Official Luthiers Forum!

I thought I asked this last week but Im not seeing it so Im sorry if this is a repeat. I am thinking about which wood to use for a bridge. I have EIR and BRW, also Ebony. The BRW sounds very musical but the EIR is quite dull and the ebony is just dead. I have looked at the physical characteristics of the rosewoods on a wood database and EIR and BRW have almost the same qualities. Based on what I hear BRW would be the best. What is it that makes for the difference? It seems that there is some quality different than is scientifically measured.

All three of those woods can work fine. Which would be "best" depends on the sound you want to achieve and the type of guitar you are putting it on. The weight of the finished bridge will have as great an effect on the sound as the type of wood it is made from.

Some have said that it matters more how to build the guitar than what it's made of. Things like carefully selected and voiced soundboard, braces that were split and chosen for maximum stiffness, etc. would make better sounding guitars. I tend to agree and I can't really say I can notice a difference between BRW/IRW/others assuming everything else is equal. I have never seen BRW before, but one customer did bring in a guitar that he claimed had BRW fingerboard, and honestly I thought it was made of cheap woods.

I will say this though: I do notice many vintage guitars use very light bridges, and they sound very good. I don't know if it's the old wood.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

That's probably quite an understatement! Most of the time, lighter is better.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

There are a number of measurable qualities of the wood that can contribute to the affects it will have on the sound of the guitar. All of them can vary within a given species; the numbers in the wood data basses are averages, and mask quite a wide range of variation. The only wasy to know what you have is to measure it somehow.

Density is one of the things that varies a lot both between species and within a species. Of the woods you have, ebony tends to be the densest, and Indian rosewood the least dense, but there's a lot of overlap, particularly between the rosewoods. Usually the denser wood will give a heavier bridge, of course. A heavy bridge tends to cut down on the sound output of the guitar at all frequencies, but has more of an effect on the high end: people usually say ebony bridges sound 'bassier'.

Stiffness can vary too. A stiff bridge will also cut down on sound overall, but usually more in the bass, so stiffer bridges tend to sound more 'treble', all else equal.

'Damping' is a measure of how fast the material dissipates the energy of vibration. BRW tends to have the lowest damping of the woods you have, so it 'rings' longer when tapped and gives a more 'musical' tone. Ebony tends to have the highest damping. Indian rosewood can be quite variable. Although damping in general tends to kill sound, and highs more than lows, it's a bit hard to say how the damping of the bridge material plays out. Some folks claim it's quite straightforward, but I'm not sure it's that clear. Does a guitar with an ebony bridge lack high end sound because the bridge has high damping, or because it's heavy? It's often difficult to isolate variables in this business.

The weight of EI and BRW are almost the same, 57 & 52. Other qualities are Specific gravity, Rupture, elasticity, crushing, shrinkage and hardness. I dont know what the scientific name of stiffness is but damping is not listed. I know of course that Ebony has higher damping. Based on almost the same readings for the above qualities between EI and BRW I believe that EI has more damping than BRW. Cocobolo is considered a musical wood but it is very much heavier than BRW so as is mentioned above it wouldnt be as good a bridge as BRW. Is that a correct assumption?

" I dont know what the scientific name of stiffness is but damping is not listed."

Stiffness is a _section_ property: it depends on the size and shape of the piece as well as the material. The relevant _material_ property is the 'Young's modulus', which is a measure of how much work it takes to stretch or compress a piece of a given size by a given amount in a given direction. Most wood has a much higher Young's modulus along the grain than across it, for example. Young's modulus is usually designated by the letter 'E'. Two parts of the same size made out of materials with the same E value will have the same stiffness.

Damping is not something that most structural engineers worry too much about, so you often don't get the measurement. It is measured in various ways, some of which are different enough to cause confusion even though they can be written down the same way. One measure, for example, is the 'Q value', which is a measure of the proportion of energy lost per cycle of vibration. Sometimes it's given in terms of 'cycles per second', and sometimes it's in terms of 'radians per second'. If it's the latter, there may or may not be a 'pi' lurking somewhere: they don't always put that in. One cycle per second equals 2*pi radians per second. Damping is also sometimes given as 'log decrement', which is 1/Q, by whichever method they're giving that, and tells you how many cycles (or 2pi radians) it takes for the signal to drop off by (sqrt 2)/2. Confused? I am.

For our purposes it's easy enough to hear any difference in damping that's likely to be significant. The measurement is simple enough in principle, and sometimes tricky in practice.

Hey thanks Alan,
I have been reading this forum for a long time and never noticed that reference. Anyway I did some reading on log decrement of wood samples and interesting enough I found some research that said EIR had higher damping than BRW. I guess we knew that but it was nice to see the proof. I found info on a lot of other woods too. One interesting species was Acacia which had higher damping than the rosewood sample they tested.

John (are you really called John Smith??), this paper may interest you. Lots of stuff on wood properties and their relevance to guitar building.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Yes, my real name is John Smith. I usually dont like my name being available to people I dont know but the good will of those participating in this forum makes me feel comfortable.

Thank you for the article. It seems to be the flip side of what is commonly believed about wood. That is, it is believed that wood of the same dimensions will sound different. Supporting that I can hear a difference between mahagany and rosewood but not between less common woods. I can hear a difference between sitka and cedar.You are saying wood of different dimensions can be made to sound the same by those that have the skill to do so. Rosewood can sound like mahogany and cedar like sitka.

That's very much the abridged version, of course, John. There's a few things that are very hard to account for, like the differing sound spectral absorption between species, but, on the whole, if you build taking the material properties properly into account you can construct guitars of different species that can be difficult to tell apart and almost impossible to tell the species in blind listening tests.

Try guessing the species for the back and sides of this guitar. (1.4Mbyte .mp3 download) Listen through decent speakers or headphones to give yourself a chance!

Some of you will already know the answer, of course, but I can tell you now, without prior knowledge, no one has picked the timbers used. So choosing between timbers for more specific components like bridges, bridge plates, etc. has a lot to do with design and selecting material properties, not just selecting species, because the within species variation in material properties and the cross species overlap is huge.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Your right I cant tell. But I can say that it doesnt sound like rosewood or mahogany and sitka. Of course if I was betting on this I would need to know the size of the guitar. As I am not your point is taken. Still since few of us will be able to purchase a guitar of yours we will increase our chances of a good sounding guitar by using BRW over ebony bridges etc and building to common dimensions.
Thank you again for the education.

No worries, John.

For those interested, that sound clip was from the guitar that can be seen here or here (bottom of the page).

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au