Official Luthiers Forum!

I'm in the process of building my second guitar which is a dread as was the first one. As part of the process this time I am documenting everything ie.. weights, thicknesses, as well as spectrum analysis. In doing so I felt it was necessary to get a baseline from my first build.
I discovered that the Air Resonance was lower than I expected at 71.4 Hz and the soundboard monopole is at 132.6 Hz which is also low. Now.. like many of you when starting out on you first build, may have subscribed to the same process I did and that's the lets just do it and see what happens process. Well, now I see!

But oddly enough I feel like the tonal qualities are pretty decent. Obviously good lows, as well as good mids and decent highs.

The question I have is, should this be the case. Now knowing that I want to hit 90-100 Hz for the air resonance and likely 180-190 on the soundboard.

Have any of you experienced this before, with numbers this low but yet seemingly good sound. Volume is certainly there and not detecting any noticeable wolf notes.

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Lots of builders including some of the biggest names never bought in at least initially to the scientific method when it comes to guitar building and measuring things in terms of response and air resonance. Ever hear of Wayne Henderson.... he may be the anti-Ervin Somogyi.... when it comes to measuring things....

The only place I ever hear it spoken of is on some forums and referencing some of the books and claims made in the books.

Is it possible to build a great guitar and never measure an air resonance or the soundboard monopole? Sure the most iconic instruments in the world when it comes to acoustics the pre-war Martins no one was doing this.....

My first sounded killer and looked like ****.... My second sounded killed and looked...... better....

One of my many builder friends has over 450 sold guitars under his belt and a waiting list for what he produces and he never measured anything either.

Is it possible to build a first guitar and have it sound better than a Martin - sure many of us have done it. Martin is at a disadvantage they have to build for the fool who is going to leave it in a black car trunk in the summer in the long term at LAX. We don't and can build intentionally light.

So not suggesting that it's hogwash.... but I am telling you that in the real world and historically speaking these things were not the measure, if any... that most builders used. They never were mine either I tapped and listened and that was the kind of Luther that I wanted to be when I grew up someone with a feel for the materials we are fortunate to have the use of.

When you are new to building there are a thousand things you need to get right because after all a guitar is ultimately a tool for musicians. That means all the playability stuff, fret work, nuts, action, relief saddles, action etc. all need to be understood and accounted for. Learning to seat a proper dovetail if that's your joint is a big thing to learn too.

If it were me I would be looking to get the woodworking and user interface stuff down, conquer pore filling and such before I start shooting for specific specs on frequency responses.

With all this said when you ask "should this be the case" my answer is sure it's the norm in my experience more people are NOT setting spec goals for these things and even measuring them when starting out than do and they, we are often pleased with the tone of the resulting instrument.

as an engineer ,like Hesh I wanted to see numbers and have a relationship of that to the tone of the guitar.

Here is what I can tell you. I learned that defection testing of a raw top tellc you little to nothing . I would pre brace and post brace thinking I could discover something. I didn't. Physics of the top and bracing is critical so yes how is the top reacting to stress with the support of the braces , one had to understand and master.
using an ear , few can master that as human ears are not as accurate as a spectrum analyzer or scope or even the strobo tuner.

weight is not a guarantee of a good guitar. Wood joinery is very important and glue interaction with this joinery is important in the exchange of energy from the strings to the top.

Here is what I did learn.

I do voice my tops to a C within a note. So B C C# are my targets. To help eliminate wolf notes and other "noise" I will voice my back so it is non harmonic to a top.

Weight is important but you must be within the structural strength. I see so many tops here that are way over scalloped , so in a nut shell if making a top flexible was key we would be bracing paper.

I also know that braces do at least one of three things, Structural , Tonal, anti split.
There was a discussion of using a top cover on X braces and there are lots of opinions. So any engineer out there knows the stresses applied must be compensated for.
I find that you can make the X brace joint too tight and this can cause things to happen tonally on a top. I want a few thousandths clearance for glue. I use the tape as this makes the braces contiguous. As a repair shop I have seen many wood supported joints fail and few cloth ones. I agree that the workmanship and joinery are IMPORTANT . So be sure you are precise.
Yes not all will agree with me and this is fine. I will say that no true discovery was ever made by status quo thinking.

When my top is braced and glued to the box there are changes as the top is no longer free state . The Hemholtz frequencies will change how the top reacts from a free state to a closed state so learn the cause and effects of this.

We all have an idea of what the guitar should be and pursue that so you make your guitar yours. I can tell you that Martin NEVER TUNED A TOP , they use templates and patterns. Most of what any production guitar company will do is to tend to warranty issues .

Find what works for you and my final thought

DON"T DO A LOT OF CHANGES FROM ONE GUITAR TO ANOTHER. LEARN TO OBSERVE SO YOU CAN SEE AND UNDERSTAND CAUSE AND EFFECT TO TONE DURING CONSTRUCTION.
Humidity is as much a part of building as wood choice so pay attention to that.

_________________
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

Not what you asked about, but I've found that string tension (in pounds) published by string makers is ALWAYS lower than real world. So I go by feel, but it's something I developed after decades of observation in the biz. Numbers are important in lutherie, but in the end - the talented players will go with feel and tone as their North Star when deciding what is special about a guitar. Good luck with your building efforts!

_________________
"Act your age, not your shoe size" - Prince

Thanks! Guys

Its funny you mention learning to seat a neck joint, Hesh.
I learned (not mastered) how to do that from Mr. John Hall's video ....Thanks, John !
But what he could do in 20 Minutes took me the better part of a full day but I got it and I guess that's all that matters.

I agree 100% with all of your advice in regards to needing to develop a deep and true feel for the sound of wood and how to manipulate it. I am working on developing my ear and Grey Matter towards that end but I'm not sure I'm there yet. Actually, I'm sure I am not there yet!

I guess I was looking for a little confirmation that I'm headed in the right direction by using the Spectrum Analysis values to verify.

Half the fun in mastering something is found on the path while getting there.

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Hey Ron very cool and I can relate. My first dovetail took that long or more It was kind of sort of... awful.....

Regarding the question about the right path that's something that only you can answer because it's a matter of where your highest interest is and it may be at least for now the tone that you seek. We just wanted to make sure that you also had an appreciation for the 1,000 other things that we have to learn to do when each of them can take some time to master. Many of these things have impact on tone too not just the top.

Clearly you do and that's cool to know.

I have a feeling that you will do just fine and we need to stay out of your way Just know we want to help and have your back if you need it.

Nice to meet you too.

I agree with all the feedback above from folks much wiser than me. That being said, in case it is not clear, you should absolutely be measuring everything you can along the way. It will be useful at some point. There is yet to be a time where I wish I had less data to review! Just don't stress over it.

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

Remember that it all works as a system. Your very low air resonance is likely the result of your low top resonance. It’d be good to know what the back is at as well. It’s not unheard of for Somogyi and his lineage to have resonances that low.

The numbers in the Gore/Gilet books are derived from the best sounding examples of traditional instruments, Martins/Gibson’s etc, but are by no means the only numbers that work.

My main concern for your case would be structural longevity. I would want to find out where the bridge rotation sits in the two degree guideline.

I’ve certainly found that using FFT measurements has been a game changer for me. But, as mentioned, it’s certainly possible to make great sounding guitars without measurements, just as it’s certainly possible that those great sounding instruments could be made even better with a little post assembly love. You’ll only ever know what dominating your resonances sounds like by doing it…

I suspect your low 'air' resonance is related to the low 'top' resonance, since those two are so tightly coupled. The back can also have a pretty big effect: getting the 'main back' tap tone within a couple of semitones or less of the top can drop the 'air' pitch quite a bit. The 'neck' resonance can also affect things, and it's often down in the range of about C~131 Hz. An impulse spectrum could help in sorting all of that out.

There are lots of ways of measuring things, and they don't all involve coming up with a number. Dr. Karl Roy, who was the head master at the Mittenwald violin making school, could judge stiffness by 'feel' to within about 3%, which is about as accurate as most shop level measurements. Apparently some tests that have been done since at violin maker's meetings show that most people aren't as good at it as they think they are, but it's not beyond possibility.

I'm a big 'measurement' guy, as I'm sure most folks know. I find it easier to communicate and remember stuff that I can write down. I remember a story George Bowden told, about making a guitar that sold out of his hop almost immediately and ended up in Australia. Six months or so later he started getting letters from folks wanting one just like it, but he could not remember what he did on that one. After that he started to keep records. To each his own.

Thanks, Hesh

Please by all means don't get out of my way it is folks like you that are actually willing to offer your insight to help another person move forward to learn the 1000 things you need to know. I have only learned enough to achieve the classification of a "Beginning Amature" but that classification wasn't on the registration form when I registered for this forum so I had to choose "Amature". lol ! So, I am relying on you and everybody else to let me know when I have officially earned the title of "Amature".

As an example, John suggested that I not make any major changes in design from the first build to the second build and that makes a lot of sense now.

Oops! Too late! This second build, which I'm about to close the body on now is the same basic design but far enough from the first that I am guessing I will loose any advantage to learn something that I might have gained had they been more similar. Now, had I thought of that before I started this one I should have and would have followed Johns recommendation and maybe have made some minor changes at best.

I went with laminated (Double) sides and an arm bevel on this one which I feel certain will change the top resonance because of the modified X-brace and the smaller surface area of the top as well as changing the air resonance because of the change in the top resonance as well as the additional mass for the arm bevel and the reduced volume inside the body.

But by hearing your thoughts about any topic just might keep me in line and out of trouble as it were.

Good to meet you to...thanks for your help.

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

meddlingfool,

Regarding the angle of the bridge. You are correct, it is at 3 degrees referenced off of the frets but the neck angle is 1.5 degrees so that would put it at 4.5 degrees total.

and you are also correct with your reference to Ervin Somogyi. I have read his books and did take his methods into account when determining the thickness for the sound Board which ended up at .092.

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

I believe what should be measured is how much the bridge rotates with and without string tension. These days I just use the level app on my iPad and stick it on with some putty. Set the guitar on its back on the bench, set up the phone, set it to zero, and then release the string tension and see how many degrees it rotates.

Hopefully it’s not 4.5 degrees, lol…although to be truthful I don’t actually know what would happen if it was. Gore suggest no more than two degrees for long term durability…

meddlingfool,

Lol ! I gave you the static position that it is currently in with the strings tensioned. There is a slight bulge / dip on either side of the bridge but it doesn't appear to be substantial. The top thickness is at .092" which by some standards may be considered thin, and I assume adequately braced.

I finished this guitar in 2020 and it doesn't really seem to have changed much if any in 3 years but it hasn't lasted 30 years or longer yet either.

You may be wondering, why 3 years in between guitar builds. Its kind of funny how plans change sometimes. My ideal retirement was going to be spending all of my free time on the golf course. I retired this year and chose to build guitars insted. Go figure!


So I will do that with my phone just to see if its in spec or not. Obviously that would be kinda important to know.

Thanks

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Hey Alan,

The back resonance is at 172 Hz, a couple of Hz below F3 174.61
The top resonance is at 132.6 Hz, a couple of Hz above C3 130.81 So, the back and top are 5 semitones apart
The Air resonance it at 71.4 Hz, a couple of Hz below D2 73.42

meddlingfool brought up an interesting point regarding Ervin Somogyi and the fact that he has been known to have lower than typical resonances and which I did follow his methods based on deflection and ended up with a top thickness of .092" on the first build and .095 on the one I am working on now.

The reason I am so curious about this is first of all I wasn't expecting these low numbers since I was fairly meticulous about what I was doing and then secondly if I got off track somewhere then I want to fix it. But now I'm thinking that these numbers fell exactly where they were supposed to be. So I'm questioning myself as to whether or not there is truly enough difference between responsive and a level or two below overbuilt. I guess there's only on way to find out.... on my next one I'm gonna build a Tank.

Thanks Alan

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Hello Ron,

I'm coming in late to this discussion and you've got good advice here so far. If you are still inclined to pursue this analysis there is one thing I'll add which should at least be ruled out. The primary air resonance is a coupling of the Helmholtz resonance and those of the box, primarily the top and back plates. As others have pointed out, lower than expected frequency here can be a result of plate flexibility, but a low Helmholtz frequency should be considered as well. We don't have any information about the construction of your guitar, but assuming it is built to a known plan and the body is not exceptionally capacious and/or the soundhole diameter is not unusually small, then this is likely not the issue. Still there are some construction details that could result in a lower Helmholtz resonance, like a large effective soundhole thickness for example. The frequency of the Helmholtz resonance can be measured of course, and doing so could help guide your investigation.

Always good to hear from someone that, after building one guitar decides they really need to build another. Welcome to the madness!

_________________
R.M. Mottola
LiutaioMottola.com

Author of the books Building the Steel String Acoustic Guitar and Mottola's Cyclopedic Dictionary of Lutherie Terms.

Hello rmmottola
I designed this guitar about 4 years ago using physical characteristics from the many articles, books that I read and videos that I watched and took what I felt was something that made sense to me so it a conglomeration of many different ideas and not really what I would consider a proven design. But everything I did was primarily with the goal of having a good base response. I think I achieved that. I drew full scale drawings on everything before I started building it.

When I was a kid in the 60's a good friend of mine had a (I believe)1976 Gibson Hummingbird and his dad had a 1960 Hummingbird which was the first year of the Hummingbirds. The "60" just had an awesome sound as compared to "76" that I really really liked. I somewhat modeled my design after the "60" Hummingbird.

16-5/16" lower bout, 12.20" on the upper bout, 11.5" waist, 5.019 depth at the lower bout and 4.09" depth at the upper bout. So its a full size Dreadnought but I wouldn't consider it to be overly capacious. However I do now think that the top and back mobility may be on the verge of being too much for a guitar even that size.

Also, the sound hole is probably a little small as well being at 3.93" and I have considered enlarging it but I only have about 3/16" before I start getting into the rosette and If I remember correctly that removing 1 mm from the sound hole will increase the frequency 2 Hz so I don't think I would have enough room to raise it as much as it would need to be raised to increase the air resonance into the conventional range.

Lol.... I kept the sound hole intentionally small thinking that I could remove some of it if need be and that it would be much harder to add it back. I never envisioned that it would need to change it as much as I think it needs to be changed, probably 20+ Hz.

Am I thinking about this correctly RM ?

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

The instrument's dimensions sound typical for the type. That being the case, this makes a low Helmholtz frequency less likely as the primary culprit here. Best of luck with your investigation.

_________________
R.M. Mottola
LiutaioMottola.com

Author of the books Building the Steel String Acoustic Guitar and Mottola's Cyclopedic Dictionary of Lutherie Terms.

Good Lord, I hope I never get this wound up in technical guitar building. I'm totally a hope and pray it sounds good type of guy. No glitter, no strobe tuners, but then, I haven't built anything that sounds great yet. Still waiting for it to magically happen. You know when the elves sneak into my shop at night and perfectly tune my braces. It could happen.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Hello Hutch !
Good to meet you, sorry for sounding a little geekish here. This is how I learn. To me its kinda like putting a 1000 piece puzzel together and your wife throws the box away, but you get many of the pieces in place but you still cant tell what the picture is and I really want to see the whole picture. I'm an all in kinda guy with an Electronics background and no formal mechanical education other than intuition so I have a lot of catching up to do.

But! I hear ya!
Praying and hoping is what I did when I built the first one and now I have two choices. I can keep building by praying and hoping and always end up with mediocre guitars or I can learn from my mistakes in hopes of improving each one there after.
I want to know what makes a guitar tick.

Thanks Hutch!

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Ronald, do you have the Gore/Gilet books?

I’ll reiterate that your low air resonance is the result of your very low top and back resonances which are 40-50hz below where they ‘should’ be for an instrument based off a standard dread. There is really no way to get your air resonance to conventional range without scabbing in wood to raise your top and back resonances.

Even with widening the sound hole to lift your air resonance, or punching in a sound port (which can be used as a fine tuning device for the air cavity resonance), you’ll only be able to change that resonance by a small amount.

Since none of your resonances are hitting a scale tone directly, and you’re not getting wolf notes, and you’re relatively pleased with the results, I’d just forget about changing this guitar and move on to the next…

As meddlingfool says, the fact that you're pretty happy with the sound as is suggests that you've got nice relationships between the 'air' and 'wood' resonances. The pitches are lower than normal, but they play well together. Adding a 'port' to raise the 'main air' resonance (which would be about the only practical way in this case, as it seems) might hurt the balance you've hit, and it would be hard to reverse the procedure. Just for kicks, you might try tuning the strings down by a whole tone or two, just to see what it sounds like.

Hey meddlingfool,

I don't have the Gore/Gilet books YET, I just found out about them about a month ago and just haven't gone online and bought them but its on my list of things to do. I did buy the Somogyi books but from everything I have read or seen about Gore and Gilet I think their books go into considerably more detail especially regarding spectrum analysis and the kind of stuff we have been talking about here.

Regarding the sound hole. I agree. It wont fix the real problem which is the Top an Back plus lets say I get lucky and increase the air resonance a couple of Hz then that would put the air resonance right smack dab on top of the of D2 and increase the potential of wolf notes.

So, you are right. It sounds ok (not great) and I think I learned a lot from it so on to the next one which I am just about ready to close up the body on now. I think I'll be ok on this one, the top is a little thicker with a smaller surface area because of the Arm Bevel. The only thing that concerns me a little bit is that the bracing on this one is pretty close to the first one with the exception of the modified X-brace for the arm bevel.

Thanks much for yours and everybody's help. Its really appreciated.

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

Hey Alan!

I play in many different tunings, Drop D, and I cringe a little when I tune to Open E but I play in standard tuning mostly . So I've been all over the Map with it and it seems like everything holds pretty true. But I don't know that I have ever tuned all strings down a full tone. I will have to try that and see what happens.

I just realized something on the Spectrum analysis graph. It shows a double Peak very near the top frequency that I assumed was maybe a reflection of the back resonance. If you don't mind take a peek at this and tell me what you think.

https://drive.google.com/file/d/1gK1-C88NXFebG-yABoGMZ6YVae1ej1VR/view?usp=sharing

_________________
Ron Riggs

I have been Bitten....by the Guitar building bug!
Not sure there is any cure for my condition
at least I hope not!

I'm with you, but I do tune things, and flex things, and tap things. Sometimes I look at the FFT graphs on Audacity to see trends as I thin. That is usually in archtops, or violins, or cellos, but it would work for bracing too. Audacity might help me see if the ring tone is the B or the D note that I hear. Sometimes they are hard to find; everywhere you hold it you find something different.

I'm just looking, and listening for a plate that sounds lively and nice. It will get better as you thin, and then start to die fast if you go too far.

I'd like the back and belly to be like a semi-tone or so apart in the main ring tone, mode 5. The belly will usually have very low frequency when you balance in the middle, and tap the edges. The back not nearly as low. That's just interesting, I don't know what to think of it. Sometimes a belly has a low, mid and high mode that rings.

When they are attached to the ribs I tune to make them even in areas, and have a lot of different tap tones. I push on them to see how stiff they feel. I thin the edges; especially on violins, cellos, and archtops, because I never finish the outside of them until they are glued together. I do the inside first.

Sometimes you KNOW you won't get them close to the same. The flame birch had a very low ring tone, and was stiff. I paired it with a very very light Englemann top. Worked great. You can go for dark, or bright in the same way I suppose. Some like reflective backs. Some like very resonant backs. I like tapping on wood, so it is resonant for me.

That may give more problems, if the overtones clash, but it seems that usually the strings will be what is dictating the resonances, so they should be good. I think. Anyway, I always go for more sound.

_________________
Why be normal?

Hey Ron me again... I wondered if you were using Peterson's Strobosoft and if not if you are aware of it. Good stuff.

https://www.petersontuners.com/products/strobosoft/