Official Luthiers Forum!

I know better
At least I thought I did.

Then I played a guitar by another builder and it blew my mind. The robust bass and midrange were amazing.
It made my own instruments sound like tinker toys... for a bit. Then I realized that my tops, being chladni tuned and all are extremely balanced. The tone of my guitars is loud and. well, balanced and the comparison came down to warmth. X brand guitar had tons of warmth. I think my guitars had all the volume, just not the warmth.

So, how does one tune a top to get more bass and mid out of it while still keeping beautiful, sparkling treble all the way up the neck?
Am I sacrificing treble for bass? (the top end on that guitar didn't do anything for me)

For reference:
My bracing has evolved over time into an A-Frame below the fingerboard extension and X around the lower bout. Typical OM size tops have two fingers and now one tone bar/leg midway between the bridge plate and the end of the X leg.
1/4" X 1/2"-9/16" stock is where I start the X.

I would think that body size or materials would be the variables I am looking for. But after building a few sets of twins, I've found that to not be the case. My jumbo doesn't have the jumbo punch I expected. And varying the materials on the tops or b/s yields subtle differences. Not the bassy business I am looking for.

Something I have been toying with lately is an idea to scallop the braces of a top and then run my chladni tuning process to see where it ends up. So I tried that and ended up without any scallops in the end.

So, what can the hive mind come up with on how to brace tops to achieve different desired tones?

Looking forward to the responses

Dave

I've have a similar experience when listening to two guitars. But my experience was even more troubling since I had built both guitars. The other fellow's guitar was a sapele with a cedar top. It was a bright but but not harsh as some can be. He loves his guitar. Mine was the same model but with cocobolo and a sitka top. It is slightly dark of tone and full bodied, just the way I like it.

The moral of the story is that whether this sound or that is better is purely subjective. Of course the exceptions are the ones with serious deficits like wolf tones or muddy highs or lows, etc.

Just my opinion of course, nothing objective here. Surely some people here will be able to offer some suggestions regarding achieving the tone you are after.

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Joe Beaver
Maker of Sawdust

Don't alter too much of your design at once. Just try to do one or two things at a time.

THings to consider - make the body a bit deeper in the lower bout. Improve the monopole movement of the guitar (drum like movement in the lower bout). You can often achieve that by lightening the top or sanding away thickness at the edges. Take a look at your x-brace and how it meets the bridge. Think of that area as a unit and how you can get the top to be more flexible but still provided the necessary structure to resist string tension. Optimizing that and then dialing in changes to the sound to achieve the tone you want is the real art of lutherie.

Best of Luck !!!

Really hoping some folks will chime in. I've built 7 acoustics.

The first few I scalloped a lot, following plans like cook books. They had great bass but got muddy on the upper strings.

Then I did less scalloping & just tapered, reducing the height as they got closer to the rims. Resulted in better note separation, more headroom, but loss of warmth & anemic if you didn't dig in.

So IMHO it appears to be a compromise like everything else

If anyone is willing to share some strategies or insights, I would be grateful.

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I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

You definitely will get more bass response, or perhaps just less treble response exposing the bass, if you scallop. You may also consider building with less of or no arch at all. I tend to like tapered bracing on larger guitars but scallop on smaller ones with exceptions of course.

As already mentioned thinning the edges of the lower bout before binding could be helpful. Read Dana Bourgeois’ monograph on voicing. It made a positive difference for me.

If you do it before binding you can roughly measure the thickness. I would say for a top around .118
I will go down to maybe .096 on the edges sometimes less if I get the tap tone I am looking for.

I usually just go with a narrow .060 purfling on the top after that to make sure there is enough top glued to the linings.

Another thing to consider is lengthening the taper on the lower X and face braces to free the edges of the top.

Did you talk to the builder of the guitar you liked? That might give some helpful information.

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

I use defection testing to voice my top and braces. Something you may want to consider as it gives a set of numbers that you can correlate to tone. That is how I developed my voicing and here are my test methods. https://howardguitars.blogspot.com/2016/01/deflection-testing-of-acoustic-guitar.html

Thoughts on tops and sound production from my perspective; The high frequencies are generated very close to the edge of the top and the lowest frequencies generated in the center of the lower bout below the bridge.There fore bass response is directly tied to the tone bars more so than the X brace. I tap tune these braces myself at the very end of voicing but they could be deflection tested with a slightly different jig than I use for the top and X. Scalloping the lower portion of the X can increase bass tones but the trade off is increased brige rotation and the fact the guitar will form a "belly" in the top quicker. Thinning the edge of the top or otherwise stiffening that area will change the high frequencies. I have found that guitars I make with a shell border have brighter highs than those bordered with wood or plastic alone.

If you do work on your X bracing to correct this work on both legs, don't fall for the this brace is the bass and that one the treble thing I see some mention, Unevenly responding X brace legs will cause a phase cancellation of sorts and wash out some of the tone with little predictability beforehand.

_________________
Brian

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

https://www.howardguitarsdelaware.com/

I'm curious as to how much credit should given to the initial wood choices and availability of same. I was watching an interview with Santa Cruz guitar owner who was building with Moon Spruce from Europe that is reserved for world class violin makers and BRW that was from some secret stash known only to a select few Luthiers and I KNOW I will never build with such exclusive woods.

I think there are a few builders who might be able to get "rich deep bass, stunning mid range and sparkling trebles" from a crap cardboard piece of dead spruce but doesn't the quality of the woods have a lot to do with the spectacular sound we hear from some of the best builders? Or is it indeed possible to make a silk purse out of a Sow's ear?

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Thank You and Best To All

You can make a suede purse out of a sow's ear, and many people prefer suede over silk.

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

IMO, Chladni tuning of the top is better at achieving clarity and fullness in the trebles than 'big' basses. Treble lives in the top; bass comes from other things, most especially box size.

Often one of the best ways to enhance the low end response of a given size guitar is to shave back braces. If you can get the 'main back' resonant mode to a pitch where it can couple more strongly with the 'main top' it will enhance the 'bass reflex' couple. This lowers the pitch of the 'main air' resonance, and enhances the output over the whole range of the 'reflex' action; up to around the pitch of the open G string, usually. Best results are usually obtained when the 'main back' mode is somewhat higher in pitch than the 'main top'. Don't get them too close together: any closer than a semitone can engender all sorts of interesting wolf tones. It's easy enough to try this out by adding mass to the back over a brace, which will drop any mode, where that brace is bending, in pitch.

Simply making the box deeper tends to spread out the 'main air' resonance without actually dropping it in pitch noticeably. It weakens the couple between the top and back, which it what forces the 'air' pitch down below the frequency that the Helmholtz equation would predict. This seems to almost perfectly cancel out the drop in the 'real' Helmholtz pitch that you get from having more air in the box. This interesting effect was first noted by Fred Dickens back in the '80s, and it's not widely appreciated. The broadening of the 'main air' peak with the deeper box does tend to sound a bit 'bassier' possibly because the (weaker) reinforcing effect of the air mode is spread out over more notes.

An interesting, but tricky, way to get enhanced bass tone is to get the 'neck' resonant mode to couple with the 'main air' mode. What's often called the 'neck' resonance is really a xylophone-bar type of vibration involving the whole guitar. It has nodes at about the first fret and across the wide point of the lower bout. As the guitar vibrates this way the top gets alternately compressed and stretched along it's length by the rise and fall of the neck. Since tops tent to be somewhat domed they 'puff up' when compressed, and flatten out when stretched this way, which pumps air through the hole. By the same token, air pressure changes in the box can push the neck back and pull it up, so there's a couple there. Or can be.

The problem is that the 'neck' mode is normally down around C below the low E pitch, while the 'main air' resonance is closer to G on the low E string. Moving the neck pitch up is a matter of making the neck stiff, and the head light. I can't say I've seen this match on a 14-fret steel string more than once or twice. It's not uncommon on Classical guitars, with thicker 12-fret necks. Making the neck taper, so that it's thicker at the body end, helps.

When this match happens it shows up in the spectrum of the guitar as a double peak around the 'main air' pitch. The peaks are lower, which reduces the chances of a 'wolf' at the 'air' pitch. The frequency response is spread out, as for the deeper box, but, as compared with a 'normal' single 'air' peak the area under the peak is greater with the 'neck' couple; the 'gain-bandwidth product' is larger, so there's more 'available horsepower'. This seems to 'put a floor' under the sound, often giving a 'dark' and 'rich' bass timbre.

Aside from that, there is always the option of scalloping braces. The whole point there is to enhance the bass reflex couple by getting the 'main top mode' to move more air, and it works. I always had a problem Chladni tuning scalloped tops: they're too asymmetric. Dana Bourgeois' trick of not scalloping the X brace on the treble side mitigates that a lot by evening out the stiffness along the two diagonals a bit (think of how the tone bars bend when when you flex the top along the diagonals). This may well be what enhances the trebles in that case; it's not 'stiffening up the treble side' so much as just making the symmetry better. Double-X braced scalloped tops are easy to tune, and may have even better trebles. You may need to use a heavy bridge to avoid 'wolf' notes, but the added mass also helps drop the 'main top' pitch.

First, if guitars were easy to make, we’d all be making perfect sounding guitars, and then where would the fun be??!!

It helps to have a robust mental model of how guitars work. In a nut shell:

• Guitars radiate sound from their vibrating parts, mainly the top plate
• If a guitar top vibrates in a different way, it produces a different sound
• The different ways a guitar top vibrates are called modes
• If the modes are tuned to resonate at particular frequencies with particular amplitudes, we can shape the way a guitar sounds
• One person’s too much bass is another person’s too little treble, so remember it’s a balance

You will already be familiar with the modes of vibration having used Chladni patterns. There aren’t that many of them up to 1000Hz: the monopoles, the cross and long dipoles, the cross tripole etc. etc. Each of these modes has a typical centre frequency, for example the T(1,1)1 monopole, AKA the main air resonance is typically centred around 100Hz; the T(1,1)2 monopole, AKA the main top mode, is typically centred around 180Hz (for a steel string guitar). As pretty much the only way that a guitar produces sound is by radiation from these different modes of vibration (sometimes well away from their resonant frequencies) it stands to reason that if we want to increase or decrease the sound radiation in a particular frequency range we need to work on a mode of vibration in that frequency range. So if more bass (for example) is required, we need more activity from the T(1,1)1 and T(1,1)2 modes. More activity is gained by making the effective stiffness of that mode less, or the effective mass of that mode less. Conversely, you’ll get less activity by increasing the effective mass or stiffness. If the mass and stiffness are reduced or increased in proportion to each other, the centre frequency of the mode remains the same, but the amplitude of the mode changes, similar to using a tone control. If the effective mass is increased relative to the effective stiffness, the centre frequency is lowered. If the effective stiffness is decreased relative to the effective mass, the centre frequency is similarly lowered. Vice versa in both cases for increasing the centre frequency, so this technique is somewhat akin to using a parametric equaliser. It is in this way that tone can be shaped, by modifying the amplitude and centre frequencies of the various modes up to ~ 1kHz. Above that frequency it becomes difficult to isolate individual modes and a different technique is required. So if more high trebles are required, for example, the overall effective mass of the soundboard should be reduced, because the vibrational sensitive to high frequencies is increased by reducing the mass, say by using a lower mass bridge.

Of course, this is a massively abridged version of a much larger document which you probably know about.


It’s perhaps surprising what you can do with pretty ordinary wood, like for example with the Shed guitar mentioned in an OLF build challenge here a few years ago. I’m not going to tell you that it has the truly spectacular sound that can be obtained from really nice, low mass, low damping woods, (radiata pine starts well behind the eight ball) but to be able to hear the true benefits of spectacular wood characteristics, the rest of the design and build has to be right first.

Having said that, it doesn’t sound too bad.

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

http://www.goreguitars.com.au

Point taken, thanks Trevor

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Thank You and Best To All

Fantastic responses! Thanks all for chiming in!

When you play a guitar that impresses you, study it and try to copy it. Tap it, squeeze it, stare at the inside with mirrors. The more of the theories you know the quicker you might be able to analyze a great guitar, but in the end most of what we do is copying.
Imagining a guitar that has the bass of this guitar and the treble of that one doesn't help much if you can't get one guitar in your hands that sounds like what you want. Trying to scientifically pioneer new guitar improvements has to proceed by making lots of experimental prototypes (hint, the prototypes should be minimally decorated and simply finished, this is no time to be trying to impress anybody, you want audible results, not visual.)
A guitar you have only dreamed of and never had in your hands is not fair competition for a real guitar.

This is huge and I completely agree. When I was building I only changed a couple things having to do with the top on each successive build so that my variables would be somewhat isolated to the changes. Even so wood even from the same freaking tree is not homogenous making every top a bit of a variable too, an understatement.

Lacking bass can be many things but often with small builder guitars it's over bracing especially if using a known, known.... design. We all did it or still do it over bracing and if you go by what say Martin or Taylor does they are braced for commercial markets. When building for the eventual unknown guitar abuser who might leave it in a hot black car in the long term at LAX these factory designs have to be over braced to belay warranty claims.

What I did to learn how to brace for the tone that my guitars all seem to have to some degree I built a "mule" L-OO with no frills specifically engineered to have tops replaced easily. I then proceeded to build ten tops braced with changes and they all spent some time on that guitar. I made a matrix and documented the changes well and back then played with Audacity and StroboSoft with spectrum analyzer and surface mount pups to see what I was hearing. Eventually one top was put on it for good and to this day it's my go-to guitar that I play the very most when playing acoustics these days.

Anything shared by Somogyi or Somogyi students got my attention and Al C's excellent posts concerning Helmholtz resonating frequencies, sound hole sizes, back braces low and wide, tall and narrow, etc. were all...... "instrumental" in helping me get to where I wanted to be in terms of the tone my stuff produced.

It occurred to me early on that if it always takes me at least 150 hours to produce a guitar I'm going to forget what ever I learned by the next build.... Hence the ten top mule that ended up being my favorite and all others that I built after it have the same bracing pattern and ideas.

Related and important is Talyor's rabbeted top perimeters and thinning top perimeters to help free the top up. Also related is the height of your X intersection as my friend Tim McKnight once contacted me out of the blue before I knew him with a suggestion on my X-intersection being too beefy for good bass response and unrestricted top movement. I took Tim's advice about not over bracing at the X-intersection and noticed immediate results on the next guitar.

Hope you are doing great Dave.