Tone Factor - lacquering the inside of a guitar

django'spooltalent

Hi everyone,

I was just curious as to the effects of lacquering the inside of a guitar's body. Does it enhance or change the tone of the guitar, if so to what degree? In addition, did Mario M, choose to lacquer the inside to protect the inside from the environment or for a tonal characteristic?

Thanks, have a good 4th

-Daniel

manoucheguitars

HI. My understanding is he did it for tonal enhancement and (more importantly) projection. This is why we lacquer the interiors of our guitars and it really does make a difference. Naturally there are those out there who might disagree, but this has been my experience with the guitars, as well as many othere I have talked to.

Robert

badjazz

This isn't an exact answer, but in Benedetto's book and videos on building an archtop, he states that he lacquers the inside of his guitars because it is just a good woodworking practice that if you lacquer one side of a piece of wood, you should do both so that both sides react in the same way to atmospheric changes. That may have some roll, at least in the original reason why thise practice was started.

Ken Bloom

Finishing a guitar on the inside will not have much affect of slowing down moisture changes. This has been shown many times. I can see the benefit from the standpoint of the possibility of increased reflection from a more polished surface. I'm sure Bob can give us a lot more here. Since all the parts of a guitar are all held together the basic woodworking argument doesn't really make sense. If youonly finish one side of the wood, a board will indeed warp but the sides have to be free to move to do that. I think I will start trying this on my next few instruments and see what happens.

Bob Holo

Howdy - I just got your PM request to respond - OK here's my take on this - keeping in mind that I've not done anything approaching a scientifically valid test...

There are at least two concepts in play - neither of which should have a huge impact, but theoretically they're in play.

1.) change of elasticity / Young's Modulus etc... In other words, does finish change the tensile strength of the material by forming a surface on both sides that is less elastic than the wood. (like fluted cardboard or those foam insulation sheets with aluminum foil on the outsides) If this did happen, it would change the frequency of resonance of the top and so alter the sound. Well... gee... not too likely unless you're putting on so much finish that you're also dampening the wood through constrained layer dampening - which would be at cross purposes with the task at hand... so... unlikely from that standpoint. It would be much easier to just alter the top thickness or the soundhole size to do this... so... again - unlikely.

2.) absorption coefficient: In other words - which fraction of the acoustic power is absorbed by the material (and the inverse is therefore reflected) This is measured in Sabine Units (after the humble guy who first measured absorption) where 1= the amount of energy absorbed by a 1 meter square of open air. Absorption coefficients for most materials range from .1 to ~.5 and are frequency dependent. It takes mass and an air seal to reflect (contain) bass. A painted European-style brick wall can reflect or contain greater than 90% of bass to treble (100hz to 2,000hz for example) but unpainted can only reflect 75%. Thin light material typically doesn't have the mass to absorb bass but can absorb treble. Painted 1/4" plywood can contain 90% of treble but barely 40% of bass. As the material gets thinner - the relationship gets more exagerated. So, what does this have to do with painting the inside of a guitar? Well... If a person assumes that the finish on the outside of the guitar is airtight (it is relative to this purpose) then putting another airtight barrier on the inside wouldn't change high frequency absorption - and the material doesn't have the mass to absorb bass... so... does anything really change? Well... I guess that relative to the inside of the guitar - the absorption would change - and likely make the guitar selectively more "echoic" in the very upper frequencies - so it may add a brightness or slight reverb. But that is just a guess.

For the record - so that you know my biases - I lacquer the insides of my guitars for three reasons.
1.) It looks coooooool.
2.) I think subjectively it does add a little reverb.
3.) It slows and evens the moisture absorption of the wood which is a good thing particularly where woods of unequal density are joined and may want to move at different speeds - also I've heard that the old timey luthiers used to do it to help keep the laminated wood from delaminating because the glues they used were really sensitive to moisture changes.

Wow... that was entirely too many words. But there it is.

PS... sorry to mod this - but - to Ken's point - there's a difference between intercellular and intracellular moisture in wood. The interfacial pathways of wood are more prone to 'wicking' moisture throughout the life of the guitar whereas the cell walls of the wood can and do become more resistant to moisture change - particularly as they age. This is particularly true in woods like Spruce & Fir & Cedar which are the wood equivalent of a bunch of soda straws tied together. So - while the moisture content in wood can change quickly - it's primarily due to absorption of water between cells. So - Ken's right that most guitar finishes really don't alter cellular moisture absorption rates a lot because they're pretty slow to begin with - but they do alter intercellular absorption to some degree - more on young wood than old wood. Anybody ever seen what a crack in a piece of spruce looks like at 100x? Cell walls, baby... and this wood is really young - fully hydrated. Those cells will flatten dramatically over time.

django'spooltalent

Ha Bob,

Thanks, all I can say is wow and detailed. I'm just curious what is your profession? Lastly, so it does make a difference, but just a little?

Thanks guys,

I'm considering to lacquer the inside of my dg-255, what do you guys think, any advice on technique or a good lacquer?

Bob Holo

Profession? Well, as Gerry Garcia once said: "What a long strange trip it's been." My degrees are in Finance, Economics, Marketing & International studies... never could make up my mind and only spent a few years doing finance anyway (thank God I left, as I worked in the world trade center for a while there...) Then I started & ran & sold a coffee company while teaching myself computers, then worked for Intel as a product manager for a decade and taught myself to design loudspeakers, now I design loudspeakers and am teaching myself luthiery. It's all very confusing, but through it all I've been fairly obsessed with physics & acoustics.

So... what profession? Well... Loudspeaker designer & Luthier. They both are this wonderful mix of acoustics & physics & woodworking... and they're opposite sides of the same coin. Loudspeaker design is all about constraining vibration and Luthiery is all about letting it rage on. (except necks... which you don't want to let vibrate)

Ken Bloom

Thanks Bob. I'm always glad to hear someone who actually knows the science behind some of this stuff. I am a more intuitive builder but I find that knowing at least the seat-of-your-pants science behind some of this makes it easier to work with the variables. thanks so much!