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The problem: I have built several of these parlor guitars to great success. In fact of the 70 guitars I have ever built one of the very best is one of these made with oak and a pine top. It is to date the best sounding guitar I have made. This one is made of BRW and a torrified Red Spruce top. The top was thicknessed using my deflection method to match the other guitars and Gore/Gillet method of tap tones and so on. The top was VERY stiff and hence was made quite thin. I could produce all that info if needed.

The guitar has brilliant clear trebles but the bass is completely lacking. I’ve made all these guitars with stiff braced reflective backs. Just about everyone who bought one of these has said the same thing, Wow I cannot believe how much bass this little guitar has… And that they are very loud for their size and so on. This one is a total dude in that regard. It’s still not a bad sounding guitar but it just not have what the others have and I want to try and fix it without re-topping it.

This is a very small parlor guitar.

Upper bout: 8 1/4in
Waist: 7in
Lower bout: 12 1/8in
Body length: 18 1/4in

To measure the frequencies of the guitar I am using an app called Spectrum Analyzer on my Android phone with a fairly decent external mic. The app is set to Log curve. I did a tap test with a soft tipped hammer. By holding the guitar by its Upper Transverse Brace with my fingers in the sound hole and thumb pinching the brace while not touching the top. The hand effectively blocks the sound hole.

This graph is NOT a tap test but rather I hum into the guitar to find its resonance frequency. The main body resonance of this guitar is 123.82Hz which is pretty much right on B


Here is the graph for the top tapped on the center of the bridge. It is 226.1Hz which is on the edge of A/Bb


Here is the graph of the back tapped in the center just below the waist. It is 360.68 which is about F/F#


Here is a graph of the back tapped with 8.2g of poster putty on the center of the 3rd of 4 lower cross brace (the one closest to the waist). This dropped significantly and registers at 285.31Hz which is on the cusp of C#/D


So….. What does this all mean? I saw Alan Carruth on another forum say to do this test but now I don’t really know what it proves. Does a significant drop in pitch with the poster putty added indicate that this guitar is a good candidate for shaving the back braces to lower the main air and give the guitar more bass?

When you say the top was cut very thin how thin exactly?

Did the poster putty improve the sound of the guitar?
When Arthur Benade wrote about adding weight to various areas of the plates it was to find what areas might benefit from thinning (or adding weight) to make those areas less stiff (or have more mass). He would move the weights around until the sound improved. Adding weights to certain areas (those of greater movement) could change the frequency of some modes of vibration without affecting others (with nodal lines at that spot). So it was as much where you placed the weight as it was how much weight you used. It also allowed you to adjust the frequency relationship of the modes to one another.
The old size 1 Martin guitar I have has 5 back braces. The lower three are wide and low rather than narrow and tall. As small as the guitar is, I doubt it was done for structural reasons (some of that model only use four) but possibly done to add mass without adding too much stiffness (speculation on my part).

The top on this guitar .071in or 1.8mm. That is thin but keep in mind it is a small guitar too.

https://1drv.ms/x/s!Av2dGV3MIkc3gjIasBQ ... m?e=TQES0Q

I did not string the guitar up with the poster putty attached. I suppose that would be a good idea

Too bad you didn't have it strung when you added the mass to the back: you might have homed in on the best back tap tone pitch.

It's hard to calculate the 'main air' pitch for a guitar because, unlike Helmholtz's glass bottles we're not working wit rigid walls. The top, being the most flexible, makes the most difference, but the back can affect it as well. In the 'bass reflex' action on the guitar the top is moving 'in' as air is moving 'out' of the hole at the 'main air' pitch, and the you can think of the top as an added 'mass' in the system, lowering the pitch from what the pure Helmholtz mode would be. At the 'top' frequency the air is moving 'in' as the top moves 'in', and the rising pressure in the box acts as an added stiffness element in the top, raising it's pitch. If the back is not perfectly rigid (and it never is) it can add to that effect.

Fred Dickens used electric circuit models on the computers at Bell labs to show that tuning the 'main back' resonant pitch close to the 'main top' would lower the 'air' pitch significantly. When the back is as far above the top pitch as yours is, you can drop the 'air' pitch by a semitone or more by working on back braces: I've gotten a whole tone or better in some cases. The effect is greatest when the half-power band widths overlap, but there are risks to that. One of the worst 'wolf' notes I've seen happened when the 'back' pitch with the hole open and both plates free to vibrate was only 7 Hz above the 'top' pitch. I didn't have the time to fully analyse it, but I believe that the rosewood back was acting as a 'flywheel', storing energy that it got from the air pressure change in the box. As the back amplitude increased it tended more and more to feed back on the pressure, slowing the motion of the top. At some point the top motion was not sufficient to maintain the air pressure change in the box, the back reverted to it's natural frequency, rather than the driven pitch, and 'dumped' it's energy into the air. Once the back stopped vibrating the top could move again, and the cycle started over. The result was a 'thud' at the difference frequency of 7 Hz, which sounded like a fret buzz. Loading the top to drop it's pitch to 11 Hz below the back pitch was enough to get away from the band width overlap, and didn't hurt the sound too much.

I tend to see a drop in the 'top' pitch of about 1/2 semitone when I put on the bridge, and can see as much again in the process of 'breaking in'. I feel that a 'safe' spacing for the top and back tap tones is about a semitone, so I'll try for something like a whole tone difference in the assembled box without the bridge. This tends to give the lowest practical 'air' pitch from that couple. YMMV

Keep in mind that the 'first corpus' resonance; the so-called 'neck mode', can also couple to the 'main air' resonance through the top; another one of Fred's findings. As with any such couple, the pitch of the 'air' resonance will be perturbed. I made a small guitar, based loosely on a Panormo, once that had the 'main top', 'main air', 'main back' and 'neck' pitches all within a whole tone. No amount of wood removal from the back braces would budge any of them; it was like an 'exclusion principle', where each of them pushed the others into a certain notch. That little guitar had the most wonderful basses...

I was hoping you would chime in Alan

I can certainly try stringing the guitar up. That will be my next step and I can get to that later today. I suppose the poster putty doesn't have to actually stick to a brace? I should be able to just stick it to the from the outside and get the same result?

I'm not a math guy at all so the whole top tuning thing is very unfamiliar to me. But my one thought would be that perhaps the top was cut too thin and thus doesn't have the mass needed to produce the bass that you're after? I think of Greg Smallman's guitars and how thin the top is on those and they typically are not very bass heavy.

The numbers can’t be measured meaningfully until the guitar is strung to pitch. I just wrap my fingers around the strings at the 12 fret while holding the guitar next to the mic and give a solid but not heavy bonk on the bridge wing, or Center of back depending on what I’m measuring.No need for fancy hammers.

Inside or out with putty shouldn’t matter. That being said, the affect you’ll hear won’t be the same as shaving the braces. You’ll be making the back slow down in freq, but you won’t be changing how stiff it is. It’ll move slower, but not farther.

When you change the freq by reducing the stiffness, the freq goes lower because each in/out motion travels farther and takes longer to accomplish, also moving more air. That’s how I consider it anyway. Point being that shaving braces on the back will do much more than adding putty.

For a guitar that size, for the way I build (which is always active back), I’d be comfy losing nearly 100hz off your posted back number. Your top number is good, but it (and air) numbers will also lower with the back. You probably can’t get 100hz change from just the braces, but you can get a lot. I’ve sent a parlour out with 1/4” or less back braces that started out at 5/8 tall. Obviously the back was too stiff.

Anyway, I’d hog away at back brace 3 and 4 and try to get the back to couple with the top. The bass increase you get from that is quite extraordinary. You’ll find that changes in numbers is slow at first, but, the closer you get to target, the faster changes happen, cubed rule of stiffness and all.

So hog away but measure often. This is a terrific opportunity to get to ‘hear’ what the back does by bringing it into the mix. Just take things down a 1/2 step at a time…

If you are aiming to use the gore method of tuning, don't block the soundhole, and dont use a tap on the back. Just hold it in playing position with the back free and strung up complete with pins. Then tap on the bridge and read off the air, main top and back from that graph. These are the coupled frequencies, which are what you get when playing and are meaningful rather than trying to isolate things.

I have both books and I thought I saw this method outlined in the books but for the life of me I could not find it. I can never find anything in those books for some reason. Anyway... Was this method outlined in the books and if so do you what page. I guess I will dig into those again.

I did string it up last night and tried the same tapping and I was getting such wild results I just gave up. I could not get a consistent result from tap to tap even in the same spots. Maybe after a day of string tension it will settle in a bit.

My little Stauffer is almost the same size as your parlor. 9/6/75/12 and about 18 long. I checked my air mode and tapped the belly and back in the center and bridge, and I get almost exactly the same frequencies. 120/230/345or so. I used an online tone generator to find the notes. But our instruments are completely different.

My Sitka top is goes from about 3mm to about 2. It has 4 ladder braces, and I curved them down to nothing on the ends to get more movement and sound. They are shaped using a formula to change each section drop by an exponent until the end was at the surface of the plate. The top did bend .25" with 7 pounds on it before bracing or anything. I was getting .035" at the bridge with 7 pounds braced, and with the sided glued on; but it is hard to be sure that everything is solid when checking that.

I just checked those movements to have some kind of measurement to have as a reference for later instruments. This is only my third, so I have to start learning something.

The Padauk back is 3mm, and I did the same thing to the 3 braces. I don't know that it really changes the tap tone much, but it opens up the sound a lot. I thinned the edges to 2.4mm so I could just use a maple purfling around the edge as a binding. I really didn't notice the tone dropping. It still taps higher on the edges than the middle.

I'm happy when the top and back seem to give a little when pressing on them.

So completely different approaches, and yet similar numbers.

As far as getting bass? I don't know. My arch top has a lot of bass, but that is a different system entirely. The air mode on that is about a semi-tone lower, but the air volume is way more!

Everything works together.

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Why be normal?

Ken a few months back I had an old Stauffer type Venetian guitar in for a restoration and the back braces on that thing were HUGE tall. Like a full inch tall. It was also maple so it must have been totally reflective. It was a cool sounding little guitar though for sure.

As Jeff said. Also the strings need to be muted.

The description is in the appendix 1 of the design book. It is not entirely trivial!

Ways to change these resonant frequencies is described in chapter 22.3 of the build book.

There is also lots of good stuff on the ANZLF in the section dealing with G&G's book. All a bit spread out unfortunately.

Good luck Dave

The blocked soundhole test is ONLY used for monopole mobility calculations.
Been a while since I did it, but you use a series of taps over 5 ? seconds.

I found the method in the books so thanks for that. I'm not sure I set up my spectrum analyzer properly. I guess I need more readings on the X-Axis? I cannot really tell were this second peak is. Here is a graph of the top tap on the bridge under string tension.



And for the hell of it here is one of the back

DonKirkland wrote:
"But my one thought would be that perhaps the top was cut too thin and thus doesn't have the mass needed to produce the bass that you're after? I think of Greg Smallman's guitars and how thin the top is on those and they typically are not very bass heavy."

The top plate on a Smallman is simply a membrane, to move air. It has no useful stiffness or strength in itself. The structure is the carbon/balsa lattice inside, and that is both very light and extremely stiff. Over all those instruments have a lot more in common with resophonic guitars and loudspeakers than they do with traditional guitars. They put out plenty of bass, but even more of everything else.

meddlingfool wrote:
"When you change the freq by reducing the stiffness, the freq goes lower because each in/out motion travels farther and takes longer to accomplish, also moving more air."

The pitch drops because the relationship between stiffness and mass has changed. The back weighs a lot more than the braces, but the braces are stiffer because of their height. Reducing the height of the braces doesn't make the structure all that much lighter, but it reduces the stiffness a lot, and tat drops the resonant pitch. Note that to have the greatest effect you want to remove wood where the braces are bending, which is in the center. lots of people thin them out at the ends, or thin out the top or back around the edges, but working in the middle is more effective. Hence 'scalloped' braces.

jfmkenna:
Is the scale along the bottom frequency? It seems to float in a way that makes it hard to compare one chart with another. I'm not really familiar with SA.

A friend of mine has written an Android app called 'Luthier Lab', that includes a very easy to use spectrum tool, and it's free.

Oh yes I have heard of the app Luthier Lab I think I will give that a try. And yes that is the frequency at the bottom. Not enough iterations to be useful. It's good for getting the max peak though. I could use Audacity too.

Using Luthiers lab app this is what I come up with. The guitar is strung up to standard tuning with the strings muted with a cloth. Tap at the bridge wing.



After shaving the two lower back braces a little bit:



And then a bit more:



So it looks like it's moving down but I have not yet hit that breaking point. I thought the point was to have the first two peaks closer together like around 50Hz or so. But they seem so far to be moving at the same rate.

I don't know much about parlors, but my experience in shaving the back braces matches what you have. The main air and top both drop in frequency.
As far as a delta of 50 Hz goes, it's going to be closer to what your showing than 50 Hz I believe. I think somewhere around the 80-ish to 100-ish is typical. It might be closer to the higher end for a parlor. I'll be curious to what others think.

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Jim Watts
http://jameswattsguitars.com

I haven't done any frequency tuning since I switched to a Mac, and the fft flute tuning program wouldn't work on it, and I can't figure out how to make Audacity do ANYTHING. Nothing at all. The look of that LuthierLab app is great.

But only for Android. I guess I'm still a Neanderthal! I can't even get garage band to record anymore, it's like the mic is turned down. You would think that when technology gets better, it would be easier.

I can still type, but not so well with just thumbs. Thumbs are just for the space bar.

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Why be normal?

Where are you getting a notion about 50hz from? What is happening is exactly normal. In the last two graphs there’s no measurement of the back peak…IMO it’s very useful to have it quantified…

Gore describes a freq response chart of a good guitar with an active back in one of the design chapters. He says that a difference of about 4 semitones makes it good.

I thought that the second peak represented the back? Maybe I am doing this all wrong. None the less the freq is dropping.

Like a dummy I forgot to make recordings before and after. But so far it has not dropped much. So I will record it tonight. That way I may be able to actually tell.

The first peak is the main air. You can verify this by covering the sound hole and it'll go away.
I read your graph as the fist peak (T1,1) being the main air resonance and the 2nd peak being you main top resonance (T1,2).
I think the 4 semi tones is the delta between the the top and coupled back (T1,3).

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Jim Watts
http://jameswattsguitars.com

so is T1,3 on that graph or do I have to tap the back?

I guess I will re-read that chapter

I think I will try with soundhole blocked just to see too.

Ken, I use Audacity on a Mac. If you're interested in pursuing it PM me your email address and we'll see if I can help. --Bob