tap tuning

[wstacy]

hello all,
#I am working on my 3rd mandolin and since I used spruce in the first 2, I decided to try redwood for the top on my 3rd. #I had no problems at all tuning with my strobe tuner on the first 2, #But can't seemed to get the desired pitch from the tone bars on this one. A luther friend told me to use spruce for the tone bars,so I did. #Does anyone udersatand what I am trying to do here? any advice would be greatly apperciated!
# # # # # # # # # # # # # # #Thanks greatly, William Stacy

[joshags03]

I just finished one with a redwood top and yes you should use spruce tone bars. The spruce has better attributes and is more suitable for tone bar material than redwood. If I'm not mistaken, spruce is a stiffer species of wood allowing it to distribute vibrations a little better and provide more support to the top at the same time.

[Dave Cohen]

I get really tired of that stuff about "distributing the vibrations". Even in an unbraced plate, the vibrational mode shapes are global. That is, all of the mode shapes encompass the entire plate. To see holographic images of mandolin mode shapes, look in Catgut Acoustical Society Journal, Nov., 2000 (Cohen & Rossing), and Acoustical Science & Technology, Jan., 2003 (Cohen & Rossing). For that matter, even the modes shapes of a membrane, such as a drumhead or banjo skin, are global. You can see images of that for yourself in Fletcher & Rossing, "The Physics of Musical Instruments, 2nd Ed.", or Rossing, "The Science of Sound".

Braces do not "distribute vibrations". They do distribute the static load, but vibrational modes shapes are not static, and are largely determined by the geometry of the vibrating object, i.e., the shape of the instrument. What the braces do is selectively change the frequency of some of the modes. Which modes undergo the frequency change depends on the orientation of the braces, i.e., the bracing pattern.

Redwood has a slightly lower longitudinal Young's modulus (elastic modulus) on average than some of the spruces. That can be completely compensated for by changing the dimensions of the brace. Typically, if you make a brace from a softer material, you would want to make it taller and thinner in profile in order to end up with the same result as with a stiffer material. Having said that, you will find that some redwood samples are extremely hard and heavy, and yes, very stiff, while some redwood samples are very soft and light, and are less stiff. There are some advantages to using redwood braces with redwood plates. The redwood braces will have the same coefficient of thermal expansion as the redwood plate material. Same goes w/ expansion w/ regard to relative humidity.

[wstacy]

thanks for the replies guys,
sorry Mr. Cohen if I opend up an old can of worms there but Im still relatively new to building and theres alot I don't understand. All I know is the procedure I use to get the outcome of the air chambers on the spruce top mandolins is not working with the redwood. Thanks again. William

[sunburst]

As I know from talking with Dave Cohen, your air chamber frequency (Helmholtz) will be pretty much the same if you build a standard sized F5.

I think what you're trying to do with the tone bars is trying to tune the free plate resonance, or free plate modes.
Apparently, nobody has been able to scientifically track free plate modes into the finished instrument and find any particular correlation to plate modes or air modes in the finished instrument.

In short, while free plate modes might be a way to gauge stiffness relative to mass in the top and back, it doesn't really matter what the notes were in the finished instrument.

In other words, don't worry if you can't get the notes you want, write down everything you do for future reference, and go ahead and build the instrument. You're developing your intuition.
I quit trying to tune parts to specific notes a long time ago, but I still write down what they are, and I don't see any correlation to the sound of the finished instruments. Others have said the same, and I think it's too bad that beginning builders are led to believe that plate tuning and tone bar tuning are as important as they seem to believe.

A disclaimer:
If anyone reading this is an experienced builder and tunes plates and tonebars successfully and can observe good results, fine with me. I'm not saying anyone's building style is wrong and I'm not saying "my way's better", I'm just saying that it doesn't work for me, and it's not the only way to build.

[Dave Cohen]

WStacy, I didn't mean to attack you peronally, nor was an apology necessary on your part.

In academia, if a speaker or questioner blows smoke in a departmental seminar, at least someone in the audience will "go for the jugular" and try to embarass the speaker for his/her error. That is certainly not called for in a forum such as this. What I DO hope for is that if someone is going to invoke physics - and "vibrations" are certainly physics - they will at least do some homework rather than inventing some Star Trek physics without checking it out first. The fact of the matter is that it is not necessary to know ANY physics in order to build an excellent mandolin. I do the physics for the same reason that any scientist does science; I want to understand.

Since I have a captive audience here, I will make my plea. If you are at all interested in understanding what you are doing, and if you want to invoke "vibrations" in your arguments, please consider reading some of the scientific literature. Mandolins are surprisingly like guitars. The scientific literature on guitars goes back about 35 years now, maybe a bit more. If you don't know where to start, your friendly local physicist at a university or college near you may be able to point you to the most appropriate stacks in the library.

[Jim Roberts]

My "friendly local physicist at a (my) university..." #pointed me in the right direction and it wasn't the "stacks in the library." #He pointed me in the direction of the closest microbrewery where I met a fellow mandolinist and we have gotten together frequently ever since to jam and to "blow(s) smoke" without the fear of some acadamecian going for our jugular.

Good luck, Mr. Stacy, on your journey toward building the perfect mandolin. #Perhaps you'll treat us with some pictures of your work someday here on the Cafe

Gotta go, Star Trek comes on in ten minutes and then it's off to work on President Garfield's Hornpipe in the key of B flat as arranged by John Reischman! #

Cheers and pray for peace (and good vibrations).

Well said Jim. I have talked to many of physicists and Doctors on the subject. In fact I have studied a little bit myself. Theories are theories and everyone can build the way they choose. Few people get it right and the rest just ride on reputation. Mandolin building is just as much art as science. The curves have just as much importance as the thickness. You may get the plate to vibrate but if the curves not right you won't get the punch. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/wow.gif

[Michael Lewis]

In general I try to use the same type of wood for braces as the top is made from. It just seemed the right thing to use.

Dave, when you were testing the mandolins with the laser appratus did you excite the structure by striking the strings or with an independent energy source like a tone generator and speaker?

[Dave Cohen]

Michael, we excite the mandolin body by temporarily placing a small magnet on the top of the bridge or on the top plate w/ a little dental wax, positioning a coil close to the magnet, and driving the coil with a single frequency excitation from a signal generator. There are a few problems with trying to excite the mandolin body with a string. First, the string vibrates at many frequencies at once (1st harmonic, 2nd, 3rd, etc.), which will also excite many of the plates modes at once. If you want to get pictures of the mode shapes, you need to excite one mode at a time. Another problem with plucking the string is that your hand or a mechanical plucker must be in front of the instrument, so you will get a hologram of your hand instead of a hologram of the instrument plate. Still another problem is that vibrating strings may steal energy from the plates if they vibrate at nearby frequencies, thereby splitting the plate modes and adding to the confusion. So we usually damp the strings by stuffing a small piece of foam between the strings and the fingerboard.

Otoh, we also record sound spectra and "playing spectra" by playing the instrument into a mic which in turn is fed to fourier analyzer (or to a computer w/ fft software). In those experiments, the idea is to excite everything at once, and to record it vs frequency as a power spectrum. That gives a kind of "fingerprint" of the instrument. The bigger problem is figuring out what it means.

To some others, please note that I already issued a disclaimer above that you don't need physics to build great instruments. On internet forums such as this, people can say anything they want to say, and often do. Of course, I can also express my opinions. Insisting on the right to tell people that braces distribute vibrations is pretty much like insisting on the right to tell people that the Earth is flat. You can do that if you like. I suppose that there are still a few flat earthers around.

On "theories". The only "theory" used in the understanding of stringed instruments is the classical mechanics of Newton, Hamilton, & LaGrange. Theories such as classical mechanics are, like all scientific constructs, not absolute. But do me a favor and try to find an exception to one of them. All of the data which I have published is every bit as empirical as is the craft of lutherie. That is to say, it is not "theory" at all, but rather is observation which has been subjected to peer review. The same is true for all of musical acoustics.

To return this thread to its original direction of plate tuning, consider the following: Saunders and Hutchins, the originators of free plate tuning, acknowledged early on that they very often couldn't tune their free violin plates to a particular frequency. It just didn't work. What they did instead with their "glitter patterns" was to attempt to establish a certain frequency relationship between two of the free plate modes, and they were usually able to do that. Knowledge of that would save a lot of beginning builders a lot of anguish, and the source of that information is, you guessed it, the literature. Reading is good stuff, folks, Star Trek notwithstanding.

[wstacy]

thanks again y'all for all your replies. I really apreciate people like you dave, I just wish someone like you would do a book on building for beginners and give us alternatives to people like simminoff,who teaches you tap tuning or voicing.This thread has really helped me alot,thanks again
# # # # # # # # # # # # # # # # # # # # # #William

[Michael Lewis]

Thanks Dave. Applying science is really a lot of work, but fascinating none the less. There are so many aspects of tone production that need to be looked into and understood, but days are so short that there just aren't enough hours to make instruments and sluth out the secrets of tone control. At least in my shop. I feel that maybe I am blindly slogging along only THINKING that I can see. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif

[oldwave maker]

Thanks to great deals on redwood from Spruce, Ive made 2 redwood topped ovalhole mandos, 2 redwood topped f hole mandos, and 1 redwood topped ovalhole mandola in the last year or so, all with fresh hot hide glued aged adirondack bracing. I think the flamed redwood works for me in carvedtops better than plain or mottled, and redwood certainly works better for me in ovalholes than f holes. The ovals seem balanced and lively, the f holes seem to lack mids, the dola is a keeper. That said, I know there are some fine redwood F5s out there by other makers. I know this is a small experimental sample, but I've worked more with intuition, flex, and a knucle bonk on the last 300 or so, rather than looking for a specific scope frequency. Its hard to make a lousy sounding mando out of really great tonewood, hard to know whats great tonewood till you make it. some help I is.....
Look for Dave Firestine at winfield with his koa/flame redwood oval a, one of my favorites.
bill
old wave- when 'good enough' costs more than a mexican liver transplant!

[Chris Baird]

When carving the braces I tap and listen to the quality of the tone. #As most who have built a mando and tapped along the way know, when you first glue on the unshaped braces the top taps like cardboard. #As you carve them down the tone gains more clarity and sustain. #If you carve too far the clarity and sustain go away again. #There is a point, and it is hard to know when, that the braces are the right thickness to make the free top more "musical". # I've done some FFT on the taps taken at various points in the brace shaping and the more bell like the tap tone the more varied and strong the partials are to the fundamental. #It gets very complicated if you try to be scientific and the variables soon overwhelm. #Luckily for most of us whom are limited in mathematical ability and inclination we can use our ear to the same end. I believe it has helped my mandolins to sound better using the tap quality as an indicator rather than specific frequencies.