Shaving a back brace on an OM-21 with good results

[BeeLectro]

I don't recommend anyone do this. Warranties don't concern me. I've got so many guitars that if I wreck one I'll cry and have a funeral but I'll get over it.

 

Anyway, here's what happened. It's a 2018 OM-21, bought it new but it had been hanging in the shop for over a year. I've had good luck buying guitars that've hung in the shop for a while, they've already opened up, they don't change much.

 After a couple years I think my tone preferences changed and I wished it had more bass, a stronger low E string. My other OM is a hog custom shop and can double as a dread.

 I remember exchanging some emails with Bryan Kimsey many years ago, he said you can tune the body by shaving the back braces. So now I decided to give it a try.

 

 This is not for the faint of heart. The tools I used are a Dremel with a 36" extension for initial deep cutting with a coarse sanding barrel. Also a 50 grit sanding disk cut in half and attached to an oak square stock as a handle. The disk is sturdy to begin with so it keeps it's shape and doesn't wear out. And a flashlight.

 

Working blind inside the guitar is a little stressful, especially with a power tool. I practiced on a cheaper guitar first, my workmanship was terrible but the sonic results were excellent. I moved on to the OM-21. 

 

Bass notes have longer wavelengths, and the notes I wanted to bring out are below open A. So the only brace I shaved is the longest of the lower bout, side to side back brace. I started at the outermost ends by the curf and gradually worked towards the middle, stopping now and then to give it a listen. I ended up thinning it down considerably, leaving about 1/8" or less at the outer ends and tapering up in the middle.

 The theory, is that shaving the outer ends will bring out the lowest frequencies.

 

It was a complete success. The bass is big and warm and consistent all the way up the neck. This consistency up the neck was a pleasant surprise. There's a few other sonic surprises that are somewhat ethereal, as describing tone is impossible, but the character overall is more interesting. Most standard series Martins produce a tone like a soft fluffy halo around each note. This mod made the halo a little more prominent.

 

Here's a few pics.

Edited September 9, 2022 by BeeLectro

[Mikeo]

OM are probably my fav.

Glad you like, and it came out well.

 

[BeeLectro]

I should probably delete this thread as I don't want to encourage anyone to try modifying their guitar only to end up ruining it.

[daddymack]

1 hour ago, BeeLectro said:

I should probably delete this thread as I don't want to encourage anyone to try modifying their guitar only to end up ruining it.

Caveat lector...

I can't imagine there are very many folks who would undertake that precarious and potentially fatal type of mod on a multi-thousand dollar Martin OM-21.

I wouldn't.

I would have traded it out for something I liked...

[Glenn F]

Come near my OM-21, and I'll get a restraining order.

[garthman]

Phew. Brave man! Or maybe foolish . . . . . . .?   

[Glenn F]

I asked my luthier friend about doing this kinda mod, and he flatly refused. I wanted to turn my D-28 into a HD-28. This guy could do it if he chose, but I expect he was thinking of his nerves and my pocketbook. Something to try on a banger. Not a Martin OM-21, jaysus,lol.

[DeepEnd]

Congratulations. Glad it worked out. You're a much braver person than I am. I wouldn't try it on my beater.

[Freeman Keller]

 

 

 

On 9/9/2022 at 3:14 PM, BeeLectro said:

I don't recommend anyone do this. Warranties don't concern me. I've got so many guitars that if I wreck one I'll cry and have a funeral but I'll get over it.

 

Anyway, here's what happened. It's a 2018 OM-21, bought it new but it had been hanging in the shop for over a year. I've had good luck buying guitars that've hung in the shop for a while, they've already opened up, they don't change much.

 After a couple years I think my tone preferences changed and I wished it had more bass, a stronger low E string. My other OM is a hog custom shop and can double as a dread.

 I remember exchanging some emails with Bryan Kimsey many years ago, he said you can tune the body by shaving the back braces. So now I decided to give it a try.

 

 This is not for the faint of heart. The tools I used are a Dremel with a 36" extension for initial deep cutting with a coarse sanding barrel. Also a 50 grit sanding disk cut in half and attached to an oak square stock as a handle. The disk is sturdy to begin with so it keeps it's shape and doesn't wear out. And a flashlight.

 

Working blind inside the guitar is a little stressful, especially with a power tool. I practiced on a cheaper guitar first, my workmanship was terrible but the sonic results were excellent. I moved on to the OM-21. 

 

Bass notes have longer wavelengths, and the notes I wanted to bring out are below open A. So the only brace I shaved is the longest of the lower bout, side to side back brace. I started at the outermost ends by the curf and gradually worked towards the middle, stopping now and then to give it a listen. I ended up thinning it down considerably, leaving about 1/8" or less at the outer ends and tapering up in the middle.

 The theory, is that shaving the outer ends will bring out the lowest frequencies.

 

It was a complete success. The bass is big and warm and consistent all the way up the neck. This consistency up the neck was a pleasant surprise. There's a few other sonic surprises that are somewhat ethereal, as describing tone is impossible, but the character overall is more interesting. Most standard series Martins produce a tone like a soft fluffy halo around each note. This mod made the halo a little more prominent.

 

Here's a few pics.

Wow, I don't come around here much anymore but I happened to stop by the other day and saw this.   Wow.

I'm not really sure where to start commenting but I will say that I am very glad your modification was a success.   But I do have issues with a few of the things you said.   Its too bad you didn't take some sound clips before and after the mod - I would love to hear each of the open strings played before and after (with the same strings, pick, recorder, yadda yadda).   Double blind listening tests are always interesting...

Something else that is equally interesting is that there is now software that will let you compare two sound notes both in the time domain and the frequency domain.    I have been using one that was especially designed for luthiers when I make modifications - here are two waveforms of the same guitar with a plastic and a bone saddle, this is A2 on the open fifth string, the fundamental is 110 hz

 

Having something like that and the sound clips would get around the inability you are having describing what you really hear.

OK, just so everyone knows what you are talking about I believe that you are shaving the lower back brace, sometimes call B4.  This is an OM that I built for a HC forum member some years ago, there are two shapes for that brace that Martin has used over the years.   One is wide and flat, the other is fairly skinny and tall like this one.

 

That brace has two main functions - it keeps the back from splitting as the plate moves with humidity and it holds the 16 foot radius of the back's dome.    Remember that the stiffness of a brace is the cube of its height, by removing one half of the height you reduce the stiffness (and strength) by a factor of 8.    

Some builders do attempt to "voice" the backs of their guitars and certainly that is a worthwhile endeavor - I've never felt that it made that much difference in any tap tones so I mostly just follow traditional designs (like Martins) and they seem to work fine.

I'm intrigued by your discussion of wave length as it relates to your brace shaving,   We know that the length of a wave is simply equal to its speed in whatever medium (air, wood, etc) divided by its frequency in hertz.    I don't know if you are talking about the note in air inside the box or propagating thru the wood - the speed of sound in air is 343 meters/sec, generic wood is 4150 m/sec.   That means your A2 (open fifth string) has a wave length of 3.2 meters in air, 38 meters in wood.   Your low E2 string would be 4.2 meters in air and 50 in wood.    Compare those with the length of that brace which is less than a half a meter, its pretty hard to believe that you are dramatically affecting the wave length - I guess I don't believe your theory.

Your comments about halos around the notes looses me completely.   If you are talking about all of the partials you can see how they changed in the saddle swap but the fundamental and first octave are pretty much the same.

 

I'll make one last comment before I crawl back in my hole.   Bryan Kimsey is (was, he is not taking new work) a very talented and somewhat controversial luthier and repair technician.   One of the things that he does is after market scallop Martin guitars from the 1970's.   This was often considered a low period for Martin's guitars - because of a lot of warranty issues they had gradually beefed up the size of TOP braces and bridge plate from what they were doing in the so called Golden Era before the war.   Bryan's claim to fame was replacing the big rosewood bridge plate with a smaller maple one and shaving braces thru the sound hole to make them more like the pre war guitars, often completely removing the popsicle brace.   Obviously it voided warranty and there were many people who were critical of his ideas.   However I happen to own a Kimseyfied 1974 D-18.   When Bryan reset the neck on it and refretted it he also replaced the bridgeplate and scalloped the braces (we left the popsicle alone).   I don't belief the work changed the sound significantly but it certainly made it a heck of a lot louder.    I did make some recordings before and after, people who have heard them agree that its hard to tell much difference except volume (and of course that can be playing style as much as anything).     As far as back braces, I've never heard Bryan say anything about modifying them, only fixing them when they break.

 

In the end it doesn't matter and I'll leave the good folks at HCAG  to think what they think.   I had a lot of good friends here and see that a couple are still around - hi to you all.   I'm still building guitars and stirring up trouble.   Life is good

 

 

[BeeLectro]

FK, I'm not sure where to start. Actually I do know where to start. This is JT from the old days. Almost 20 years later. (18 years, I think)

Thanks for the gracious and thoughtful reply.

 My recording equipment has been in storage for several years. I had no plans of posting a thread about this until after the fact. Otherwise yes, a before and after recording would be fitting.

To all, acoustically, I'm a CFM player exclusively, mainly because they are accessible, there's a lot of them out there, and for the most part they sound good. But I don't see them as sanctimonious. I currently own 5: CS-OM, hog bs; 000-18 14fret; 000-28 14fret; 000-16 black walnut limited; 000-16 Granidillo.

My biggest complaint with CFM is I can't find a dreadnought I like. The bass is strong at the expense of everything else. All these other notes are contributing important information and shouldn't be buried under the bass. My logic is: If CFM wastes that much wood making infurior dreadnoughts, I'm not gonna feel bad about modifying one. Or two. Maybe even three. 

 Sonically this experiment was a success in some frequency ranges (or tunings), less so in others. To make the discussion even more convoluted, I don't tune to pitch, but half step below. I'm old and I don't want to work that hard. Light strings, low action, half step below pitch.

So, this OM-21 before being modified, lacked strong bass in the range below open A, G#. I spent a long time with the guitar before buying it, a couple hours at least and decided I was good with the balance, and I was for a couple years. My preference changed, I guess. Two of my 000s produce a warmer bass than does/did this OM-21, and I think this influenced me.

As for technicalities like frequency lenths, I can't speak to it other than using the logic that the bass notes use the lower bout primarily.

 After posting I shaved two more back braces to a somewhat lesser extent, and sanded the "tone bars" also.

It wasn't a complete success because tuning as I usually do, half step below, the balance is off in other areas of the bass. But if I tune down to D it works well.

Bryan Kimsey: I no longer have the emails I exchanged with him. I think he should make his mods and secret tools public domain, give the info away like Denny Rauen did with the compound radius.

Halos around the notes: this metaphor also works well for old Fender amps. For acoustics it's caused by scalloped braces. The braces divide the top into different sized sections. Each section vibrates differently when a note is played. The scallops cause the adjacent section's vibrations to mix with eachother. Some people hear it as additional bass, and to some extent it is. It's actually extra harmonics produced within the top wood due to the differing sizes/shapes of adjacent sections handling the note differently, and mixing at the scallops. Where adjacent sections meet at the scallops is the source of it. It's the magical thing we love about acoustic guitars. My 000-28 does it especially well.

Again, I don't see guitars as sanctimonious.. To me they are like model T Fords and can be tinkered with in order to improve the musical experience of the player.

FK, thanks again for the incredible reply. I'll need to read it a few times to get the most out of it.

-JT

I'll probably add more to this later.

Edited September 20, 2022 by BeeLectro

[garthman]

Hmmmmm.

Perhaps it's just me but before I buy a guitar I sit down and play it for quite a while to determine its playabilty and sound. I do make some allowances for the strings and understand that the guitar will sound a little brighter with new ones - but, HST, I have found that new strings lose that initial brightness pretty quickly. When I am satisfied that the guitar has ticked all my boxes I buy it and cherish it deeply thereafter.

Now why would I want its sound to change?

[daddymack]

20 hours ago, garthman said:

Hmmmmm.

Perhaps it's just me but before I buy a guitar I sit down and play it for quite a while to determine its playabilty and sound. I do make some allowances for the strings and understand that the guitar will sound a little brighter with new ones - but, HST, I have found that new strings lose that initial brightness pretty quickly. When I am satisfied that the guitar has ticked all my boxes I buy it and cherish it deeply thereafter.

Now why would I want its sound to change?

the sound will change, over time as the wood 'breathes'.

[garthman]

23 hours ago, daddymack said:

the sound will change, over time as the wood 'breathes'.

People say this ^^^^ but I seriously doubt that it happens. I think it is the listeners' ears that change.

[BeeLectro]

I sanded some off the other back braces, with more good sonic results. My goal this time was to see if I could make open D louder. So I tapped around on the back to try and get an idea where it would most likely be sympathetic to that range. It worked. It also brought out other frequencies around that range, but nothing detrimental.

I'm starting to believe these things are overbuilt for the sake of the lifetime warranty. I'm also convinced a luthier could "tune" a guitar body after it's been built. Some special tools would be required, and maybe some kind of analytical software, or just a good set of ears.

Oh, I also reinforced the sides of this guitar just below the waist, visible in one of the pics. This was done several months ago. My theory is CFM is making the sides too thin, and they tend to vibrate at annoying frequencies. I've done this mod to another CFM, a 16 series in another thread, with good results. This reinforcement may help channel energy to the back.

I'm okay with the consensus thinking I'm nuts. I think forums tend to find issue with those who think for themselves and don't ride the bandwagon.

This guitar is sounding wonderful.

Pics.

[BeeLectro]

50 minutes ago, garthman said:

People say this ^^^^ but I seriously doubt that it happens. I think it is the listeners' ears that change.

Both. I have a CS-OM that sounded pretty good when new, but sounds incredible now, and can almost double as a dreadnought. It changed a lot over time. OTOH, I had good luck buying a 000-18 that had hung in the shop for over 2 years. It hasn't changed at all since I bought it. The "opening up" happened while hanging in the shop.

As for the guitar in topic of this thread, my preference changed. So, instead of selling at a loss, shopping, spending more money, I decided to give modding it a try.

[garthman]

46 minutes ago, BeeLectro said:

Both. I have a CS-OM that sounded pretty good when new, but sounds incredible now, and can almost double as a dreadnought. It changed a lot over time. OTOH, I had good luck buying a 000-18 that had hung in the shop for over 2 years. It hasn't changed at all since I bought it. The "opening up" happened while hanging in the shop.

As for the guitar in topic of this thread, my preference changed. So, instead of selling at a loss, shopping, spending more money, I decided to give modding it a try.

Ears change a lot over time. As we grow older we lose more and more of the high frequencies so guitars begin to sound more "bassy" / "mellower" than they did when we were younger.

And maybe the 000-18 hanging on the wall didn't change (or "open up" LOL) at all. How would you know?

Fair enough about the change of preferences.

[BeeLectro]

1 hour ago, garthman said:

And maybe the 000-18 hanging on the wall didn't change (or "open up" LOL) at all. How would you know?

Fair enough about the change of preferences.

It was at a shop I frequent, close to home. I go there when I'm bored. When new guitars arrive I notice them, maybe try them out. I tried the 000-18 when it first arrived and didn't think much of it. Two and a half years later, out of pure boredom, I asked the clerk if I could play it again, I bought it that day.

Edit: I asked the clerk if it was the same 000-18 as had arrived two and a half years earlier. He checked the arrival date. I'm certain it's the same guitar.

Edited September 22, 2022 by BeeLectro

[BeeLectro]

As for preference changes, mine haven't changed. One guitar that does it all doesn't exist. Having a selection is nice. My preference in regards to this OM-21 definitely changed.

[garthman]

22 hours ago, BeeLectro said:

It was at a shop I frequent, close to home. I go there when I'm bored. When new guitars arrive I notice them, maybe try them out. I tried the 000-18 when it first arrived and didn't think much of it. Two and a half years later, out of pure boredom, I asked the clerk if I could play it again, I bought it that day.

Edit: I asked the clerk if it was the same 000-18 as had arrived two and a half years earlier. He checked the arrival date. I'm certain it's the same guitar.

I still think it's not the guitar that has "opened up" (which I don't think happens - I've seen no scientific proof of it anyway) but one or more of several other causes. First, as I mentioned above, people lose high frequency hearing over time and two and a half years is quite enough time to lose quite a bit. The strings could either have aged (guitars hanging on store walls don't often get a string change) and lost lots of their high frequency harmonics or, conversely, a miracle could have happened and the store may have fitted new strings (it happens!!!! - very occasionally) hence the better sound. The temperature, air pressure, relative humidity, etc could have been different, etc, etc, etc.

The only way to determine if "opening up" actually occurs would be to conduct proper scientific experiments where all other possible variables have been eliminated. I ain't seen one yet.

[BeeLectro]

9 hours ago, garthman said:

I still think it's not the guitar that has "opened up" (which I don't think happens - I've seen no scientific proof of it anyway) but one or more of several other causes. First, as I mentioned above, people lose high frequency hearing over time and two and a half years is quite enough time to lose quite a bit. The strings could either have aged (guitars hanging on store walls don't often get a string change) and lost lots of their high frequency harmonics or, conversely, a miracle could have happened and the store may have fitted new strings (it happens!!!! - very occasionally) hence the better sound. The temperature, air pressure, relative humidity, etc could have been different, etc, etc, etc.

The only way to determine if "opening up" actually occurs would be to conduct proper scientific experiments where all other possible variables have been eliminated. I ain't seen one yet.

They used to say playing a guitar causes it to open. And some people would leave a guitar on a stand in a room with a loud stereo playing for several days. I think that's all bs.

I'm a witness that it does happen, although I have no proof. It's just time that does it. After a while they loosen up and get accustomed to the tension and the guitar shape they've been made into. Wood and glue are both somewhat flexible. Add 100 pounds of tension and the whole thing distorts. No wonder you'll see some finish separating at the neck/heel joint after a while.

The real proof for me came when I custom ordered an OM from CFM. At first I wasn't completely happy with it, the balance of bass mids trebs was too heavy in the bass. It's much more even now, which I think is pure luck. I'd never recommend anyone custom order a CFM, no telling what it'll end up sounding like.

Hearing. I suppose that's a valid argument. But I think we grow accustomed to our handicaps.

[BeeLectro]

Garthman,

Also, the tonal aspect one would listen for is how the body resonates. The body will still resonate the same regardless of the age of the strings, as long as the same gauges are always used. Discerning what the strings are doing vs what the body is doing can sometimes be problematic.