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Some of you were kind enough to offer me some advice about brace heights for an acoustic bass guitar, which is now finished. Here's the bass in the wild. (Proper pics will have to wait till I've solved the problem of a stuck shutter on a Nikon D90..anyone?)




I now want to refine the top and back bracing to make sure I'm getting the best response possible. I will put in a pickup at some point, but for the moment I want to push the acoustic responsiveness as far as I can. It already sounds great, but let's push the envelope....

Here's a frequency chart, tapping reapeatedly on the bridge.

The resonant frequencies, as far as I can tell are:

Air: 98hz
Top: 200hz
Back:267hz

Here's the front bracing:
Top braces: 13.5mm at the X, then...
Lower legs: Straight taper down to 8mm, 75mm from the edge, feathered to nothing at ends.
Upper legs: Straight taper down to 8mm, then down to 3mm let into sides.



I haven't got a picture of the back bracing, but it's ladder braced, 4 braces each 16mm high.

Total string tension is approximately 70kg.


I've read from builders like Gore that the bridge rotation should be up to 2 degrees. At the moment it's at 1.2 degrees.

So what do you all think of this plan:

1. Shave/sand the lower legs of the X braces, halfway between the X and the end of the taper. Do a bit then re-tension to check the bridge rotation.
2. When the rotation hits 2 degrees, take a resonant frequency snapshot.
3. Shave the lower back braces (the lower two?) till the back resonance is 4 semitones (a major 3rd) above the top resonance.
4. Crack open the champagne and party hats...

Does that make sense? I've never done this before so any pitfalls I should be looking out for?

Thanks so much for your advice. I realise this is a bass guitar, so not in most people's interest zone, but there's very little bass-specific stuff online.

Put the pickup in before you start shaving as it will impact the numbers. If you get happy numbers and then add a pu your numbers will change.

Instead of sanding x-braces, I suggest starting with the tone bars.

I would not do this on a guitar that had a good sound. Do it on one that can only be improved. But, your guitar, your gamble.

So brace shaving is a bit risky but I suspect that John already knows that and is looking to proceed anyway.

I've only shaved the braces on one guitar that I built. It made a tremendous improvement and took that guitar from being a bit of a tonal disappointment to one of my favorites to play and enjoy today, 19 years later.

I posted a pic it's the one on the left.

So the story is I went full on parabolic bracing aka "tapered" bracing about 5 guitars into my building. Back then on the OLF, like today there was no one here who was able to provide guidance for the dimensions of tapered braces.

Of course these days I recognize that guidance would have been simply a safe starting point anyway so what I was looking for really was not necessary.

Anyway this guitar, the sunburst one has an Adirondack top, very stiff and Adi bracing and sure as shootin just like most newer builders I over braced the sucker.

Off the bench it sounded like a Takimine..... which to me is a failure.... It was tight, lacked low bass and was average when it came to being responsive.

I had not yet learned to voice guitars which I started to learn as a result of my brace shaving of this one. I already had Audacity and the optional spectrum analyzer and contact pick-ups from the premium Strobosoft program.

Since I wanted more bass and lower end that meant in my thinking opening up the entire top so I went after the tapered X-brace.

I used finger planes and it was all done through the sound hole and then I would tap the top and check frequency responses. I was able to watch the main air resonance drop lower and lower before my eyes as I loosened up the top.

Addressing the tone bars and finger braces did not yield a noticeable difference like shaving the X-braces did.

A problem area was the X intersection and I had used a cap instead of a patch which I later went to recognizing that we are not building ottomans.... or at least I wasn't. That intersection is incredibly stiff so much so that one of the best builders I know once advised me to lower the hell out of mine and that would open them up considerably. He was and is right.

For those who are learning the cloth patch is not intended to be super strong but instead it is intended to persist some flex and that it does.

Anyway I could not address the intersection through the sound hole, sadly it is what it is. Sure I could have scraped the cap off and lowered the intersection but now we are talking about a pretty substantial operation for through a sound hole....

I was mindful to not go too far and today this is a great guitar that got played yesterday by some friends who came over to jam. Because of the koa back and sides it has a darker tone anyway and is not a candidate for lots of high-end sparkle like a James Olsen. Instead this guitar has strong fundamentals and came alive with careful, incremental, verified with frequency monitoring brace shaving.

I followed up my finger planes when I was done removing material with some quad folded sand paper to clean things up and then of course I vacuumed the inside of the box being careful to not block the sound hole and permit the vacuum to collapse the top.....

I would think that a bass like this one, your's John would respond quickly and well too since you want the fundamentals and are not looking for a lush, overtone tone.

Just keep in your mind at all times.... that going too far is not reversible (at least easily...) and what will result is too much break-up of the top and distortion.

If it were me I would decide what frequency spectrum with your tunings you plan to use and then take the lowest note, mid note and highest notes you plan on playing on this bass and use those notes over and over again while monitoring progress to watch and listen to it gradually open up.

Nice job on the building too John she looks great, well done.

[quote="Hesh"]



Anyway I could not address the intersection through the sound hole, sadly it is what it is. Sure I could have scraped the cap off and lowered the intersection but now we are talking about a pretty substantial operation for through a sound hole....

/quote]

Hesh, on the surface this seems like a rather risky operation in that the intersection is a lapped joint and by shaving it down you are disproportionately weakening things...e.g. if the height of the intersection is 1/2" and you take it to 7/16" your lapped material is going from 1/4" to 3/16" while the lower piece is unchanged...just something that popped into my head as I read your post.

On the flip side, if it sounds better, then why not?

Good point. I've got a couple of stick on piezo's (Schaller Oyster). My plan was to move them around on the outside of the soundboard to find the best locations for them. They don't weigh nothing, 12g each (x2) which is 2/3rds the weight of the bridge! So once I've found the right location a K&K may work better....

Thanks Barry for he suggestion. Is your thinking that working on the tone bars will loosen the lower end of the sound bars without compromising the stability of the top?

Yes it is risky and I said I could not address it. It can be done but until you spend time with an arm in the sound hole and realize how incredibly limiting your movement (and arm) is you tend to want to stick with the simple stuff that you have a full range of movement for what you seek to accomplish.

There are builders who take the X intersection to 1/2" or less and one builder does around 3/8". He's built over 400 the last time I looked and makes excellent guitars.

This guitar was one of the last ones I built with an X intersection over 1/2". Not necessary and it really kills responsiveness to make the top that stiff.

This forum has always suffered from all of us over building our stuff at least initially. The height of the X intersection on our plans too is way too high and the wooden cap makes the intersection even stiffer.

We want to the top of a guitar to pump like a fire place bellows which was Mario P's analogy that I read here one day and it completely changed my understanding of how a guitar works, an acoustic guitar.

Lastly you're right it is risky and it's risky in the sense that you can't put material back, easily....

But when you are faced with a disappointment such as lots of hours on a project and you are not pleased with the tone what do you have to lose?

And as for our friend here John he posted about this before and seems very clear headed where he is going with this. So that's my hopefully helpful 2 cents, 3 with inflation

Thanks Glen the concern. This is (hopefully) just a stepping stone to a greater understanding of what makes acoustic bass guitars tick. One of the troubles with building basses is that there is no virtually no information online about what I should be aiming for. What the resonant frequencies should be, sound hole sizes, scale lengths etc etc. It's either that or make a start on putting up curtain rails for the wife.....

Thanks Hesh for the extensive advice! I shall proceed as you suggest, shaving the x braces first and comparing the effect on the same low/mid/high notes. That's a really a great suggestion to keep the notes the same so the comparison is easier.

As you realise, my plan is shave the braces because it's risk! I made electrics for 10 years then a couple of years ago the acoustic obsession hit me pretty hard. But I play bass, not guitar. So I'm trying to understand the lessons learned by you six stringers, so see whether the same lessons can be applied to basses. The mechanics are identical, the construction is the same, even the string tension is similar (on mine anyway). So I'm guessing there's a lot of overlap. But the only way to find out is to build to the point of failure. If it does literally implode I shall be disappointed, but what a data point! The next one is already in progress, so I shall sweep up the very expensive firewood and crack on with the next one. No looking back

Did you use bridge rotation/top deflection as a measure of how flexible your tops were?

(Lovely builds by the way!)

@Hesh I was intrigued by your comments about the Tacoma Thunderchief in my other thread. I happened to come across one locally for sale, so grabbed it and and some point I'll do some reverse engineering to see how it works.

When I got it I thought that it sound pretty good. But since finishing my own bass, I have to say mine gives it some pretty stiff competition. Despite it being a lot smaller:


The resonances of the Thunderchief are as follows:
Air: 85hz
Top: 200hz
Back:360hz

Interestingly the Top resonance is the same on the Tacoma as on mine. The air resonance is obviously a lot lower than mine, and you can sense it on the low E. But the body is almost double the depth....I can feel an audio shoot out coming...

Edit: I also need to measure the area of the soundhole of the Tacoma. I wonder how much the slightly smaller offset sound hole has on resonant frequency?

If it implodes or you destroy the bracing you can always take the info you've learned and retop it. I have retopped one from the 1940's that was damaged beyond repair and also have two of my early guitars on the shelf to retop. It's not a lot of fun but very doable and still easier than building a whole new guitar.

Now that's the spirit John you're more keen to learn and find out and what's a little top explosion/implosion... Anyway when you go too far in material removal you will know it far before things explode, hopefully. I did find that it takes very little material removal at some point to see changes so do be gradual and check often.

Yes I did deflection testing which is simply a controlled, repeatable way to set up a top and see how much it deflects given a specific weight. It's been a while but I think I spaced my fixture I used at 18" and used a five pound weight and measured the deflection. You can do this a million different ways the point is to always set-up the jig and test the exact same way whatever you choose to do. When I deflection tested I also prethicknessed my tops to the same thickness for the test. I don't remember what I used though might have been 0.130ish".

Deflection testing is useless for most builders because it relies on populating a database of results and correlating that to how we believe the resulting guitars turned out. So to populate the database by a factor of one you have to successfully complete a single guitar. As such for many the deflection testing database and the real value of same is never realized unless they stay with it and perhaps complete at least a couple dozen instruments.

Cool you are doing some great work John and your approach is excellent to all of this. Good going!

There is a thread right now about sound hole size that may be of value to you. Back around 2008 on the OLF in the archives when we discussed sound hole size there are some good threads here too that might be useful to you. Al Carruth is someone to pay attention to for sound hole size and the affect on Helmholtz response he's done more work on this subject than anyone else I know of.

I was intrigued back then to learn that a smaller sound hole can produce more bass in certain circumstances. Lots of stuff that is counter intuitive in Lutherie.

As for the Tacoma if you reverse engineer please also know that most of these guitars fail.... They end up in the trash heap and they tend to fall apart. Customers like them when they are still in one piece and they hit the sweet spot when all the unplugged albums, Alice In Chains, Clapton, etc. came out.

But the design is not very serviceable and a rare example of a factory guitar being built too lightly and not holding up. We see braces peeling off, cracks, bridge lifts. With the sound hole north of the typical position we need to enlist the Greys from outer space to borrow their long arms to work on these things....

OK well so far this is been interesting, if no fun at all....

One thing I hadn't throught through was the slightly thinner body (80mm) means that it's really really hard to shave/sand down the braces. I can get my hand in ok, but once in there's no space to actually do anything at all easily.

So the first lesson learned is the body needs to be a bit deeper. Second, it's a process that I could imagine doing again to deal with a specific problem like a wolf note, but not as part of my standard build procedure. I like to follow plans and have specific measurements - fiddling around removing wood when I can't see what I'm doing just doesn't feel right.

Anway, I shaved some height off the lower legs of the x brace. Veeeery slowly the bridge rotation increased, but it did seem to take an age for any change at all. As it changed I noted the change in frequencies as follows:

Bridge rotation: Air resonance: Top Resonance
1.2deg.....98hz.....200hz
1.3deg.....98hz.....193hz
1.4deg.....97hz.....190hz

I then tried to shave the back braces with a view to bringing down the back resonance. This were even more difficult than the top. Something about their location meant I just couldn't really reach them. However, after an age of frustration the resonances are...... (in brackets are what they were before I started shaving):

Air: 94hz (98hz) - between F# and G
Top: 182hz (200hz) - between F and F#
Back: 252hz (267hz) - between B and C

Here's the chart:


And at that point i gave up, exhaused!

Was it worth it? Yes, I think it was. This thing is loud! It's also got an incredibly complex voice, much more so that the Tacoma Thunderchief, which is bland in comparison (though to play fair I should put new strings on it).

There's a bit of wobbly wolf note on the F#. The air resonance is at 94hz, and F# is 92.5, maybe a bit closer than is ideal.

If I had free access to the braces I'd keep on going - 2 degrees is still quite a long way away. I may give it a couple of days to settle then have another go. I'm a sucker for punishment.

So all in all, I'd say this was a success.

But with my next (current) build I'm going to glue the top on first and use top deflection as a proxy for bridge rotation. I saw a presentation by Giuliano Nicoletti recently who said that according to his research top deflection and bridge rotation correlate very closely. Hopefully I can get the final resonances where I want them before gluing the back on.

Nice work John and a nice approach to the work too.