Official Luthiers Forum!

Hank, Maybe we should try using "Jets" instead of ports. We could tune the pilot jet to control the low speed errr low frequency bass circuit, use an adjustable needle to control the mids and then a main jet to control that almighty ham fisted flat picker.    I just can't figure out where to put that darn air screw.[/QUOTE]

Aw, just put an old SU carburetor on it. That ought to cause no end of "tuning maladies"!

_________________
Hank Mauel

[QUOTE=Hank Mauel]
......Aw, just put an old SU carburetor on it. That ought to cause no end of "tuning maladies"! [/QUOTE]

SUs?!?! What a nightmare!! "tuning maladies" wouldn't even begin to describe the pain. How about triples??

Yer really dating yourself, Hank.

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

Had the twin SUs on my hot rodded '72 B, Huffaker gas shocks, blueprinted engine... Got it up to 115 one day on a back road in Sonoma County, and that scared the s... out of me.   The car started to lift and I couldn't feel the road anymore through the steering wheel.   Next step would have been an air dam, but that was just going too far.

Al,

   As i said earlier, Fred always had something new to show me anytime
we met...and it was usually something so far from anyhting that I'd ever
thought to try that it would seem almost radical to me.

   Once, he had with him about a dozen tops of different woods braced in
different ways. Some were fan braced and others glued up with
complexed and really intricate lattice patterns.

   He was working on a system that would enable him to switch tops on a
guitar and compare them with reasonable efficiency. I never did get to
see if he'd completed it, but at a year later, he still hadn't.

   It's very true that Fred could pinpoint a shortcoming in another builder's
work, but we always had great conversations as we threw them around. I
was happy to have him let me know that something that I'd suggested
had worked for him and became part of his regular practice on a few
memorable occasions.

   I'm sure he's deeply missed by all who had opportunity to meet him,
but not nearly enough builders were able to be exposed to his freindly
and thorough way of making great points.

    He was on the same search that we all are and had the resources and
motivation to try lots of things as he traveled that road to a better guitar.

    Science and intuition were both embraced by Fred and we need to
follow that example as well.

Thanks,
Kevin Gallagher/Omega Guitars

Kevin, are you coming o Healdsburg?
lance

Lance,
   I was hoping to be at Healdsburg just as a visitor or spectator, but with
my year off we won't be able to make the trip. I'll most likely not do a
show in the near future as a vendor or exhibitor. I'm just going to focus
on chipping away at the orders standing in my shop. Once they're all
complete, I'm planning some big changes in my business and approach to
it.

   I look forward to being at a show soon to be able to see and play
guitars from all the guys that inspire me.....oh, and to meet the builders,
too. I'd like to be able to sit down and hear some of your thoughts on
building and tone.

   I'm still an old school guy and am really challenged and inspired by all
of the cool innovations alot of builders are introducing.

All the best,
Kevin Gallagher/Omega Guitars



   

Rick,

   I was actually surprized to see you lend more credibility to a bass reflex
enclosure as a Helmholtz resonator or oscillator than an acoustic
suspension enclosure in your post above.

   Since Helmholtz technology typically calls for the activation of the
contained air to be created by activity at the opening in order to resonate
the enclosure while still offering a limited or restricted exhaust capability,
the acoustic suspension enclosure presents the more readable
environment. With a known resistence provide by a consistently
manufactured speaker and its predetermined coil throw or travel, the
amount of compression and its effects can be much more closely and
accurately read and documented. No exhaust is about as predictable as it
can get and it limits the movement completely to the compressibility of
the air in the enclosure.

   The guitar falls far from the typical activation criteria as does the bass
reflex enclosure, simply because the activation of the contained air
comes from a source away from the opening and does not use the air at
the opening to create the subsequent compression of the air behind it
and insde the body. in this application, the soundhole becomes nothing
more than an exhaust and intake channel so the action and result are far
from the those of a true Helmholtz resonator. That's the reason that I'd
abandoned applying the technology to my instruments...or at least in its
fundamental method...a while ago. I still use EAW's Smaart acoustic doc
program and a few others to make superficial frequency responce,
findamental resonance and balance notes for my own benefit so I've not
gone completely in another direction.

   While working with several engineers at both Lehigh University and Bell
Labs, they had resonators that would generate base data for them to refer
to and to calibrate instruments with and they usually in the shape of a
sphere. Temperature and relative humidity were very closely monitored
and tightly controlled in their lab situations to eliminate ambient
interference and effects. Many of their regularly used resonators even had
small plugs in the openngs. The weight resistence and the tension and
action of the spring that he;d them in place were closely documented to
prvide for as many contants as possible. I know that the lab environment
falls on a path for from our applications in the real world, but it's
fascinating to see the measure they will take to ensure accuracy for
reference and data sake.

    I appreciate your work with the Dead and the prestigious list of folks
you've worked with and have been a fan of you and your work for a long
time, but i've worked with some of the finest acoustic engineers in the
northeast over the years and have found that they agree that it is a bit of
a stretch to include a guitar body, with its minimally restrictive exhaust
capabilities as a Helmholts resonator.

    It's all really impressive stuff to be able to discuss and the engineers
that i know can just numb your brain with data...even us math nuts...and
all of the variations of the technology's application and were always
interested in having me bring different guitars in for them to shake and
vibrate with tones because it was so far from anything else that they gt to
work with.

   I'm going to have to grab the ear of a few new guys in New Jersy who
are on the cutting edge of acoustic technology to have them set some
tones to guitars to see what they think. Things change as the personel
does and i'm always up to learning. New ideas, new equipment and new
discoveries keep it all fresh for us. Bell has a state of the art research lab
about 45 minutes from me that a close friend oversees so I should be
able to slip there pretty easily. Time for some fresh schoolin'.

    Sorry for another long post, but a few of these have me thinking about
ways to further my insight into my own instruments and those of other
builders. We've been thinking about opening a guitar building school
much like that of Charles Fox here in the Northeast and it will be great
stuff to touch on to have experts come int to do so when we do.

Thanks,
Kevin Gallagher/Omega Guitars

Kevin (and others), please check this out:

http://www.phys.unsw.edu.au/jw/Helmholtz.html

Also, typical acoustic suspension loudspeakers are made so the chamber is as non-resonant as possible with fiberglass or wool or horsehair stuffing.   I see the air in an acoustic suspension cabinet as being a very highly damped spring, much like having great shock absorbers on a car. I just do not see the analogy of a Helmholz resonator with sealed box speaker enclosures. Now if you put a port in it and tune it right, what you have is a Helmholz resonator that is helping to control the impedance curve of the loudspeaker itself by harnessing the back wave and creating two lower Q resonant peaks than a simple infinite baffle would exhibit. The lower of these peaks is higher in amplitude than the speaker would otherwise put out without it.   

So I'm not necessarily advocating big time for the guitar as a Helmholz resonator...unless you put a tornavoz in it.

However I'm also not going for the idea that a acoustic suspension loudspeaker is a Helmholz resonator. There is no entrapped air mass connecting inside to outside, and I don't see how you can call a highly controlled electrically driven speaker cone that mass.

'Pure' Helmholtz resonators are rigid, with spherical walls, no damping to speak of, and a single opening. nothing we work with approaches that very closely.

All the guitars I've measured certainly have at least one 'Helmholtz-type' resonance, with air flow through the soundhole and pressure change inside the body that is in phase everywhere.

So far, the closest thing I've seen in a 'guitar' to a 'pure' bass reflex enclosure is an Ovation, although I guess the Smallmans can come pretty close. Obviously, with soundboards that are dynamically much more complex in the low range than speaker cones things get out of hand pretty quickly.

Still, as a way to begin to make sense of these complicated beasts the 'bass reflex' cabinet is a good start. Most of the modeling work I've seen, from Fred Dicken's equivalent circuits to Wright's thesis and the FEM studies of Elejabarietta et al, starts from there.

Whatever it is, the guitar is not a 'pure' anything, and I don't think it serves much purpose to insist that it must be in order to use the terminology, so long as we understand the limitations.

Ricardo asked:
"Any thoughts on the affect of two holes in the upper but instead of a traditional center hole, on sound? "

Sorry I missed that one for so long.

I built one like that some years back: we called it 'Casper the Friendly Ghost', since the bridge looked a little like a smile...

One thing I found was that the 'crosswise sloshing' air mode in the upper bout tended to radiate pretty strongly, since the two holes on mine were near the edges. The radiation was out of phase, of course, and thus tended not to amount to a lot of sound right out in front. However, it could 'beam' in certain directions, one being toward the player. Any note that had a strong overtone at that pitch (around 700 Hz, iirc) sounded loud to the player, and you tended to back off on those notes. This made the tone 'uneven' for the folks out front. I imagine that putting the holes right up next to the fingerboard, as has more commonly been done, would have less of that problem. I did not feel that there was any huge advantage to the 'extra' soundboard area, but, of course, that was just one guitar.   

Al, this probably deserves a separate thread, but I have found that there is a lot to be said for "extra" vibrating area of the top the way I'm getting it with the cantilevered fingerboard.   What I'm hearing is more harmonic sustain than I experience with more normally built guitars. There's a kind of woofer/tweeter relationship going on below and above the normally located soundhole that is very interesting.

I have the guitar that I made for Henry Kaiser in 2001 back in hand; Henry is loaning to me to take up to Healdsburg, and I can only hope that all my guitars turn out like this in six or seven years. It's doing everything in the tone department that I had hoped it would. This one is a pre-side port guitar, and the other reason I have it back is to drill a couple of holes into it, one on either side of the waist as per HK's wishes. I'm going to do that after H'burg.

Thanks Rick. That works for me. Fred Dickens referred to the acouctic
suspension system a few times when we spoke, but also mentioned the
bass reflex rechnology in much the same context as you had. His
reference to the suspension enclosures was simply that they provided a
clear indication of compression and its extent because of the accurate
and consistent function of the speaker and the known behavior or its
materials.

All I had ever seen as Helmholtz resonators were those in the labs at Bell
and Lehigh that Al described above. Spherical containers of glass and
metal with a single small opening. They resembled a Christmas tree
ornament with no hook.

Thanks for the info ad your patience.

Regards,
Kevin Gallagher/Omega Guitars

Kevin, I'm just going for clarity here.

I've been involved directly and indirectly with loudspeaker cabinet design for many years, and I went through a lot of the issues re. ported vs. non-ported cabinets vs. horns, etc. in very real world situations where tens of thousands of dollars (or maybe hundreds of thousands) were at stake.   There were some very interesting reasons why we chose to go with sealed (quasi-infinite baffle) cabinets for the Dead as opposed to using bass reflex (ported "Helmholz" style) cabinets. Phase response was one of the big issues, and though the sealed cabinets didn't go down as far in apparent bass when measured individually, in the line array format they were utterly incredible in launching coherent bass frequency waves.

My current interest in loudspeaker design is with quarter-wave transmission line designs, and someday I'm going to make a guitar on these principles.   It will involve a folded tube behind the top that winds up about six feet long... I have no idea if it will work or not, but there's only one way to find out...

Rick,
I'll look forward to your efforts on that guitar. It'll beinteresting to see if or
how it works.

I've always been amazed at the advances that come so quickly in the
loudspeaker enclosure industry. We've come a long way since the 60s and
70s, needless to say, and future designs will likely be no less impressive.

Thanks for your time.

Regards,
Kevin Gallagher/Omega Guitars

The basic principles in loudspeaker design haven't changed much if at all, but the materials technology and the electronics have advanced a great deal. There are new solutions to overcoming some of the limitations of physics, and now, especially with digital power amps coming on strong, it is possible to throw a lot of very well controlled power at speaker cabinets. 1,000 watt amps are no big deal now, and with digital power supplies, they're light.

One of the things that I find interesting is that the latest trend in large PA systems is line arrays.   Hmmm....that's exactly what we were doing 35 years ago with the Wall of Sound. We didn't have the trick hardware to hang the array and do a vertically curved front, but since the whole thing is based on being able to control the directionality of all frequencies by paying attention to wave lengths, the principles are exactly the same.   

Likewise, too, there are certain issues that we deal with in guitar design that have constraints based on physics on the one hand and available materials on the other.

Rick,
   We installed six banks of EAW SLAM (KF761 I believe) units with four
enclosures in each and a big rack full of Crown digital amps and they work
great for getting all of the sound to all of the people. The installers even
flew some of the sub cabs which was something that I hadn't seen before.

   I never got to see the Dead with the wall of sound, but heard that it made
for an incredible listening experience.

Thanks again,
Kevin Gallagher/Omega Guitars

The Wall of Sound was incredible. It was all there, clear, balanced, without blaring. Never heard anything like it.

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

Im not sure the air moves "out only", It may move out and in with the vibration of the top.

Try tissue paper or maybe wax paper over the soundport to see what happens to it. Tape it on 2 adjoining sides.

Not sure I caught the "out only", but that would be quite impossible unless one sat on the top.

The air doesnt go "back in" when the guitar vibrations lower to equalize?- I dont know, I have some wax paper handy though

Also, where (Fretboard, soundhole, bridge) you pluck the strings on a regular top will affect the sound. How does that factor in to an off center soundhole?

 

I would think the soundport just lets the sound out, pretty simple. - just putting your ear on the side while playing allows you to hear the 'real' guitar.

I think Rick just said it would be impossible for the air to only move out unless you only compressed the box. The top is going to increase then decrease pressure in the box with every full cycle of vibration. Of course the sound board is vibrating in a very complex way so it is not as simple as push and pull. Some parts of the top may be vibrating one direction while another may be heading the opposite way. This is what makes it so hard to understand and find simple answers. Thinking along the lines of the basic function is helpful, but leave room to look at the big picture as it is a complex system.

Well, I suppose it would be possible for the air to move 'out only' from the port, if it moved 'in' at the soundhole; sort of 'circular breathing'.

BlackHeart wrote:
"Also, where (Fretboard, soundhole, bridge) you pluck the strings on a regular top will affect the sound. "

That has more to do with the actual force signal of the string on the bridge. The transverse force waveform, which is most of the power in the string, will be a square wave, more or less, with a duty cycle that reflects the plucking point. That is, if you pluck the string 1/5 of the way along its length, than the wave will be 'up' for 1/5 of the time, and 'down' for 4/5.
If you look at the frequencies that are in the signal, you'll find that the power of the partials will rise and fall off in a regular pattern. For the example given, of a pluck at the 1/5 point of the string, every fifth partial will be missing, or nearly so. That's because those frequencies 'want' the string to be stationary at that point, and you made it move (think about sounding the 12thm, 7th and 5th fret 'harmonics'). As you move the plucking point closer to the bridge you get fewer and fewer missing partials, and relatively more energy in the high frequencies compared to the fudamental.

As always, there are limits to this, set by the stiffness of the string, and other things. There is also the question of how much of that high frequency stuff the guitar can actually use, so to speak. Altering the plucking point also alters the phase relationships of the various partials, and this might have an effect on how well the box can put those sounds out.

One of the great things about the guitar is that it allows you to so easily alter a fundamental parameter of the tone. Not only the plucking point, but the exact shape and nature of the pick/finger doing the work, and the direction of the string vibration alter the timbre. The tone of the piano is much more constrained by the mechanics of the keywork, and even the violin seems to lack some of the flexibilty of tone of a good guitar. Players, of course, make these changes without thinking of them, or even being aware that they are doing it sometimes. Certainly most of them don't think about the physics of the thing at all, and never while playing. But for us builders an understanding of where the tone color comes from, and how to bring it out, seems to be a useful thing. All part of being a better tool maker....