View Full Version : Acoustic Mysteries and methodologies

02-06-2011, 09:57 PM
In another thread there was a bit of discussion going on about arching plates and their effect. So not to hijack that thread any more we can start one devoted to the how and why's of bracing patterns and methods.

Shaping the top and back and even how the sides are built effect the tone and projection you will achieve with an instrument. The combinations are virtually limitless. And I'm afraid than none of us will have enough years left to figure out what works best for us, as there is always the possibility of improvement. It's why I'm so addicted to this craft.

Anyway, to get the ball rolling, there is some good reading at the University of New South Wales.

Some other good work has been done by Ervin Somogyi (http://www.esomogyi.com/). You may be able to find some Youtube videos though much of what he has to say is rather cryptic. He does also have 2 books available called "The Responsive Guitar" and "Building the Responsive Guitar." There is also a DVD along the same lines. I've got all of them, and they are a great read with lots of info to digest.

02-06-2011, 11:11 PM
Thanks Allen,

Lots of reading there. I hope this thread becomes as interesting as it potentially could. I am personally on a quest for the enveloping sound in my ukuleles.

Take care,

02-06-2011, 11:42 PM
Let me preface this with I'm certainly no expert on all of this. It's a very long learning curve.

Here's what I know so far. There are 3 main modes of oscillation that a top goes through when the string is excited. There are lots of other modes, but these are the main three.

The first is the monopole. This is the action of the bridge moving up and down much like a speaker cone does. It's responsible for bass response and volume, as well as balancing and blending the effects of the other modes.

Then there is the long dipole. This is when the bridge rocks from neck to tail block. This mode is a good for treble response and is predominately responsible for producing a sound that projects to the back of the room. It's one that you will find predominately in carved arch top instruments. Hence those types of instruments have a sound that carries vary well.

The other is the cross dipole. It's when the bridge rocks side to side. It's mode is responsible for producing a rich enveloping sound that does not project well.

No instrument will display only one mode. All will be present, but one will usually be the dominant one and thus the instrument will either produce a enveloping sound that does not project well. Sounding good to the player and those close by. The other will through the sound to the back of the room, but will sound thin and hollow to the player.

Now the mystery and magic comes in designing a bracing pattern that accentuates the particular modes and hence the tone characteristics you are wanting to achieve with any given instrument.

Think about the standard bracing patterns you see in various instruments and try and figure out what those patterns will achieve in terms of how the top will react.

02-07-2011, 02:50 AM
Thanks Allen,

That is some enlightening information. I spent some time sifting through the links you provided, and my head was spinning from all of the information. The words you wrote above make perfect sense though, and reinforce some thoughts that I have had while experimenting with my bracing. Now I can go back and read from those links a little more, and it will probably make more sense. I now have a vague understanding of how these 3 main modes work with the top and bridge. Now to try to figure out where the back fits into all of this.

Take care,


02-07-2011, 09:03 AM
Now, if you want to get a little more scientific (as if this stuff wasn't already) then some of the work that David Hurd (http://www.ukuleles.com/) has done has applications here. His Left Brain Lutherie book gets pretty deep into some of this stuff.

Deflection testing may be able to help determining if the top is deflecting in the areas that you've designed it to, or stiffer in areas that you want it to be. Thus accentuating the vibrational mode you are after.

02-07-2011, 06:02 PM
Hey Allen, this is a great discussion so far. As you mentioned at first, what we don't know far exceeds what we do. I have yet to hear a definitive explanation of how much the back of a flattop instrument contributes to the overall sound produced. I have heard a lot of theories by some very good builders, but they are all over the map. I would be most interested in your source of info concerning the primary vibrational mode of arch top instruments. It seems to me if the primary mode of vibration is the long dipole, it would make a great deal of sense to have the bridge rigidly attached to the top rather than floating.


Liam Ryan
02-07-2011, 07:10 PM
I have yet to hear a definitive explanation of how much the back of a flattop instrument contributes to the overall sound produced. I have heard a lot of theories by some very good builders, but they are all over the map. Brad.

Unfortunately Brad, there is no clear answer to this question. Every different instrument's back will contribute differently to the instrument's sound/timbre/volume/sustain/attack/etc. By definition, as soon as you change the back you change to way it contributes. There is no difinitive answer.


02-07-2011, 09:57 PM
Sorry Brad. I've trolled the internet for years looking at this sort of stuff, and unfortunately we were robbed several months back with both computers and all the bookmarks I had saved now gone. So a lot of this stuff I'm trying to find again.

One good source for a lot of this info has been The Guild of American Luthiers (http://www.luth.org/). I've been a member for about 4 years now, and have the Big Red Books 1 - 4. All articles that they've ever published. Some pretty in depth stuff in them covering all kinds on instruments.

A topic that Ervin Somogyi talks about in his video's and books is about how in his style of building he incorporates the back which is built light and lively in order to help drive the top. At the polar opposite Greg Smallman makes his guitar back and sides extremely rigid and heavy to help drive the top. His guitars come in at over 8 ilbs which is like an elephant when you are talking high end Classical style guitars

I've not heard Ervin's guitars but I've heard a couple of Greg's, and there is no denying that he makes a great guitar. And I've no doubt Ervin does as well.

Ervin is a member of the ANZLF and pops in now and then for a bit of discussion. He has said that his guitars are built to enjoy playing in the lounge room. Not in the concert hall. Greg's guitars are found in lots of concert halls though.

So we're left with polar opposites of building styles producing something that works but in different ways.

I've tended to build a light and lively back, but do notice that if it is rested against my body that the sound does change. Thus the back is obviously contributing to driving the top, or radiating sound itself. Does this happen when you build with 8 ply laminations as Greg does? I don't know for certain, but I suspect not. I'm not sure there is a market for ukes that weigh in at 5 ibs.

The source for arch top instruments I believe was from a violin book, web site / paper. They've been studied far more intimately than guitar family instruments, but comparisons were made to arch top guitars and mandolins as the principles are still the same. I do recall some discussion on x-bracing the arch top guitar as compared to the braces running from neck to tail block and how that affected the long dipole mode, as it obviously would when you think about it.

I'm a complete novice when it comes to arch top instruments, but I do know that without sufficient break angle over the saddle, the sound is seriously compromised. Thus you need the string tension to be pushing the top down. Exciting the string sets the bridge in motion, transferring that energy into the top and then the top doing what it was designed to do. Would it benefit from being glued....? I don't think that it's required with a well fitting bridge. Bridges on flat top instruments are glued on because string tension is trying to pull them off.

Violins have been around for a very long time and they seem to be able to pump out an awful lot of sound that carries across the neighbourhood (I've got a couple of people that play several blocks away and I can still hear them).

02-08-2011, 04:36 AM
I would be most interested in your source of info concerning the primary vibrational mode of arch top instruments.

You might want to check out the scientific and trade publications of left-brain mandolin builders Dr. Dan Cohen (http://cohenmando.com/acoustics.html) and Roger Siminoff (http://www.siminoff.net/index.html).

Steve vanPelt
02-08-2011, 07:27 AM
There is no difinitive answer.


As far as I can tell that's pretty much it in a nut shell.

For every well thought out acoustic theory there's another one to debunk it. Like the Smallman vs. Somogyi theories, it just doesn't get more opposite than that. I've heard one of Ervin's guitars, briefly when he spoke at a woodworking club I kind of belong to, last year. I've not heard a Smallman, but I'll take it on faith that it, too, is a fine instrument.

Gourmet Guitar did a video series with Ervin that's pretty close to his voicing video, on YouTube, here... http://www.youtube.com/watch?v=cRTRhwDprjY&feature=related

And for the Secrets of Archtop (and making Italian pizza) John Monteleone gives it up here... http://www.youtube.com/watch?v=nA_C0C21yJQ&feature=channel

Yes, an ukulele is not a guitar, and some folks believe that the small size of an ukulele isn't worth the effort of all this acoustic mumbo jumbo, I still believe that it is. After all, some just sound better than others, so it seems to be a matter of finding the right combination of all those little parts that work together to produce a quality sound. I haven't found that yet, but they are getting better. I'm afraid that this is one of those things that only comes with experience, so I keep doing it hoping to gain some.

All of the builders I've talked to, all the interviews and articles have a common thread...they can't really *explain* how they do what they do. I used to think they were all holding back to protect their secrets. Now I'm beginning to believe that the 'secret' must come from within yourself as you work with the wood. And that the plans and information available these days can only get you 90% there, at best, if you're lucky, but that last 10% is undefinable except by the spirit of your own hands and mind.

I still can't help but to spend way too much time in my own head trying to figure it out, but I find the most successes, which include failures ( finding what doesn't work is possibly more important than finding what does, imho) on the work bench. I'm trying to discover what the wood can do rather than think it into submission. Hope this makes sense.

Anyways....enough babbling by this frustrated wannabe acoustician. I hope this thread goes on to be one of those long ones with a be-jillion opposing view points. I've come to the end of the internet, as it relates to plucked string theory and need more....

Pete Howlett
02-08-2011, 01:56 PM
After 500 instruments I couldn't agree less - trying to elevate the humble ukulele into some sort of acoustic stratosphere is to create a 'new' instrument. I also believe that the gains achieved by such finessing are so minimal as to be academic when it comes to building. In my experience people listen with their eyes and not their ears. Also in many instances, with ukulele at least, volume and intonation are king - relatively easy goals to achieve. David Hurd's work is the only authoritative source since all others deal with guitars - an entirely different instrument... I know of one maker who uses and swears by David's method of deflection tuning and if I am not mistaken, this process is concerned with optimising the dynamic output of the ukulele.

Liam Ryan
02-08-2011, 10:38 PM
Pete, what exactly is it that you don't agree with? I'm not following.

Two Dogs, I don't know that it's a Smallman vs. Somogyi situation. Perhaps more a Smallman and/or Somogyi. Both builders are leaders within their given guitar design paradigms. Somogyi building light, resonant, enveloping instruments best suited to the lounge room or recording studio. Smallman building heavy, stiff, reflective instruments perfectly suited to unamplified solo performance in a 5000 seat concert hall.

As for Somogyi's voicing techniques, I've seen that youtube performance, as well as his DVD and I also own his books. It is my understanding that all of these purposely avoid detailed explanations of his voicing technique so as to give his students a reason to pay what they do for his classes.

02-09-2011, 12:46 AM
Pete, what exactly is it that you don't agree with? I'm not following.

My question too.

The discussion has to do with what shapes sound in an instrument. Not whether one is better than another, nor justifying the way any one person builds.

You've stated in other threads that you build wit a radius in the top. Others do not. Therefore there is something that you are doing that modifies the tone that you get as compared to others. Setting yours apart from the "humble ukulele"

You also use a bridge patch. Some do, and some don't. Changing the dynamics of what that instrument does. In larger instruments you will use tone bars / finger braces. They certainly won't be exactly the same as I use, or Chuck or Liam. Thus there is something different that all of us do to shape our sound.

As you state you've got more than 500 instruments under your belt, so I'm presuming you've obviously figured out a bunch of this stuff.

A more useful post would be for you to elaborate what it is that is different to the tone that you get from putting a radius on a soprano, as compared to not. How does the tone change? Does it project better. More volume? Is there more presence? How about note definition and clarity with a radius on the top as compared to a true flat top. Does changing the radius on the top also affect tone. How about using a barrel shape as compared to a dome?

With so many instruments built I'm assuming you've explored and exhausted all these possibilities.

I don't know if you've read and studied David's book, nor met him and sat through a couple days of instruction with him. And while there is a lot of measurement, calculations and science behind his work, it all comes down to how the top reacts. And yes, part of it has to do with volume. But it also has to do with quality of sound. And he'll be the first to tell you that none of those measurements will mean a thing until you've collected a great deal of data in order to make some assumptions about what works, and what does not for your own circumstances. The essence of a scientific approach.

Pete Howlett
02-09-2011, 12:06 PM
Don't take offence - my reactionary views are unique and well known, offering no criticism to others who like this stuff. To put it bluntly, I plainly do not believe in a scientific approach to a simple folk instrument. That for me is all this wonderful instrument is. Working on sound construction principles, I put a radius in the top to put tension into it and counteract the 'Hawaiian Belly' effect. Instruments that go together under tension seem to me to have a great response. I use a bridge patch because, absent on my first 2 ukes, the bridge flew off. Bob Gleason did the repair and gave me the sage advice to use bridge patches.

As for analyzing why some instruments perform better than others I don't have the intelligence or wit to work it out. All I know is that over building like over analyzing doesn't provide the answers for me. There is truly a mojo to this that took me 12 years to get the soprano right! So I am no fan of trying to find the perfect uke through scientific means - as I see it, there are far too many variables, the most important one being client perception... and that changes with each uke.

02-09-2011, 12:47 PM
To me, the arched soundboard and back are also insurance against future humidity effects. If an arched top flattens slightly due to low humidity, it's not that big a deal; if a dead-flat top cracks, that's a problem..

As an ex-engineer (albeit a chemical engineer) I can see how the arch might alter the balance between your long-dipole & cross-dipoles motions, but I have no way of knowing if it's a good thing or not a good thing!

My own personal bit of mojo is this: lightweight bridges. When you want to mute the volume of a banjo or a violin, what do you do? You attach a dead weight to the bridge, right? So to try to avoid muting my ukes, I prefer to make the bridges from lightweight mahogany rather than denser rosewood etc. Does this work? It'd be very hard to prove definitively, but I like to think so.

Liam Ryan
02-09-2011, 09:06 PM
Thanks Pete. Your first post makes alot more sense with that explanation. Certainly no offense taken. I thought I'd maybe missed something.

Next question though. What were you doing during the 12 years it took to get the soprano right. Were you experimenting with instruments in order to gain understanding of what elements could be changed in order to get a better sounding instruments? Also, what is a 'right' soprano?


Pete Howlett
02-10-2011, 03:42 AM
I left them alone, didn't build because I just couldn't face building something that didn't work. I then had a 1926 Martin in for repair and noted why it was so strong and copied it. I am not an innovator - just not academic enough. I also find that there is far too much of the scientific in this humble craft. At the core of what I do I have a Shaker ideal - Hands to work, Hearts to God. For me, trying to keep it simple and making it well is hard enough without also trying to explain it.