View Full Version : New Cohen C#

Dave Cohen
Apr-21-2004, 8:44pm
Here are some photos taken by the new owner and passed along to me. This mandolin was commissioned to sound great and not have any sharp points, so the points were all "softened". This mandolin has a redwood top, curly maple back/sides/neck, flatsawn ziricote headstock overlay and accents, and rosewood and maple binding throughout, i.e., no plastic. The tuners are gold Waverlys with ebony buttons. The open scroll is completely hollow except for a small wedge where it is glued together at the end.

The first shot is a straight-on shot of the front.

Apr-21-2004, 9:24pm
I like it, mace mando.

Michael Lewis
Apr-22-2004, 1:05am
Hey Dave, very elegant! You taking anything to IBMA in Oct?

Dave Cohen
Apr-22-2004, 9:13am
The owner sent me some other photos with great detail. I tried to post them last night, but they are too big. I will see if I can get my wife to crop them w/ photoshop tonight.

Hi Michael. Y'know, IBMA last October was such a bust for me that I haven't decided yet whether I will go this year. I was really ignored in droves last Fall. Btw, I finished the F that I had there in pieces. It looks good and sounds good. I will try to post some pictures of that one as soon as they are ready. How goes it w/ you?

mad dawg
Apr-22-2004, 9:29am
Absolutely gorgeous -- nice work!

John Zimm
Apr-22-2004, 9:42am
That looks great. I am betting it is nice and light and doesn't tend as much to fall forward when the player has it over the shoulder. Beautiful work.


Apr-22-2004, 8:40pm
Dr. Dave
i have admired your mandolins for some time, this one is beautiful. love the sound holes. I know you do alot of work on sound properties. is this a result of your research?
is the inside of the scroll maple?

Apr-22-2004, 9:38pm
Looks good Dave!
I've been getting ready to start a couple of "A"s with a solid color stain (no burst). I was going to use Sherriton(sp) brown, but after seeing the Indian corn color and now this, I might be changing my mind.
Did you try anything new in the sound/structure department with this one?

If you want to, send me the other pics and I'll shrink them in photoshop.

Dave Cohen
Apr-23-2004, 7:55am
Here are the photos with greater detail for better shameless self promotion. This one shows the oil-finished neck, nice figure in the back, and a different approach to carving the heel area of the back.

Dave Cohen
Apr-23-2004, 8:00am
I forgot to mention the different approach to binding the scroll. This next one shows the neck join detail, binding detail, the "softened" point, and the scroll interior. Yep, the interior of the scroll is curly maple.

Dave Cohen
Apr-23-2004, 8:09am
...And finally, a similar shot, a little closer in, w/ perhaps a little more bridge detail.

Dryseptember, the soundholes have conventional area and positioning, so are pretty conventional in that respect. I tried to deliberately simplify in order to bring out the very sculptural aspect of the archtop mandolin.

Sunburst, I used a mixture of transtints - honey amber, golden brown, and reddish brown. I will have to look in my building notes to remenber the proportions. When I do, I will try to remember to post it. I used very little on the redwood top, more on the maple to bring the colors closer together. I use the Benedetto regimen; one clear coat, color coat of lacquer w/ dyes in it, followed by remaining clear coats.

Dave Cohen
Apr-23-2004, 8:21am
I forgot to mention; I will be presenting the paper at the ASA meeting in NYC on the afternoon of May 27. No attempt at determining mandolin quality yet - that is really difficult. But we have some good insights into the effects of soundhole configuration, bracing, etc. I think that luthiers will find it really valuable. Since I tend to think like the science weenie that I am, I cannot objectively say whether it is in plain luthier english or not. There are no partial differential equations in it, though all of the stuff about normal modes and eigenfrequencies sorta implies pdes.

Apr-23-2004, 8:47am

Is there a place where one can get a copy of this paper? I'm actually a big fan of PDEs, eigenvalues, and normal modes of vibration.

I''d also like to say thank you for letting me noodle with some of the mandolins you used in your last study. I'm still feeling pretty stoked from playing my first vintage mando!

Rob M.

Tom C
Apr-23-2004, 10:53am
Nice flavor! No hard points but that scroll can really hook ya.

Apr-23-2004, 2:41pm
Please, tell us about the bridge. Never seen one like that before.
Beautiful work all over!!

Apr-23-2004, 8:19pm
BEAUTIFUL!!!!!!!!! Yeah, what's up with that bridge? I love the look of it. Does it function better than a traditional bridge? Please let us know. thanks!

Dave Cohen
Apr-23-2004, 8:52pm
That is my usual bridge. It too is intended to be sculptural, rather than a simple rectangle. It is adjustable in the usual way. But Gibson-type adjustable bridges weigh between 14 - 17 grams, whereas mine weigh about 8 - 9 grams when carved from ebony. With a maple base and an ebony saddle, I can carve a bridge to weigh about 7.5 grams. An all-maple bridge of mine weighs about 5 - 6 grams. Do they function better than a traditional bridge? I think so, though I don't have any hard data to back that up. I don't think that there is any advantage to a heavier bridge; all it can do is act as a low-pass filter. Other things being equal, the impedance of a heavier bridge should be greater at higher frequencies, and so should attenuate the high frequency response. So I generally try to keep 'em as light as possible. My saddles are taller and thinner than the typical, to counteract the tendancy to sag under string tension while still keeping mass as low as possible.

One local player had me fit one of my bridges to his F5 copy. He claims that the bridge really woke his mandolin up. That is just one anecdote, though.

I am working to get the paper in pdf form, so that ASA can post it as a lay language paper on their site.

Apr-23-2004, 9:37pm
Very nice design!

I wonder about the F holes... same area and position, except they go closer to the right and left edges of the top. I have wondered what the affect on tone would be if the traditional F holes were moved farther out, or curved as yours are, to achieve more surface area for the vibrating top (the top wood on the outside edges of the F holes doesn't do much, I imagine). Does this mando have a different bass character? Stronger than normal?

Not sure I follow the bridge weight argument... changing different bridges out has led me to believe that a heavier bridge will attenuate the bass response and make the treble sound tinny and harsh. That's the opposite of a lowpass. I do see that in a simple spring-mass system, adding mass will reduce the natural frequency (by root of the mass, if I remember correctly), but there must be something different going on here.

Dave Cohen
Apr-24-2004, 5:42am
Impedance has a first power dependance on the force and frequency, and since F = ma, a first power dependance on the mass.

Another local player asked to try one of my bridges on a loud but particularly harsh sounding F style mandolin. With my bridge on there, it sounded, if anything, even harsher. It was every bit as loud, but even more shrill in the treble. We both agreed that that particular mandolin sounded less offensive with a heavier bridge. But again, that is just another anecdote. Less anecdotal is the violin mute. It is a wood clip that is slipped on the bridge. It mutes the violin by virtue of its added mass stealing energy from everything else, particularly the higher harmonics.

One of the things that doing the holography has reinforced is that soundholes are not a barrier to plate vibration. The mode shapes seem to go right through the soundholes, as if they weren't even there. So I don't think that moving the soundholes out toward the perimeter necessarily causes a larger plate area to vibrate.

This mandolin has good bass, but also good balance. The owner said he would be taking it to Mandofest.

Ted Eschliman
Apr-24-2004, 10:42am
Very interesting!
I would treasure a chance to see this at Mandofest. Hope I can hook up with the owner.

Jim Hilburn
Apr-24-2004, 10:59am
I played it last night, and it sounds great. Very full and rich and powerful. And the owner can really get the most from it. A real unique tone.
I could only get a pinky on one very light brace. What's up,Dave, is it an X or is it top secret.

Mandolin Enthusiast
Apr-24-2004, 11:13am
Yeah, I played it last night too; (what a party!) #The setup was immaculate, the tone was fabulous, but it sounded best in the hands of its owner! #A fine mandolin!

I have it on good authority that the Cohen will be at Mandofest 2004. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/coffee.gif

Dave Cohen
Apr-24-2004, 1:16pm
Hi Jim. Nope, there is no secret; it's just my bracing. I keep changing it incrementally, trying to find out what does what, but it is basically my bracing. It is easier for you to look in there with a dental mirror than it is for me to describe. Imo, it is NOT magic. A few other builders have tried it, but haven't had as much success with it as I have. I have built a few w/ tone bars, and they still end up sounding like mine. It leads me to think that there are other factors equally important, or even more so. Like f'rinstance the arching, and the relationship of the top plate stiffness to the back plate stiffness, and the overall plate stiffness.

One thing that my bracing does for me is to bring down the frequencies of the various longitudinal bending modes. With tone bars, the (0,1) mode (longitudinal rocking) seems to be found at a higher frequency. Same goes for the (1,1) (twisting) and (0,2) modes. Is that "good"? To be perfectly candid, I don't know!