F-Hole Size Question

[Tailgate]

I was reading a thread about the Northfield Big Mon and it mentioned the larger F-holes... I seem to remember the Gibson Goldrush having smaller F-holes. Just wondering what the benefits whould be of going with one over the other, as well how they compare to 'standard' sized f-holes? Is it a tone thing? Better volume/projection?

[MikeEdgerton]

You might check out the links in this (http://www.mandolincafe.com/forum/showthread.php?59937-f-hole-sizes) thread while you're waiting for an answer.

[Tailgate]

Thanks Mike. I probably should have tried the search function first.

[Marty Jacobson]

Size of sound ports is one of several variables which may be adjusted, and which as part of a complex system results in what we call "tone". So this one variable, in and of itself, does not allow us to draw any conclusions. In much the same way, looking at the specs of the torque rating of an engine does not, on its own, tell us anything conclusive about the towing capacity or acceleration potential of a vehicle, unless we know what else is present in the configurations we are comparing.

[Willie Poole]

As a young mandolin player I always thought that enlarging the FF holes would make a mandolin sound bassier, then I was told that it would make I higher pitched after I had enlarged the sound holes, when you take material away you can`t put it back so that mandolin was sold with a slightly higher pitch than I liked, but then some people like them that way....It has been said here on the Café that covering the F hole on the treble side will make the mandolin sound bassier...try it and see what you think...

Willie

[Tailgate]

Hey Marty, what I was getting at is, all other things being equal, what would be the reason going with a larger or smaller f-hole? It seems there are those who believe that smaller f-holes equal more low end and larger equals more high end. Then again others believe that any difference is negligible and that the temperature and humidity and day of the week and pretty much any other factor may play a bigger role in the tone of any particular instrument.

[sblock]

Hey Marty, what I was getting at is, all other things being equal, what would be the reason going with a larger or smaller f-hole? It seems there are those who believe that smaller f-holes equal more low end and larger equals more high end. Then again others believe that any difference is negligible and that the temperature and humidity and day of the week and pretty much any other factor may play a bigger role in the tone of any particular instrument.

Here is the basic physics:
Enlarging the size of the ff-holes will raise the frequency of the air cavity (Helmholtz) resonance. Reducing their size will lower this resonant frequency. The Helmholtz resonant frequency is roughly proportional to the area of these holes (their shape and location also matter somewhat, but much less so).

So, for a given mandolin's sound, having smaller ff-holes will tend to make it sound a bit bass-ier, and having larger ones will make it sound a bit treble-ier. These are noticeable effects, but they are not all that dramatic, in practice. Furthermore, the Helmholtz resonance of a mandolin is by no means the only instrument resonance that contributes to its overall tone -- there are many other resonances (a whole spectrum of these, in fact)! Other factors that can exert even stronger effects on the mandolin tone include the size of the air cavity (for example, with thicker ribs on the sides, or with a larger top and back surface), the mass and position of the bridge, the types and gauges of strings used, the types of tonewoods used, and -- especially -- the thickness graduations (and associated stiffnesses) of the top and back, and the location and types of the tone bars and style of bracing. That's a whole lot of factors to consider!

But no, the "projection" (i.e., the tendency to radiate sound in the forward direction) of a mandolin is not closely vrelated to the size of its ff-holes. Projection is more related to their actual shape (oval, round, ff) and their location on the top (or a side port). Of course, if you block all the sound holes altogether, the sound pretty much goes away, but that's trivial.

[Marty Jacobson]

Hey Marty, what I was getting at is, all other things being equal, what would be the reason going with a larger or smaller f-hole? It seems there are those who believe that smaller f-holes equal more low end and larger equals more high end. Then again others believe that any difference is negligible and that the temperature and humidity and day of the week and pretty much any other factor may play a bigger role in the tone of any particular instrument.

Oh, ok. I don't change the size of my f-holes when designing an instrument. I do vary the depth of the instrument. I don't believe there is a single optimal ratio of sound port area to air volume for "proper" mandolin tone. Anything that looks reasonably like a mandolin and isn't outlandish, say 30% bigger or something, is going to sound roughly like a mandolin. That said, bigger instruments in a given tuning tend to emphasize the bass and slimmer ones, still tuned the same, tend to be more sparkly, which is exactly what you would expect from what Prof. Block has stated above. In practice it takes a lot of experimentation, because it's not just bass and treble balance, there are subtler characteristics in play as well.

My instruments vary in depth, very intentionally, over a range of about 3/8" (9.5mm).

Here's comparison of two different redwood-topped mandolins I built in 2013. The body volume isn't the only thing which is different about their design, so this is far from an apples-to-apples comparison. But one has a more typical mandolin body depth, and the other is 1/4" deeper. The sound ports are both identical in size, cut from the exact same CNC code.

I can hear the influence of the body depth here, or at least I think I can. I also think I can hear the influence of a different bridge placement and dramatically different back plate designs (floppy on the first sample and stiff on the second). So all else is not equal here.

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[Tailgate]

Great stuff! Thanks for the info guys.... and Marty, those mandos sound great!

[sunburst]

Did you click Mike's link in post #2? If not, click it and then click the second ("Another thread") link that appears there, then go to post #19, and you'll find the beginning of an explanation from the guy who knows more about this stuff than any of the rest of us here, Dave Cohen. Reading the rest of the thread is worthwhile too. Lots of good information there, and plenty more in other places if you want to do the research to find it.
In short, it takes about a 30% change in f-hole (or other hole) size to make a 7% change in Helmholtz resonance. Small variations in hole size make such infinitesimal changes in Helmholtz frequency that they barely qualify as changes at all. Add to that the fact that many other factors are at work affecting the sound of the mandolin, and it is an oversimplification to say that smaller holes = bass boost and larger holes = treble boost.

[Tailgate]

Hey John, I did have a chance to go through the posts in the links earlier on. The reason I asked the question is because I was curious, with so many other variables at play, why would a builder choose to make a model with f-holes that are sized different sizes than their other models and use that as a selling point (I.e. Northfield Big Mon)? If it makes such an insignificant difference compared to other factors such as wood type, thickness, body depth, etc., why bother adding another variable to the mix?

Is it simply that the builder is honing in on a very specific sound and making as many slight modifications as possible in hopes of achieving the sound they're after?

[sunburst]

I think it's more of a B.T. Barnum kind of a thing. I think it has less to do with sound and more to do with sales.

[fscotte]

I tend to think enlarging the f-hole would reduce the stiffness of the top more than affecting the air chamber frequency.

[trevor]

A few years ago Rosta Capek enlarged the sound hole on his oval hole mandolins. There was a noticeable loss of bass. I don't know if the same applies to f holes.

[mandroid]

Perhaps its like the Ported Speaker's Port tube size, length,

and the back side of the vibrating speaker, acoustic suspension,

instead of the Air volume Retained within the body of a Mandolin ..