body resonance effects?

[Brian Evans]

What affects the natural resonant frequency of an archtop, and what can you do about it? My archtop has a fairly definite resonance at D (4th string open D). When I sing into the side port and hit that note, it really resonates. The has the effect of making that note somewhat dead sounding. More dead fifth string fifth fret D than open D, if that makes sense. So I wonder what are the parameters that cause this? I suspect (based on tapping for tone) that my back and top are too close to the same note (they were at least a tone apart before I glued up the box, though). I have the traditional f holes and a small side port. I don't really feel like re-tuning the back or anything like that, the guitar sound phenomenal otherwise, to my ears. The bridge is a pretty light (I recall around 23 ounces) one piece Jatoba. I haven't experimented with tail piece weight or anything like that, because I just don't know what might have an effect. Any thoughts?

Brian

[Alan Carruth]

Normally the 'main air' resonance is the one that responds most strongly when you sing into a hole. This is generally the lowest pitched resonance on a guitar that can produce sound effectively, and that's a pretty high pitch for it if it's the open D string that's affected. It usually comes in somewhere between F# and A on the low E string, although it can be a bit higher on archtops. OTOH, a port can raise that pitch quite a lot: I've never used one an an archtop, since that's usually already high on them and I don't want to kick it up any further.

One most flat tops the next major resonance up is the 'main top' at about the open G string pitch. Again, this will normally be higher on archtops, but all the way up to D is a stretch for that too.

There are ways you can check it out. The 'air' resonance pitches are functions of the size and shape of the box, the sizes and locations of the holes, and how flexible the walls of the box are to some extent. If the pitch of this mode changeswhen you cover over a hole it's most likely an air resonance. Generally, the smaller the hole(s) the lower the 'air' mode pitches will be.

You can find out if the top or back are contributing to it be adding some mass to them. Poster adhesive is a good thing to use, since it's fairly easy to remove and won't leach oil into the wood as modeling clay can. Slap on a wad somewhere near the middle of the top or back and see what happens.

One complication on arch tops is the tailpiece, which can have it's own resonances. Try loading that as well and see what happens. In some cases the back strings between the bridge and tailpiece can get into the act too, although that's not usually such an issue.

Tracking down 'wolf' notes like this can be challenging. On the other hand, once you've found them they are often fairly easy to fix.

[Brian Evans]

Thanks for your thoughts. I've also read the paper you did on body resonances, quite helpful. The idea of the port being involved had occurred to me, but your indication that it can raise the resonance pitch is very interesting. the resonance definitely changes when the port is covered, as does the overall sound of the guitar. I will try the adding weight thing as well.

edit: well, that didn't take but a second! singing into the F hole, with port open - quite strong and noticeable resonance at D, as before. Cover port with hand, less intense resonance at low B. How interesting! I will play with this further! I wonder if a tunnel on the port would have an effect?
Brian

[Alan Carruth]

A tube or sleeve of some sort on the port will have an effect similar to making it smaller. It's a useful way to fine tune the pitch. Even a paper tube would work. In fact, I saw a report once of a fellow you used cellophane to 'deepen' the F-holes on small violas to drop the 'main air' pitch. Of course, the mode tends to become weaker when you do this.

[Eric Knapp]

This sounds like tuning ports on speaker cabinets, something I used to do a lot. One of the options that was often used is a sealed box. I'm sure that has been tried on acoustic guitars and I'm wondering about the result. Are there any resources on that?

Thanks,

-Eric

[Beate Ritzert]

This sounds like tuning ports on speaker cabinets, ...

It is. The soundbox of a stringed instrument is to 1st order approximation a Helmholtz resonator. Which means that as soon as You know the volume of the box and the area of the sound holes and their depth You can estimate the fundamental resonance of a guitar.

[Randolph Rhett]

This sounds like tuning ports on speaker cabinets, ...

It is. The soundbox of a stringed instrument is to 1st order approximation a Helmholtz resonator. Which means that as soon as You know the volume of the box and the area of the sound holes and their depth You can estimate the fundamental resonance of a guitar.

Out of curiosity: Assuming I know all of those things, what is the formula?

[Eric Knapp]

Out of curiosity: Assuming I know all of those things, what is the formula?

I haven't needed the formula for a long time as I've been using software that does it for me. This looks right, though.

http://www.diysubwoofers.org/misc/portcal.htm

This one looks more complete with better math.

http://www.jlaudio.com/header/Support/T ... rts/287541

The big difference I see between speaker design and instruments is that speaker boxes need to be as dead as possible. They have to not resonate at any frequency. Of course instruments are designed to resonate, that's the whole point. The size of the ports of various instruments seems to have evolved to be pleasing sounding depending on what type of sound you're after. Combining the resonances of thin wood with bracing and ports makes for a very complicated calculation. There's no wonder why lutherie is considered to be part high craft and art. It's hard to do it right. Kudos to you all who already can. I'm itching to make my first awful instruments so I can advance to the mediocre level.

-Eric

[Barry Daniels]

Eric, you are very correct about the differences between an ideal Helmholtz resonator and a guitar, and that difference makes such calculations not very relevant to the design of guitars.

If you want to get up to speed with current science on guitars you might want to get the two book set by Trevor Gore and Gerard Gilet called "Contemporary Acoustic Guitar, Design and Build".

[Alan Carruth]

Gore & Gilet is very good, but pricey. A free resource to get you started would be "Acoustics for Violin and Guitar Makers" 4th edition, by Erik Jansson. You can download it as .pdf files from:
http://www.speech.kth.se/music/acviguit4/ Get a\part1.pdf through part9.pdf.

A lot of this is old info, and not nearly as detailed as G&G, but it does show a lot of useful stuff and the price is right.

[Eric Knapp]

Gore & Gilet is very good, but pricey. A free resource to get you started would be "Acoustics for Violin and Guitar Makers" 4th edition, by Erik Jansson. You can download it as .pdf files from:
http://www.speech.kth.se/music/acviguit4/ Get a\part1.pdf through part9.pdf.

A lot of this is old info, and not nearly as detailed as G&G, but it does show a lot of useful stuff and the price is right.

This is great, thanks. When I was in college I took a course from physics professor Jack Fry at UW-Madison. It was "Acoustics For Musicians". It was a required course for masters music students. Jack was great and it was what really started my desire to make instruments. He made violins that nobody liked but his bows were something else. He was able to tune his bows by moving the center of moments to exactly where people wanted it. He found through experimentation that that value was how violinists judged the "weight" of a bow. It was cool to see.

-Eric

[Alan Carruth]

I made a matched pair of violins once to check out Fry's hypothesis about asymmetric back graduations. They ended up sounding almost the same, but the small difference was opposite to what Fry said it should have been. That's one data point, of course.

When I made my first 5-string viola I borrowed a decent viola bow from a friend, and measured the mass, the pitches of the lowest three bending frequencies, and the CG and center of moment of the stick from both ends. Then I made a stick that matched it as well as I could. It didn't look like the original, but it played like it.

[Beate Ritzert]

I made a matched pair of violins once to check out Fry's hypothesis about asymmetric back graduations. They ended up sounding almost the same, but the small difference was opposite to what Fry said it should have been. That's one data point, of course.

Which (by application of mathematical logics) means that You have falsified that hypothesis.

[Alan Carruth]

I'd like to think I falsified it, but I'm sure a clever person could find all sorts of extenuating circumstances. For example, the only two 'matched' backs I could find were from a plank of birdseye figured Rock maple; very different from the curly European maple that Strad et al used. Maybe if I'd used the 'right' wood they'd have turned out differently.