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In Todd Rose's thread regarding recommendations of back and side woods for loudness Al Carruth said the following:

"Nothing will help in the volume department as much as using a low density piece of top wood, and leaving it a bit thick, although the low damping of Red spruce seems to help on the 'cut' end of things. And, of course, a shallow box and a large sound hole moved a bit up toward the neck end will help."

Well, as always when Al said this I was fascinated and wanted to learn more. So I PMed Al and asked if he would mind if I started a separate thread to discuss the following:

1) Top density and thickness for volume/loudness.

2) Shallow boxes and volume.

3) Shifting the sound hole forward for greater volume.

And anything else related.

These are topics of great interest to me and I suspect to many other OLFers too.

Thanks in advance to Al for his anticipated participation here.

Uhhhhhhhhhhhhh...........

I'm definitely waiting for this 'movie' to start...

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There is no difference between the man that thinks he can....and the man that thinks he cannot.

When I think about volume/loudness I also think about where the sound goes. Some different scenarios would be volume focused at the player, volume focused straight out of the guitar but not so much to the sides, and volume that is strong and enveloping around the instrument in all directions. I also focus on getting the most out of different parts of the neck. Some guitars are cannons in the low open positions and duds up the neck while others (like selmers) are sometimes the opposite. I would also be interested in the difference volume wise between tops of equal strength where one has a thinner top and thicker braces and the other has a thicker top and lighter braces. Thanks for bringing this up Hesh (and Todd).

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Burton
http://www.legeytinstruments.com
Brookline, MA.

Hesh, thanks for starting this thread. I will be watching it closely!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Some of the loudest guitars I've ever heard are Bluegrass Dreds played by those who are trying to compete with the banjos in the group. They almost always use medium strings and are very aggressively plucked and strummed with rather heavy picks. The guitars that Mario brought to the Michigan gathering were very light, deep and stiff. They were some of the loudest and sweetest sounding guitars I ever played.

As far as the smaller instruments designed for finger style...In my limited experience, the tops that were stiffer were also louder and favored the treble side of the spectrum...even with B&S wood that tapped like cardboard.

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JJ
Napa, CA
http://www.DonohueGuitars.com

I would think that bridge mass should be on that list as well, or at least I'd really like to know for sure if bridge mass has a big influence on volume or whether its mostly affecting tone and sustain. In my experience so far:

Higher bridge mass =

less volume
less bass
does not affect high end much
more sustain

The one I don't feel too strongly about is the less bass thing because the instruments I'm thinking of have way under-sized sound holes and over built tops so I'm not sure the exact cause of the lack of bass.

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Very interesting thread, Hesh. Thanks for starting it. I'm specifically interested in soundbox depth (was thinking about it yesterday).

Stiff backs produce more projection. That can be interpreted as greater volume from someone standing away from the instrument.

Projection is a term bandied about often by luthiers and enthusiasts but seems to have a pornographic type definition - I know projection when I hear it!

By what objective criteria are you judging "projection" and do you have any data to back up the statement that stiff backs produce more projection? (I will have to admit that I only have anecdotal evidence on my bridge statements).

On my latest instrument I've been told that it "projects" wonderfully. It also has a basswood back that is anything but stiff. I, by the way, suspect that what a lot of people call "projection" is not volume related but more spectrum related. i.e. there's something about the energy spectrum of the instrument that makes it "stick out" or easier to hear in a crowd compared to other instruments.

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Build it light, set the action high, and hit it hard.....

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Eschew obfuscation, espouse elucidation.

Nice thread Hesh! I to am interested in volume .I think big frets and strings would have an effect on volume as well as wood selection.

On a recent build that seems to be the loudest I’ve built. I put a 15inch radius on the top, hard steel bridge pins and MOP nut and saddle. Other than that it’s a standard Dred pretty much the same as I’ve built before. My plan is to replace the pins and see if that makes it less loud and then the nut and saddle. I don’t know if this helps but I thought I would throw it out there.

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Say what you do, Do what you say.

Not to drop a fly in the ointment here, but loud where?

In the envelope around the player and the immediate vicinity, or loud as in carrying projection? They seem to be different beasts in how approach each kind of "loud"

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Hi Andy

See Brock's follow-up of sorts for the differentiation. Projection is volume away from the guitar. I'm not sure what you'd like in terms of data. I don't carry a sound level meter around. Best evidence is to try it for yourself or search the archives for Rick Turner's input on projection.

Hesh asked me to enlarge on what I posted in the other thread, so here comes. I'll note that this in my opinion, based on my own experience, reading, and research.

"1) Top density and thickness for volume/loudness."

Plucked strings don't have a lot of horsepower in them; that's why the fiddlers can usually drown us out. If you don't have much horsepower in the engine, the only hope you have for acceleration (= high end) and top speed (= sound power) is in keeping the car light.

All of the sound has to go through the top, so that's the most important part to keep light. The limit on how light it can be is the torque on the bridge; if the top's too thin or too lightly braced to hold that for any length of time, you're fried.

Most of the weight of the top is in the top plate itself. That might weigh 110-150 grams, depending. ALL of the bracing together is typically 30-50 grams, and the bridge will be almost as heavy as the bracing; something like 20-35 grams. The place to look for weight savings is in the top itself first, and then the bracing.

I simplify things by only considering the stiffness of the top wood along the grain. Since the top will 'cold creep' over time it seems to me that the crosswise stiffness, as much as it effects the tone, is not as much use in countering the bridge torque. I _know_ that's not strictly true, but it's a useful simplification and probably won't get me into structural trouble.

Assuming you're talking about a single design, the stiffness of the top plate will depend on the Young's modulus of the wood, and the thickness. Basically, as has been discussed in another thread, all else equal, the stiffness is proportional to the cube of the thickness multiplied by yhr Young's modulus.

Now, it turns out that the Young's modulus (E) along the grain of all of the top woods (seven varieties) I've tested scales with the density. If the wood weighs 300 kg/meter^3 the E value along the grain will be close to 6000 megaPascals, and if it's at 500 kg/m^3, the E value will be close to 17000 mPa. ALL of the top woods fall near the same line, regardless of species, and the scatter is something like 10% plus or minus, for the most part.

The important thing to see is that E scales more or less linearly, while stiffness goes as the cube of thickness. Making a given piece of wood twice as thick makes it eight times as stiff. If you make two tops of different density wood, but make them to the same thickness, you'll find that the one with the lower density will be a bit lighter in weight. It will be thicker, of course, but not enough thicker to add so much weight as to make it heavier. Lighter=easier to push. There may be other benefits as well, but they are harder to demonstrate and more subtle.

This is where I need to say something about bridge weight. Obviously, the bridge has to move if the top is to move, and a heavy bridge will slow things down, no matter how light the top is. There goes your treble response. We could get into a long discussion on 'impedance' here, but I'll try to resist in the interest of covering some other ground. Suffice to say that the higher impedance of a heavy bridge has a lot to do with the added sustain.

"2) Shallow boxes and volume."

So you've got a top=bridge system of a certain weight and stiffness, and you stick it on a box. Given a certain amount of input energy from the strings the top is likely to vibrate with a given amplitude. As it moves 'in' to the box, the pressure inside goes up, and some air is pumped out of the soundhole, and the opposite happens when the top is moving 'out'. The shallower the box the greater the pressure change for a given amount of top motion, and the more air is likely to be pumped in and out at the 'air' resonance pitch.

At this point, we have to talk a little about 'loudness (or volume)' and 'power' and 'projection'. I tend to use 'power' to mean the actual horsepower output, regardless of what the thing sounds like. 'Volume', for me, is how loud the thing sounds close up, and 'projection' is loudness at a distance. 'Power' can be measured, although it's not as easy as you'd like it to be. The other two are subjective, and have very little relationship to the power in the signal. In fact, I've got one acoustics text ('Fundamentals of Musical Acoustics' by Benade) that spends a whole chapter on the different ways of calculating 'loudness' from power and spectral measurements, and ends up concluding that none of them works well in all cases.

One of the many things that seems to correlate with close up 'loudness' is the strength of the lower frequency part of the spectrum. This is the 'bass reflex' range of the guitar, where the 'main top' and 'main air' resonances dominate the output. Sounds below the 'main top' pitch (often around the open G string) tend to radiate in all directions, while higher pitched stuff is more and more directional. A guitar with a strong bass reflex range will usually sound loud close up, and to the player, and this is particularly true if the attack is fast. A shallow box helps with that fast attack, but doesn't (surprisingly) alter the 'main air' pitch much.

'Projection' can benefit from a strong bass reflex, but not a lot. If there is twice as much power in the bass reflex range (3dB stronger: a huge change) the sound won't be much 'louder' close up (because of the way your ears work), but, owing to the inverse square law, it will be audible about 1.4 times as far away. However, good projection seems to have a lot to do with the high frequency content of the signal, between 2000-4000 Hz, because that's where your ears are most sensitive. Sound in that range is coming off the top and out of the hole (mostly) and heading away from the player, often in 'beams' in particular directions. I have seen and played guitars that were barely audible to the player, and absolute cannons thirty feet out in front, becuase of this.

"3) Shifting the sound hole forward for greater volume."

What I said, iirc, was: "making a larger soundhole closer to the neck end". The larger soundhole obviously 'breaths' easier, and tends to give more volume within reason. There does seem to be an 'optimum' size, but I suspect it's a little bigger than the usual. Making the hole larger raises the 'Helmholtz' air resonant pitch, and thus probably the 'main air' part of the bass reflex. You may not want that, as it tends to reduce the 'fullness' of the bass notes (that's why the 'standard' soundhole is not larger). Shifting the hole up toward the neck helps to drop the 'Helmholtz' mode (and loses you some useful frets: it's all a compromise). See Allen's article, 'The Basics of Air Resonance' in American Lutherie #1 or the first Big Red Book. Again, there may be other, more subtle effects, that could add to the 'loudness' as well, but the research remains to be done.

"And anything else related."

This post is long enough already.....

Guitar design 101 again (for what, the 50th-100th time Al?) by Alan Carruth, never is dule or lacking in information.

Thank you again Al for giving away your knowledge and extensive wisdom gained by all the research and testing you've done.

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Yeah I’ll second Rod’s comment, though I’ll probably have to read Alan's input a few times to start to understand it.
Thanks Alan
Geordie

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Are we lucky, or what? Great post, Al, as per usual! Clear, concise and quite understandable.

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Waddy

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Sound Clips of most of my guitars

This is exactly what I thought that we would receive if we asked Al for his very valued thoughts and knowledge.

Thank You Al!

Let's talk about bridge weight some more here and possibly even get into impedance.

It totally makes sense that a heavy bridge will slow things down, but in my experience with banduras, a very heavy bridge doesn't seem to affect high end nearly as much as it does mids and lows.

There is a type of bandura that uses either brass or brass/steel quick re-tune mechanisms on the bridge and almost without fail they sound decent to great in the upper registers and get thinner and thinner as you go down in register. Now, these instruments tend to have over-built tops and way undersized sound holes too.

Interestingly though just last week the day after the Ann Arbor OLF gathering I was A/B-ing two instruments and it illustrated this exactly.

I understand that guitars aren't banduras but certain principals can be rather universal (bandura does use a tailpiece type string capture btw).

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How about a hypothesis as to why stiff backs would "project" more. Greater mid and high frequency content?

To me, your original statement is very bold and very confident. I'd hope that you have something to back that up with. Not saying your wrong or anything just that it my book a statement like that needs some back up.

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this is not to be a cut on anyone and certainly not on our beloved OLF, but this is the kind of post that first hooked me on this forum and one that it seems we have seen less of in recent times. i was just considering some of this today and was delighted to come home to a topic on the matter. thanks hesh, todd and others for drawing out the conversation. and a big thank you to al for sharing his vast knowledge.

i have no personal data to back up what james mentioned concerning a stiff back and projection, but i do believe i agree. perhaps because we have discussed this here on a few occasions and i guess i have taken the "why" for granted. my thinking is that less energy is lost in the reflection of sound waves off the back. just writing those words down makes me question my logic, however. thats why its nice to discuss.

i will throw out one last thing that i know nothing about, but have heard discussed briefly in the past both here and by ervin somogyi. i dont remember what does what, by i know that the manner in which the soundboard moves has an effect on how loud a guitar is and how well it projects. in other words, monopole and dipole movement of the top. like i said, not something im knowledgeable of, but something i would love to see discussed.

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sweat the small stuff.

My first build....an OM with a sitka top was built under guidance from Dave Freeman and Paddy Burgin. During the 3 weeks it took to build the instrument Paddy and Dave tried to cram as much information as possible into my head but one comment stood out from the rest and stuck in my head..."build light and stiff".

Ive pretty much built all my guitars so far that way and theyve all been "loud" guitars.....even the classicals.

Heath, I completely agree! I love these topics.

Andy, run a search for Rick Turner project. If my memory is correct, you'll find detailed conversations on this between Rick and Al that go into some detail about it, particularly in a conversation where Al even goes into the purpose of arched plates in arched instruments (the string family and archtop guitars).

You might also enjoy a thread I started on stiff vs. loose backs if you can find it. A lot of this information was in it, also.

I believe you'll get some richer discussion that way than if I were to just say I tried it and it was true to my experience.

Firstly I would like to echo the sentiments of others in relation to Al Carruth and his ongoing willingness to take the time to share so generously his vast knowledge and experience. Thanks again Al .
With respect to the issue of volume /loudness I have to agree with a number of others who have said that lighter is a key. I have only completed two instruments thus far and No 2 is definitely the louder. I purposefully aimed lighter everywhere and succeeded except for the ebony bridge. I was intending to scallop the bridge behind the saddle but am quite happy with the overall responsiveness and tone so will leave it as is and experiment some more on No 3. Something that did intrigue me and this might be worth discussing in another thread but is sort of related to volume /loudness is the affect of varying string height above the top. I understand that the greater height the more leverage and consequently the greater top movement for the same energy input . I installed an under-saddle pick-up which raised the string height but did not noticeably affect the volume. If anything I think the sustain dropped a little. Maybe it is just my ears but do you think there is a point where the extra rotational forces on the bridge outweigh the greater leverage? Is there an optimal string height?

Regards

Craig.

Thanks, Al!

Just to clarify, when you said, "If you make two tops of different density wood, but make them to the same thickness, you'll find that the one with the lower density will be a bit lighter in weight" ...you meant stiffness, not thickness, right?

Okay, so that all makes perfect sense. But it throws a wrench in my method of buying tops! I flex them and select for stiffness. Now, it seems that if I want to make loud guitars, which I do (not that loudness is all I want from my guitars), I should select the floppy tops and make them thicker.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/