give a bit louder sound but can have a negative effect on the tone. So you wouldn't have all the complex overtones that a well made active back guitar can have, which tend to be more sensitive for fingerstyle playing. However active back guitars need either a lot of experience or a lot of calculations to get perfect though and if you get it wrong can lead to the guitar having wolf notes or dead notes.

Note, this is all gathered from my reading online, not from experience, so please correct me if I'm wrong!

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"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

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A live back is one that vibrates using energy from the top, reflective back is not a term I like as it implies the sound is bounced back to the top. The wavelength of sound kind of says this is not really likely to happen. I prefer to think of it as being more acoustically dead as the sides are. I see the live back as more pleasing to the player as the sound is coming from the front and back of the guitar. It also adds a resonance or two to help color the sound. It also gives mechanical feedback to the player, the guitar feels alive. This also goes with lighter builds. A heavier back and sides may make for a louder guitar, but loudness is not the only measure of a good guitar. Making a guitar out of particle board and having a light top may give you a loud guitar (hey, I might try that) but it may not be as interesting a guitar to play.

Which would be preferred for an acoustic bass guitar? I am presently working on a U-bass and hope to improve the acoustic performance (I hate amplifiers).

Bob

it´s not all about the volume.
thickest i´ve bent indian rosewood was 2.4 mm, but maybe it was that particular piece that wassn´t too stiff.

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member of the guild of professional dilettantes

Loud is not a tonal quality.

I would say that perceived volume is actually a product of the tone due to the complex way in which our ears and brain process sound.

For all my most recent builds I have been going for light and loud. To keep things light is always my first goal. So I am in the thin sides and back camp here. To stiffen the sides I use full width (under the lining) side braces of spruce. I should mention I build for a lot of bluegrass players. It seems that for these bluegrass pickers loud is what they need to cut through. I build a lot of F style mandolins and if they are not loud they are not wanted. Flat top pickers seem to like loud guitars as well. Tone is hard to define. It is easier to build for volume than tone. Tone is subjective.

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http://www.cushmanguitars.com/.

A man who carries a cat by the tail learns something he can learn in no other way.
Mark Twain

'Loud' is a subjective impression, related to the actual output that you'd measure with a dB meter, but more complex. For example; normal ears are much more sensitive to frequencies above 1000 Hz than they are below, say 350 Hz, where the fundamentals of the open guitar strings are. Doing things that increase the low end output often don't make the guitar sound louder, even if they add a lot to the power as measured by a meter. Increasing the high frequency output can add to loudness, even when it costs something in terms of power.

On most guitars an 'active' back only actually increases the output in the very lowest range; below the pitch of the open G string. This is useful in putting a 'floor' under the perceived sound, and also in terms of 'projection', but may not make the instrument sound louder close up.

Gore shows (and my experiments back him up) that adding to the mass of the back, and more so the sides, can make the guitar louder by 'keeping the energy in the top'. Technically what happens is that the vibrating area of the top increases without a commensurate increase in the effective mass. This is particularly true of the 'main top' resonant mode, which actually produces most of the power. Even though this resonance is (usually) someplace near the pitch of the open G string, this mode accounts for a lot of output up to at least 1000 Hz, simply because it's the most effective sound producer on the guitar. Thus increasing the ratio of area to mass of the top can make the guitar 'bright' as well as increasing the low end power.

This is not a free lunch. There's a particular timbre associated with this, that might be hard to describe, even though we're all familiar with it. Basically, the stringed instrument with the highest ratio of A/m is the banjo, and the more you approach that the more of that sound you tend to get. Going the other way, toward a low A/m ratio takes you through the timbre characteristic of a resophonic guitar, then a Flamenco, a normal Classical or steel string, and, eventually, into 'piano' territory.

That said, simply making a normal guitar with heavy sides only takes you a little way down that road. In that case you end up with an instrument that is loud and projecting, and often quite 'directional'. Fred Dickens used to make Classicals that way, and I remember one that was nearly inaudible to the player, but a cannon out in front. All the sound was heading in the direction of the audience. It was actually a bit disconcerting to play. It would have been helped by a side port, but that was long before the 'port' craze got started.

I find that I can bend my sides without much difficulty if I keep them in the 0.08-0.09 range. That's also where I like the backs, because I like the lighter weigh. No other reason for me.

I'd probably go with heavy and stiff back and sides to give the soundboard something stationary to push against. You're never going to get low resonant frequencies out of a small box, so make the soundboard as light and loose as possible so the strings can just yank it around directly.


I build my guitars ultra-light all over just because I like the feel of it, and they still seem to sound good. But there's definitely some logic behind heavy sides. All comes down to Newton's third law... the soundboard needs something to push against when vibrating in order to move air effectively.

Ervin Somogyi talks about two different types of sustain... type 1 comes from the soundboard impedance, and is similar to solid body electric guitars where the energy stays in the string because the soundboard is hard to get moving, due to high mass and/or stiffness. As you make the soundboard lighter and looser, you lose this kind of sustain because string energy is converted into sound more quickly. Type 2 is what you get with a live back. The back soaks up some energy from the soundboard and then passes it back later, which slows down the conversion of string energy to sound. If the back has high damping, then some of the energy will be wasted as heat rather than passed back to the soundboard.

The Somogyi (and Gore/Gilet) recipe is to make the soundboard light and loose so it responds very quickly to string vibration, use a live back of low damping wood for long sustain, and heavy sides for the lightweight plates to push against.

If you make the back heavy, then the side mass is mostly irrelevant, because the back won't be very active anyway so it doesn't need anything to push against, and the soundboard has the heavy back+sides unit to push against.

If you make everything light, then your body holding the guitar is ultimately what the soundboard is pushing against when vibrating, and being that your body is made of soft flesh, the guitar is able to move around some, plus some energy is damped out. But as said earlier, this is the style I build. The back and sides are still far from massless. But I do need to try heavy sides sometime and see if the boost in bass power is worth it.

I've been kind of able to gauge side thickness with sales. Something that tells me that maybe it imparts a quality to the sound that appeals to a lot of folks.

Since I started doing double sides with basically a reflective back and a little stiffer top stuff seems to move out of the store a lot faster and I get more compliments on how they sound. I like the way they sound as well for the most part.

I really don't know how to describe it. Louder for sure and maybe a little edge or sparkle to the mids and highs as well as clarity.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Did Bogdanovich not describe the effect of his laminated sides as making the sound from the guitar more "focussed"?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

after 200 guitars I can say that from an engineering and physics point of view the heavier the guitar the less responsive. The top doesn't push against anything but the air. Once you set the back against your gut that pretty much kills that. Martin guitars are pretty much the base line and they are .075 sides tops vary some from the hand sanding process as thin as .105 and as heavy as .120 backs .095 to .105
It is all about the box frequency. Learn to control than and understand the top physics you will master this craft. The more I build the more I learn I have more to learn, the heavier it is the more it takes to get going. An object at rest tends to stay at rest and object moving tends to stay moving so heavy may add sustain but you may loose volume. Also a guitar will sound different when being played and when you are just plucking a string.
As you build and learn don't do a lot of changes at a time. Make small changes and note the differences between guitar A and guitar B. One thing you may quickly learn is how those small changes affect tone , volume , and sustain.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

That depends on what your target sound is.

Put very simplistically, a non-live back produces more volume and a live back produces more tone. Details of the whys and wherefores in the usual place (it's not simple!). A live back makes for a softer Helmholtz vessel and so tends to drop the frequency of the main air resonance. Soft sides head you in the same direction. But live backs add more peaks to the guitar's frequency response chart, so it sounds more interesting (more "tone"), but at the expense of most of these peaks being of negative effective area (i.e. out of phase with the T(1,1)2 main top mode) so net radiated sound is reduced (less "volume")

Both live and non-live backs have there places depending on what the player wants. I build both. Two of the guitars in the workshop currently are pretty much opposite extremes. One live back using low density Aus. blackwood (540 kg/m^3) and one non-live using another Aus wood with a whopping 1300kg/m^3 density (yes, you read that right).

Very true. For a sound wave to be reflected off the back of a guitar, a sound wave has to propagate internally from the top to the back, then bounce. But the longest wavelength (lowest frequency) sound that can be established in the cavity (bar Helmholtz type effects) has a half wavelength the same as the cavity depth, i.e. about 100mm, so a frequency of ~1500Hz. At those frequencies, all backs are reflective. So the "reflective back" term is not very illuminating.

Certainly projecting. I've not had a problem with overly directional, though, like you can get out of some parlour sized guitars which laser the sound at you. But then I'm probably not making "normal" guitars!

A bit of differentiation required, methinks: Somogyi goes for modal frequencies that are too low for my tastes, a combination of low stiffness soundboard with a high mass bridge. I pitch mine somewhat higher using lower mass bridges for a more balanced, responsive sound. Somogyi sees live backs as a sort of flywheel, though he provides no rationale for that. For the abridged version of my view of live backs, see above. Somogyi is silent on side mass, as far as I'm aware. G/G has whole sections on the effects of side mass (with the testing and modelling).
Yep; I know that sound!

Yes, whatever he meant by that, but he doesn't provide a rationale.

I couldn't disagree more. Ever played a Smallman (all 3.5 kg of one?). But where the mass is matters significantly.
The top/bridge etc. have mass and when in motion (i.e. producing sound) have velocity, therefore momentum. To satisfy Sir Isaac, there has to be a momentum equilibrium with something else, i.e the rest of the guitar, which moves in opposite phase; which can be expressed as the top pushing against the sides/back. If this wasn't the case, the sound of a guitar wouldn't change as nothing other than the side mass is varied. See Al's comments here.
Amen to that.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

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The words are "reflective or responsive". Since most bluegrass players stand, reflective backs are the usual, but most of the time, they are mic'd anyway. Fingerpickers mostly sit and responsive backs are the usual. Same with mandolins. Bluegrass players mostly look for loud because they have to compete with ban*os. They are mostly standing too, so the back is reflective and the ff holes help project the sound. But then, they are mic'd too. Seems as though most folks that tell you they want loud need to listen more...
Want loud? Build a mandolin with sugar maple. It'll sound like a drawer full of silverware tossed down a flight of stairs. Want Loar? Build with red maple...
There is more to this world than bluegrass. I built a LOT of mandolins in my time, mostly F5 style and "If they are not loud they are not wanted" is not true. Loud is quantitative, tone is qualitative.

I have played many heavy guitars and few are that great but I agree you have to look at who is building that. As a rule few beginners understand the physics and even you have to agree a heavy guitar that is brace in the Martin style will not hold up to one that is building with the weight in mind.

Let us be honest that the bracing system and bridge is working together a guitar that is built light and braced the same way and made heavier will certainly respond differently.

Learning these concepts is what makes builders guitars sound differently. The end result of a guitar , while employing sound mechanics and physics , still relies on an empirical data for the final result. What it is that the builder likes or the customer wants. It takes more energy to drive a heavier topped guitar than a lighter one. Energy cannot be created or destroyed only transformed. So how your take the string energy and apply that to the top and body is the end result. Find what works best your you as a builder . Kasha also like heavy instruments and I agree some people did well with them .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for the clarification And right you are about Somogyi on side mass. I had to go back and check the part of the book I was thinking of, and he does only speak of rigidity, not mass. And doesn't specify whether he makes the total thickness of his laminated sides any more than non-laminated, which he says is 5/64" (.078") on average, which is medium by my standards.