Precise Measurement Of Body Resonance

Lee Baucum · Post by **Lee Baucum** » 14 Oct 2002 7:30 am

Not too long ago, Karlis Abolins performed some very precise measurements with regard to the various issues surrounding "cabinet drop".
http://steelguitarforum.com/Forum5/HTML/004390.html

Has anyone performed such a study on the way a steel guitar body resonates when a string is plucked? There has been so much discussion over the years about push/pull tone versus all pull tone. The theory (among many) seems to be that a push/pull changer tends to transfer more of the strings' vibrations directly to the guitar's body, which (in theory) translates to "better" tone. Some will claim that with an all pull guitar, too much of the strings' vibrations are transferred to the end plates, where they are then transferred to the legs of the guitar and on down to the floor. I would think that if the end plates were securely attached to the body of the guitar, the vibrations would be transferred to the body, as well as the legs. (What do I know? I'm a banker!) I've heard the "leg vibration" theory used as both a pro and a con. One person says to strum a guitar and feel the legs. If you can feel the vibrations in the legs, it's a good sounding guitar. Another person says just the opposite.

I was wondering if someone could use some sort of precision measuring device, perhaps coupled to a piezo pickup, to measure just how much sound is being transferred to various parts of the steel guitar. Measurements should be made on the body and neck, as well as all of the "hardware" that is attached to the body. These measurements should be performed on several brands of guitars.

Something tells me that some of the steel guitar builders have done this, but have not published their findings.

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Lee, from South Texas

Donny Hinson · Post by **Donny Hinson** » 14 Oct 2002 7:51 am

A real scientific audio analysis could be done, either with piezos (as you mentiomed), or with precision laser shearography. It would be expensive, though, and would only relate to that specific guitar. The cost of doing this, even for something produced in limited quantities, such as the new MSA, would probably be prohibitive.

And to what purpose? In the final analysis, it's the players that determine what sounds "good", and all the scientific evidence in the world wouldn't change most people's minds. You see, we steelers are a bullheaded bunch, myself included.

Everett Cox · Post by **Everett Cox** » 14 Oct 2002 9:54 am

On the other hand, such an analysis would be very interesting and, perhaps, provide a 'benchmark' useful to many of us who attempt to build instruments. I had been contemplating something less hi-tech, myself. Like borrowing my daughter's stethoscope and trying to record from various places on an instrument into an audio wave analyser. It would seem particularly helpful in the design of non-pedal steels have an idea of the effect body shape/material has on the end tone. -Everett

patrick donovan · Post by **patrick donovan** » 14 Oct 2002 12:51 pm

Here's a radical idea...plug them into an amp and listen. If it sounds good it's a good guitar! Too many musicians listen with their eyes rather than their ears.

Regards, Patrick

David Doggett · Post by **David Doggett** » 14 Oct 2002 12:56 pm

This question of "body resonance" puzzles me too. In the physics of acoustic stringed instruments, the vibrations of the strings are amplified by the resonation of the top of the sound box. But there is a trade-off between string sustain and resonator volume. At the extremes of this spectrum of sustain-volume tradeoff are a banjo, which is the loudest acoustic string instrument, but which has no sustain (it's just a loud pluck sound, right?), and a solid-body electric guitar, which has the ultimate sustain, but has no acoustic amplification. Acoustic guitars and resonator guitars are near the middle of the spectrum, with some of both volume and sustain. The volume gained by the resonance takes energy from the strings and shortens the sustain. Of course, a mushy top without optimum stiffness can also drain off string vibration energy, but without increasing resonance and volume.

Now, according to this simple volume-sustain tradeoff, the ideal body for an electric instrument that needs no acoustic volume, but needs all available sustain (i.e., a steel guitar) would be something very hard and rigid, like a block of granite, or metal (if it weren't for the problem of thermal expansion). Very hard wood, or synthetic materials like plastic, or carbon fiber, also work well. But to the extent that any of these bodies are so thin that they resonate, that would seem to be robbing some sustain from the strings. Whatever acoustic volume is gained by this resonance is irrelevant, as all of the useful volume comes from the magnetic pickups, which only react to the metal vibrations of the strings, not to any body resonance. Any endplate or leg resonance would also seem to be undesirable.

So according to this logic, the nut and bridge (changer) of a steel guitar should be anchored in a body so hard, rigid and massive, that there is no resonance draining off string sustain. Softness or inconsistencies in the body material would seem to drain off sustain energy without providing any resonance (the mushiness factor). Also, interfaces between changer parts (i.e., fewer interfaces in the push-pull system) and/or the body (i.e., anchoring to the neck rather than directly to the body) may also drain off sustain energy without providing resonance. Therefore, the fact that one feels resonance in a body (or legs, or whatever) at least indicates there is little mushiness factor, which probably is why some builders point to it with pride. But that resonance itself would seem to be robbing some sustain energy, and would seem to indicate that materials are too thin and light-weight.

Does this reasoning make sense? Does anyone have any real experimental data on this?

Bobbe Seymour · Post by **Bobbe Seymour** » 14 Oct 2002 2:54 pm

All this goes back to "backfeeding" the string, period. A swinging string needs loops and lodes along the string (over tones)to create the pleasant, warm, sweet, richness that we all call "good tone" . Any method used to aquire this effect is acceptable.
I used a "Voice-Print" machine , borrowed from a police station once,a few years ago to "see" what tone really looked like! I was supprised! We looked at a MSA, Emmons P-P, a ZB and a Sho-Bud Pro II. I know now what to look for in finding a good tone. Or better put, what to hear for!
Bobbe [This message was edited by BobbeSeymour on 14 October 2002 at 03:55 PM.]

David Doggett · Post by **David Doggett** » 14 Oct 2002 6:59 pm

So Bobbe, are you saying that the right kind of body resonance feeds back to the strings to give desirable overtones? What about leg resonance? And in your sample lineup of guitars, which was the model for "good tone" and which for not so good tone? And after that experiment, what is it you "hear for?"

Wayne Cox · Post by **Wayne Cox** » 15 Oct 2002 1:08 pm

Re:Patrick Donovan,
Patrick,here's another radical concept I've used for many years. DO NOT plug it in.
Find a quiet room or quiet place and just strum across the strings a couple of times w/a thumbpick. Then,place your bar on the strings and do the same thing. It might sound crazy but it actually eliminates many of the variables such as pickups,amps,chords,effects,etc. If it sounds good unplugged,it can be made to sound good with the right pickups and amplifier! I bought the best sounding steel I have ever owned,using this method. I bought it without ever hearing it through an amp! After hearing the basic cabinet resonance, I knew it would sound good with most any decent pickup or amp. Perhaps this is not the most scientific method but it works for me.~~Now back to the original posted query.~~During the middle 1970's I was visiting the MSA factory in Dallas. While there, I was introduced to two gentlemen from Holland who were using some sort of graphical analyses of sound waves & vibrations as they pertained to acoustic guitars. They were there to offer (or sell) their services to MSA. Whether MSA followed through with this or not,I simply do not know.

W.C.

Wayne Cox · Post by **Wayne Cox** » 15 Oct 2002 1:18 pm

Re:B.Seymore,
Bobbe, did you happen to speak into that "voice print" machine? What kind of picture did it print? Did it resemble an amplifier or a steel guitar?

he,he, I steel love you man! You are still the most delightfully sarcastic person I know!
W.C.

David Doggett · Post by **David Doggett** » 15 Oct 2002 3:07 pm

Okay, forget those last two posts. I didn't know what I was talking about. Here's what I really meant to say. Granted that in acoustic string instruments the resonance in the body contributes overtones that give the sound its texture (what we call tone, as opposed to the pitch or volume). But in a solid body instrument with magnetic electric pickups (not transducers, piezo-electric, etc.), does body resonance really do anything except take away some sustain and overtones? When people speak of softer woods giving darker tone, and harder wood and metal giving brighter tone, doesn't that just mean the softer wood was absorbing some of the high string overtones, and the hard wood and metal weren't. In other words, something can be taken away, but not really added. What did your voice profile instrument say about that Bobbe, anybody?

Doug Earnest · Post by **Doug Earnest** » 18 Oct 2002 5:00 am

Here's my simple Ozarks take on it --

A pickup makes a sound because of the movement of the string in the magnetic field. If the string were stationary and you moved the pickup fast enough that would also make a sound. So, a pickup mounted on a body that has resonance is going to add to the sound of the vibrating string (backfeeding, as Bobbe said).

It stands to reason to me that, up to a certain point, a body with lots of resonance will produce a more rich tone than a body with little resonance.

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Doug Earnest
The only Zum Keyless U12, Fender Cyber Twin

Sage · Post by **Sage** » 18 Oct 2002 7:47 am

A great thread, all. First, I'll get the known out of the way. If it sounds good, it is good. What we hear as good sound is made (mostly) by the hands of the player. That said, we will always strive to have or make the best instrument possible.
As folks above have described, it is the particular overtone "signature" of the resonant body which feeds back the energy of the string after modifying it in it's own way. This is what makes the same note played sound different on a piano, banjo, mbira, or steel guitar. You are right, David about the relationship between stiffness and acoustic output (impedence). When you move air, it requires energy which results in damping, reducing sustain.
Acoustic guitars take advantage of natural resonant frequencies to amplify the string sound, and to color it. Resonance relies on the relationship of stiffness and damping, which are different functions. You can have high stiffness with low damping or high stiffness with high damping. If there is minimal acoustic coupling, the sustain will be similar (but I think that low damping results in more energy feedback and sustain). The difference is in the overtone profile, the effect of resonant energy that gets fed back into the string (or not). Notice: I'm going to toot my own horn here a little. If you have a piece of railroad track or granite with strings mounted on top (or foam core carbon fiber), you will have high stiffness and high damping. The overtone profiles will be very flat. That doesn't mean that it is bad, it is just neutral. In my guitar body, which is very stiff, light, and well coupled, the resonance is very high because the damping is very low. The difference is that it resonates above the range of most of the played notes (around 1300 hz and above). This gives it a lot of "life" IMHO, without it barking out at one spot or going dead in another like solid body instruments which resonate in the playing range do. As to the question of meaurement, it is a simple thing to do and we do it. A good mike into a computer with a spectrum analyzer and "tap" tests will tell you about the body. As to the rest of the steel, the changer interaction, legs, etc, it is too hard for me to figure out how to do a proper control test. Anyway, it is fun to do.
T. Sage Harmos
Harmos Steel Guitars [URL=http://www.harmosmusic.com][This message was edited by Sage on 18 October 2002 at 08:55 AM.]