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Bridge wood and various pitches
https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=37650		 Page 1 of 1
Author:  Steve Frady [ Wed Sep 05, 2012 8:33 pm ]
Post subject:  Bridge wood and various pitches
I was making some bridges last night and began tapping on them with another piece of wood. Indian Rosewood was the lowest pitched. Padauk was the highest and Osage Orange was in the middle. That was the extent of my little experiment. I would like to hear some opinions or even measured results from the type of bridge wood used in guitar building. I am interested to know if some builders choose the bridge wood to enhance a desired tone response. I was thinking that in order to support some extra bass on a parlor guitar, that the lower pitched wood might help.

Steve
Author:  Trevor Gore [ Wed Sep 05, 2012 9:56 pm ]
Post subject:  Re: Bridge wood and various pitches
I've used all sorts of wood to make bridges from, many also CF reinforced. It's very difficult to hear any difference related specifically to species, but very easy to hear differences related to bridge mass. I prefer low mass bridges, which leads one to low density woods, but the bridge still needs to be hard enough and strong enough to support a saddle and not be sliced up by the strings.

If you want bass out of a parlour guitar, keep the T(1,1)1 and the T(1,1)2 as low as you can and its monopole mobility as high as you can, without the thing falling apart.
Author:  James Ringelspaugh [ Thu Sep 06, 2012 12:07 am ]
Post subject:  Re: Bridge wood and various pitches
Lower mass = more responsive, generally louder, less headroom, usually considered better for fingerstyle. Higher mass = slower attack/decay, more sustain, mellower tone, can dig in a lot more with a flat pick.
Author:  Eric Reid [ Thu Sep 06, 2012 2:15 am ]
Post subject:  Re: Bridge wood and various pitches
Damping? Nothing scientific here, but I pay attention to the sound of a bridge dropped on iron. Torres used Brazilian bridges on many of his cheapest, as well as his most expensive guitars. When I worked for a production shop, I watched classical guitars with Pau Ferro bridges (clunghk) sit on the rack, while the same model with Brazilian bridges (klinnggg!) marched out the door. I made a belly bridge for a 50's Martin out of very black, very resonant Brazilian...Ebony to a casual glance. Everyone who played that guitar wondered if it was for sale. I still wish it was.
Author:  miramadar [ Thu Sep 06, 2012 10:00 am ]
Post subject:  Re: Bridge wood and various pitches
Quote:
If you want bass out of a parlour guitar, keep the T(1,1)1 and the T(1,1)2 as low as you can and its monopole mobility as high as you can, without the thing falling apart.


What does T1 and T2 mean? Can you explain this to me??
Author:  jfmckenna [ Thu Sep 06, 2012 10:18 am ]
Post subject:  Re: Bridge wood and various pitches
YEah Trevor what is this formula you speak of?

I think if the bridge pings well then it's good to go. Not so sure it matters what note the ping is. I've used all kinds too. One of my best classical guitars ever has an ebony bridge, apparently a big no no in the world of luthierie.
Author:  John A [ Thu Sep 06, 2012 1:31 pm ]
Post subject:  Re: Bridge wood and various pitches
Quote:
keep the T(1,1)1 and the T(1,1)2 as low as you can and its monopole mobility as high as you can, without the thing falling apart


Information is most useful when written in a language or terminology which is easily understood, or a definition key provided.


For bridges, I try to stick to a preferred weight - on classical that is 15-18 grams. My last bridge was walnut and sounds pretty good - my next bridge will be EI Rosewood - and I plan to keep it around the same weight. I can;t speak for steel strings (yet).
Author:  mqbernardo [ Thu Sep 06, 2012 5:07 pm ]
Post subject:  Re: Bridge wood and various pitches
I know i´m not Trevor, but out of my head:

T(1,1)1 corresponds to the lower air mode (also called "Helmholtz" resonance), generically around 100 Hz.
T(1,1)2 is the top monopole, very generically around 180-190Hz.

IIRC, that´s the terminology he uses in his (excellent) book.

cheers,
miguel.
Author:  Trevor Gore [ Thu Sep 06, 2012 6:12 pm ]
Post subject:  Re: Bridge wood and various pitches
miramadar wrote:
What does T1 and T2 mean? Can you explain this to me??

It's one of a number of nomenclatures for naming the modes of vibration of musical instruments. It's a nomenclature that's been around for probably at least 25 years and is a bit more descriptive than some of the other methods. For example, a system in use in the violin world just assigns a number to the modes in the frequency order in which they appear. Unfortunately, the order in which specific modes appear isn't always the same on guitars.

The top modes start with a T, the back modes with a B; then you count the number of anti-nodes across then down, so a T(1,1) is a top monopole and a T(2,1) is a cross dipole. The T(1,1)1 is the first (in frequency order) of the monopole modes the T(1,1)2 the second. Otherwise, as Miguel said.

The resonant frequencies of the various modes of vibration, their relative amplitudes and the width of the peaks (Q) essentially define the sound of a guitar (or the sound of any other vibrating body, for that matter). So manipulating those modal parameters is how you alter (design) the sound of your guitar.
Author:  Alan Carruth [ Fri Sep 07, 2012 12:35 pm ]
Post subject:  Re: Bridge wood and various pitches
Assuming you've made all of your bridges to a standard size, the pitch of the main resonance is telling you something about the ratio of stiffness to weight of the bridge. A wood with higher density and low stiffness will give a low tap pitch, and low density and high stiffness goes the other way. Since this pitch is determined by a _ratio_, it does not say anything much about the absolute stiffness or mass, but, of course, if you know what all of the bridges weigh and have the tap pitches you can figure out the ranking there, too.

The bridge does two things on the guitar: it tells the string how long it is, and it's also the heaviest, and one of the stiffest, braces on the top, so it has an effect on the way the top vibrates. Since these two functions work at cross purposes to some extent, you're looking more at a balance of things than at some absolute definition. You want the bridge to be massive enough and stiff enough to define the string length pretty well, so you don't end up with 'wolf' notes, for example. However, the heavier and stiffer the bridge is, the more it's going to restrict the top movement, and particularly the 'monopole' modes.

(The nomenclature that Trevor uses is pretty common, and is often used even in the violin world when talking about some modes of the assembled instrument. Since fiddles are pretty standardized, they do normally use numerical designations, as 'mode #1' and 'mode #2, for the _free_ plate modes, which usually occur in the same order. Guitars are more variable that way.

What Trevor calls the T(1,1)1 is often referred to as the 'main air' resonance, and the T(1,1)2 is the so-called 'main top' mode. These are actually the two halves of a 'bass reflex couple', where the 'Helmholtz' air resonance and the top 'monopole' mode interact strongly. The result is two resonances that _both_ involve air motion in and out of the soundhole [like the Helmholtz' mode], and the entire lower bout of the top moving like a loudspeaker cone and producing sound [the isolated T(1,1)]. In what we term the 'main air' resonance most of the energy in the mode is tied up in air motion, and the top and air are 'out of phase', so the top is moving inward as the air is coming out of the soundhole. In the 'main top' mode, the situation is reversed: most of the energy is in the top and the air and top are 'in phase', both moving outward at the same time. Due to the 'phase cancellation' the main air mode tends to produce less output in front of the guitar.

All of this can get confusing. The rule with resonances is that you can get two or more resonant mode shapes at a given frequency in a simple system, but you'll never see the same mode at two different frequencies unless there are two different oscillators involved that are coupling. Using the top mode shape to talk about both parts of the bass reflex couple is accurate enough, but might obscure the fact that there are two potentially independent vibrating systems working. I think it helps to keep that in mind.

If you think that's confusing take a peek at the violin world sometimes. Between the somewhat more complicated structure, and the way the terminology has evolved, they have all sorts of terms for modes that are hard to see, and may or may not affect the timbre; things like the 'CBR' [corner block resonance], and the' B1+'and 'B1-' modes. There's a lot of discussion about just what the relationships between these mode does to the sound, and not a lot of agreement. We're a lot better off that way.)

Anyway, since the top 'monopole' mode bends a lot in the center, a stiff bridge can restrict that motion, and cost a lot of sound. Similarly, since the top moves the most in the middle for that mode, too much mass can cut down on power as well. You need enough combined mass and stiffness to get the end of string to be more or less stationary, so that it will make the right note, but either high mass with low stiffness, or high stiffness with low mass, can potentially work well.

A light, stiff, structure will, of course, tend to have relatively high resonant pitches, and guitars with light,stiff tops tend to be more 'treble balanced'. I'ts more complicated than that, though: since the 'bass reflex' action is in the low range of the guitar making a light, mobile top can give very high output in the bass range, often as 'punch' in the attack. This is part of the characteristic sound of a Dread with scalloped braces. The dense ebony bridges they often use make a lot of sense there, using mass rather than stiffness to 'nail down' the end of the string, and lowering the resonant pitch too. Some of the 'lattice' and 'sandwich' top guitars have a sound to my ear that speaks of a light and stiff structure.

This all can get very complicated, and tends especially to do so on good guitars. Ultimately, each design exists as a system where all the parts have been balanced to get a desired result. On my own guitars, which tend to be fairly, but not totally, 'traditional', I've found that using bridges that are as light as Trevor likes tends to give a sound that is too 'sharp' and 'trebly'. I have not noted any particular trend in the sound related to the damping factor of the bridge wood, as some have. No doubt altering one or another property of the bridge will have a predictable effect on the tone that will be similar from one design to another: mare mass> less power and a larger drop in the treble range than the bass, for example. However, this is likely to sort out differently for different designs, and you need to work out how it's all going to work for _your_ system.
Author:  Darryl Young [ Fri Sep 07, 2012 4:42 pm ]
Post subject:  Re: Bridge wood and various pitches
Alan Carruth wrote:
A light, stiff, structure will, of course, tend to have relatively high resonant pitches, and guitars with light,stiff tops tend to be more 'treble balanced'. I'ts more complicated than that, though: since the 'bass reflex' action is in the low range of the guitar making a light, mobile top can give very high output in the bass range, often as 'punch' in the attack. This is part of the characteristic sound of a Dread with scalloped braces. The dense ebony bridges they often use make a lot of sense there, using mass rather than stiffness to 'nail down' the end of the string, and lowering the resonant pitch too.


I understand the part about Ebony being heavy.......but isn't it also stiff? Are you saying the top isn't stiff and the Ebony bridge (that is heavy and stiff) has a moderate stiffness to weight ratio........and the system works since the top is mobile and the bridge is heavy?
Author:  Alan Carruth [ Sat Sep 08, 2012 5:27 pm ]
Post subject:  Re: Bridge wood and various pitches
Ebony is denser than, say BRW, but the Young's modulus is not higher, I don't think. It's easy to tell this sort of stuff if you start out with billets of the same size and check the tap tones. In my experience, ebony bridge tend to drop the pitch of the main top mode more than rosewood ones.
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