Official Luthiers Forum!

I'm getting ready to start number 6, which will be mine. While I think my last few have been generally successful, I think they could be accused of being "bright". Loud, punchy, bright, sustain are words that have been used. I would like to balance this one a little more toward the warm side. I don't need a formula, because there isn't one, but what are the "toggles" you would use to move the tone toward the warm side? I've been scalloping, and have just the slightest bit of perceptible belly behind the bridge, which I'm guessing is a good thing. Braces are .280 by .6 in at the X, about .3 at the low point, about .5 at the lower peak. OM size, 24.9 scale.

The lower peaks would seem to create nodes around the rim - would reduction in the height of the lower peaks, or moving them away from the rim buy me anything?

My first change would be to take the X down to .5 .... I don't even build Jumbos with .6 in the X .. I do use tapered bracing not scalloped, but I still think .6 is too high, especially for a 24.9 scale ...

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Well this is going to be interesting --- I've had good success reworking completed instruments by thinning the perimeter all along the lower bout -- feathering in about an inch or so. The outer edge top is .0625" I use a Dremel in a router base to poke a series of holes the inner being a bit less deep. Then blend it with an orbital sander. Actually this is now pretty much my standard procedure I definitely end up with a more responsive sound board and to my ear the ones done after they are completed do have less of an edge and are more to the mellow sounding side. This is not my idea another Don Teeter practice.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

fwiw, tapered braces, thinned perimeter. i go tall and thin on my braces, parabolic shape, non tucked. basically concentrate on firming up the area around the bridge and plate and make the rest flexible. go for a barely tubby tap tone before finishing. make an active back but not a dampened one. tune the box before putting the back on. cedar reversed kerfing, mahogany back braces. i don't think you're going to find a lot of agreement here. i'm afraid you'll need to experiment. somebody's bass is often not somebody else's.

I have heard that on classicals a smaller sound hole encourages bass.

Like I said Arie, just fishing for ideas - not looking for someone to give me a solution. I had thought about the soundhole though, mine are large at 3.9-4 in.

Perhaps you could fit in a temporary reducer in the sound hole on one of your existing guitars and see if it helps.

Soundhole experiment on #5 proved interesting. I downsized it fairly dramatically for effect, but it definitely took some of the edge off. Softer, rounder notes. It also seemed to lose some of the "enveloping" sound that I've appreciated in this guitar. More direct, better for singing, softer, smoother, but less interesting. It appears nothing is free

The better the top can pump up and down the better bass response. Bringing the X down and scalloping will help but in my opinion nothing helps bass like loosening up the perimeter of the soundboard. You can thin the top around the edge, carve all the braces below the waist to nothing along the edge, or both. As others have said a smaller soundhole will lower the air/helmholtz resonance but I really haven't fiddled with that too much so I can only speak in theoreticals there.

I've had good luck bringing up the bass by thinning below the bridge. In fact, for some years now, I've been tapering the top thickness from the start. Once I've figured out what thickness the wood should be I make it about .25mm thicker at the upper block, and .25mm thinner at the tailblock (so a half millimeter total taper, usually). This adds stiffness where you need it, and thins it out where it doesn't hurt so much structurally. Thinning the edges outside of the bridge wings brings up the treble, in my experience. I've known Classical makers who did that as a matter of course to brighten things up, and, again, I've done it that way myself when it seemed called for.

Theory says that thinning in the center is more productive at bringing up bass than thinning around the edges. That's what scalloped bracing does for you. I generally use tapered bracing, because that's the sound I'm more interested in, but sometimes I, or a student, will do one that's scalloped, and it does indeed seem to bring out bass and 'punch'. Our perceptions can play tricks on us, of course: it often seems to me that 'punchy' guitars sound 'bright', and, of course, there's more to brightness of tone than just treble to bass balance.

A smaller sound hole can help bring out bass, by lowering the 'main air' pitch, and also by reducing punch. There are probably a few other things going on with that, too. A deeper box can have some of the same effect.

Whenever I want to move the sound in a certain direction, what I try to do is a little of all the things that I think might help, rather than a bunch of one thing. This helps keep things in the 'comfort zone', so you're not so likely to get confused, and also ensures against going overboard with any one thing. All of the individual things that bring up bass also have other effects, which vary. Doing a little of three or four things keeps the side effects from getting out of hand, since they tend to cancel each other out.

It would seem that freeing up the perimeter by thinning the top surface (I understand that Wayne Henderson does this too) or the Taylor method of routing a channel has to also free up the center of the sound board -- to me the sound board is a consolidated unit and it would be impossible to prevent that effect. In other words you can't just free up the edge of a diaphragm if you go all the way around. Even partially the middle will move more too.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

Part of this derives from a study done by Evan Davis a couple of years ago. Evan did his PhD thesis on guitar acoustics, has built some guitars and other instruments, I think, and his day job was with noise control for Boeing, so he knows a bit about sound. He showed that thinning around the edge does help the top displace more air, but that loosening up the middle is far more effective. This makes sense if you think of what happens when you scallop the braces on a straight-braced guitar. Of course, anything you do to the top will have some effect all up and down the spectrum, but if you're going for more bass, the middle is the place to look for it.

One of the guys I communicate with and bounce off guitar related stuff is John Greven I think most in the field will know of him, those that don't might want to Google. Anyway forums are not his thing. But he has his views on this topic too. Yesterday I asked for his thoughts regarding this issue -- as follows.

Ken;
Coupla things....

The basic physics of any guitar regarding bass mids and highs is as follows:

long bass frequencies require either a large top or one that is limber and flexes easily
high frequencies require a small top or a very stiff one that is much more rigid and inflexible

bass frequencies are better supported by a physically softer (not talking stiffness here) rather than a hard top
high frequencies prefer to very dense hard top material versus the softer
(highly mineralized Adi or Sitka are the hard tops and Euro and soft Sitka are at the soft end)

Whatever the top material, thinning the perimeter of the top increases its overall motion, power, and responsiveness by taking away some of the absolute rigidity of a top as it approaches the sides thus creating a kind of speaker cone effect.

using smaller, softer brace stock on a very stiff top will help balance bass and trebles
using harder stiffer and larger braces on a soft to will do the same

It is all about balancing structure and materials to effect top motion one direction or another and choosing the correct starting top material for a particular voicing. Many builders go for the stiffest tops and then make them thin (on the order of .115" or less), which structurally you can get away with, but the top becomes bright and thin sounding, lacking the woodiness and low end of the same top left thicker in the middle and thinned to about .110 only along the edges. It is in the archives at Martin that their starting top thickness was .130-.135 for all tops going into the system and then they were taken thinner based on the specific physical properties of that batch of tops. Some of the early 20s super hard Adi was down to .095 on the edges and .110 in the middle while the 30's Adi were averaging .110 on the edges and .120 in the middle. The much softer Sitka of the 60's required huge bracing to stiffen and they were left thicker overall. Even with the big rosewood bridge plates, these soft tops moved more than one would like.

If you have a finished guitar that is too dark tonally, a harder saddle and nut will brighten it a bit. With an overly bright guitar, a very soft saddle and nut (like Micarta) will help kill some of the high partials, but these measures are only somewhat effective. Trying different strings can also help, like regular bronze for the brightening the dark tone and Elixers to kill some of the brightness.

It's all about tricks and seat of one's pants tweaks. Nothing is a sure thing in this work. Best guess is better than nothing.
John

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

Yes I've read the Davis paper -- heavy!! The PDF file is too large to attach here's the link

https://digital.lib.washington.edu/rese ... /1773/7116

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

Tony, while I agree with your assessment, I think its also important that the builder make those changes in the context of their building, namely, what was his top thickness and what material? I know a .625" brace, tapered with scalloped ends on a jumbo or dreadnaught is fine, depending upon the materials and the thickness of the top. For example, Ervin's tops are extremely thin and his braces start at .750 for the X, tapered, unscalloped with scalloped ends. So its important to contextualize the idea that .5" is right for an OM and .6" is for a jumbo.

$.02,

David

Al, wouldn't leaving the mass in the top but lowering the brace height accomplish the same thing as thinning the top?

Tjp, if I wanted an om with a better bass, I'd use a longer scale length. Just speaking from my own experience, nothing else. I've never built a smaller body guitar with a short scale and been satisfied with the bass. Just just might be the way I build.


Sent from my iPhone using Tapatalk

David:
What they're talking about in the 'effective mass' of the various modes, and the 'effective area' as well. All of the mathematical treatments are based on models that look at the modes as flat topped pistons moving air, and use the mass, amplitude, and area to calculate how much. This is much like a loudspeaker (they're only conic to stiffen them up, so that they won't have lots of mulitpole modes at low frequencies, and act more like a rigid flat piston). A real top doesn't move like a flat piston, so the analogy is inexact, as all analogies must be. Davis' model actually looked at tops that flex, and that may be why it gave somewhat more useful results.

In the real world the relationships are less clear cut. For example, if you shave braces and thin out near the edge, the area that moves might be enlarged, but the mass that is moved could also go up, andf the average amplitude over the whole area might not be increased all that much. Overall the ratio of area to mass could actually go down if the mass increases faster than the effective area. This is what tends to happen as you enlarge the lower bout; the stiffness has to remain about the same to keep the top from folding up, and you have to beef up the structure to do that over the larger span. Mass tends to rise faster than area, so the ratio of A/m drops. That's one reason it's easier to make a loud small guitar than a loud big one.

Loosening up the top in the center, as you do when you scallop braces, increases the effective piston area rather quickly, without adding a lot of mass that has to be moved. A few grams off the braces can give a big increase in output, particularly in the bass range. You haven't changed the mass of the top plate at all, and only made a small reduction in the overall mass of the structure
(which is what counts), but the product of average amplitude times area, the amount of air moved, has increased significantly.

Of course, if the model contradicts reality, then it's the model that's wrong. However, we have to be very careful about what we take to be 'reality'. As Richard Feynman said: "The easiest person for you to fool is yourself". If you go in thinking that thinning the edge will increase the bass then you're likely to hear that. You might even be right! That, in the end, is why we try to do careful experiments using objective measurements to figure out what's really happening.

There is also the complication that perception and reality can be different. When presented with a sound that consists of all the overtones of a fundamental that is not there, we tend to hear the fundamental. If you started with a few scattered overtones, and added in some more at higher frequencies, the result might well be a perception of a stronger bass, even though all of the added energy was in the higher octaves. If you believe that 'perception is reality',that what counts is what we think we hear, then you've got a stronger bass. There's nothing wrong with this in a pragmatic way, except that if you're trying to make a better guitar there will be times when it won't work: when adding trebles just makes it more 'treble'. There seem to be all sorts of instances of this in the sound of the guitar. For example, it seems to me that sharp starting transients ('punch') often come across as 'good treble response', even when they are mostly gotten by increasing the bass output. It's possible that stuff in the 'time domain' can influence our perception of the 'frequency domain', and vice versa, and we don't know a lot about that. This stuff sure keeps life interesting.

Here is how I do the final adjustment for better bass on a finished guitar that already sounds good but seems to have more potential:
Ideally I have another guitar at hand which (1) sounds fantastic and (2) is built aprroximately the same size and wood densities as the guitar being adjusted. Direct comparison of the two guitars is useful in many ways, but mostly it keeps me from greedily believing that I can infinitely adjust the guitar. I'm just trying to make the lowest octave of notes be as equal to each other and full and rich as they are on the proven guitar.
I identify the wood note of the guitar by putting a booklet over the soundhole airtight and thumping the bridge with my thumb. Once heard, you can identify the same sound without the air hole being blocked. The other note I want to identify by name is the air resonance through the soundhole. I blow a huge puff of air toward the soundhole as if it were a flute mouthpiece, and after many tries, I can get the air inside to sing me a low note.
If the two notes are a perfect fourth or fifth apart, there will usually be audible loud and soft notes in the bottom octave. For example, air resonance = low E and wood resonance a fourth higher at A would be expected to make the G sound weak playing big fat bluegrass chords. I compare it with the known good guitar which has D for the air and Ab for the wood. I make the soundhole just a teeny bit smaller, first by taping a cardstock crescent at the edge. I lower the wood resonance by looking for areas of the top that don't ring the bass as much (when thumped) on the adjustee compared to the great guitar.
This shows me where thinning a brace a tiny bit will lower the pitch of the wood note. Danger; if you take off too much the tone will turn loud and nasty. only take out wood flakes equal in weight to a very thin guitar pick at a time, restring the guitar and play it.
Notice that this is not an overall theory of tone; it is simply fine tinkering on instruments built in the mainstream of traditional design.
The things Alan mentioned about time performance versus frequency response have been discussed very much in the theory of ported loudspeaker design. The conclusion there is that it is all complicated, ones that sound different can both sound very good, and that very small adjustments are worth making once the system is up and running. Copy something you know that you love.

It occurred to me reading Carruths most recent post in this thread that in actuality couldnt one say that the low notes on some level actually push the higher frequencies out of the instrument? Thus in manipulating low frequencies as he said, we could in effect alter the projection of the highs, uppermids...... Or that all we ever really hear are blended hues, if Black is low and White is high, we only ever comprehend varying greys, or silvers....

My opinion, for what it's worth, is that good trebles come more from the top, and rely primarily on getting the proper balance between the bracing and the top itself. I've seen large bodied Classicals with cedar tops that were like a marshmallow that had surprisingly good trebles because the top worked well.

Add weight to the rim/sides (solid lining using more dense wood, more dense or thicker tail block, etc.).

_________________
Formerly known as Adaboy.......

you can also add bass by using a smaller sound hole .

_________________
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

A smaller hole increases the bass response by dropping the 'Helmholtz' resonant pitch, which, in turn, drops the 'main air' resonance. It has the drawback of reducing the power available, though. Moving the hole closer to the end of the box can also drop the 'Helmholtz' pitch usefully. Putting a normal sized hole up in the corner of the upper bout can drop the 'main air' pitch down to E or below, so I'm told. This sacrifices some other interesting things that the hole in the normal position does, but there are few free lunches.

A larger soundhole can increase bass too. The air frequency will shift up of course, but the amplitude will go up too since there is more area. However, since more mids and highs from the inside of the soundbox can escape (less effective low pass choke), the sound can easily end up subjectively brighter. Six of one half dozen of the other...

IME it really comes down to: stiff and light = bright; flexible and heavier = bass.