top thickness at soundhole - does it matter?

[hat]

A question for the engineering types on the board - does the thickness of the top at the sound hole have an effect on the sound/air being pushed out? ( think of a whistle effect). I have an old Harmony that I am about to start work on. The top has been sanded down, that much is obvious. I haven't taken any measurements, but I can tell around the sound hole it's thinner than it should be. One idea I have is to just reinforce that area with a thin spruce laminate on the bottom side as needed.
So - to my question- as the air goes past the lip of the sound hole - how much does the thickness of the lip affect the tone? or does it have any effect at all?

[John Arnold]

I doubt that it will have a noticeable effect.

[tadol]

Well - of course! Everything matters when it comes to tone! ;-)

Numerous builders reinforce the area around the soundhole, even laminating a crossgrain ring directly under, or adding a hardwood ring right at the edge of the soundhole. I don’t think you would ever be moving enough air to have any concern about any type of “whistle” effect - and the air would be moving in both directions anyway -

[Edgar Poe]

Very minor. You can compare the much denser inlays on some guitars at the soundhole, For example Martin D-45, Perhaps Martins most costly guitar, if it did effect the sound, I'm sure Martin would have eliminated it.

Ed

[yellowesty]

The "simple" model of a Helmholtz resonator treats the air in the port (the soundhole) as a mass and the air in the cavity (body of the guitar) as a spring. That means that more air in the port should lower the resonant frequency. But, you say, we know that a smaller soundhole yields a lower resonance. That's because, given the thicknesses of guitar tops, the "air in the port" is almost entirely due to end effects (the transition between the air directly in the soundhole and the larger body of air in the guitar and on the face of the guitar). Thickening the guitar top will, very slightly, increase the amount of air that acts as a mass, but edge effects will dominate as long as the top is thin compared to the diameter of the soundhole. (Helmholtz resonators used as loudspeaker enclosures often have ports that are several times as deep as they are wide.) And guitars aren't "simple" Helmholtz resonators, as the volume of the body is not rigidly fixed (the top and back flex significantly) and, it's been claimed, the shape of the body can influence the magnitude and stability of standing waves in the body, affecting the "springiness".

So, no, to any practical effect, top thickness at the soundhole does not matter.

[Alan Carruth]

The only time you'd expect there to be much flow velocity through the hole would be at the 'main air' resonant frequency. It's likely that, at that pitch, there's enough drag from the air trying to turn the corner that the losses are higher, and it could change the pitch some. However, the problem is that rounding that edge off enough to make a useful difference is probably impossible; you'd need a radius of a couple of inches, I would think, and the strings are too close to the top, if nothing else.

Adding in a cylindrical or somewhat conic sleeve to extend the hole into a 'port' tends to increase the drag through the hole, lowering both the pitch and activity of the 'main air' resonance. One such device is the 'tornavoz' that has seen cycles of popularity and obscurity over time. Generally it's metal sleeve, wider at the bottom than the top, that extends from the sound hole quite deep into the guitar, sometimes almost touching the back. At that depth it practically kills the 'main air' resonance, dropping the pitch well below the fundamental of any played note, and the activity to almost nothing.

Recently a device called the 'O-Port' has come out. This is a flexible plastic ring, the can be folded up to snap into the sound hole. When installed it has a rounded flaring profile that extends about 1-1/2" into the guitar, and a similar distance outward around the sound hole, so that the inner lip in parallel to the face of the top. I installed one on an OM-size guitar and made before and after 'impulse' spectra. The 'main air' pitch dropped from 103 Hz to 92 (G# to F#), with about a 20% reduction in measured peak height. The 'main top' resonance showed a similar pitch drop, but was reduced in amplitude by 60%. Keep in mind that the 'main air' and 'main top' resonances are halves of a 'bass reflex couple', so it's not unexpected that both would be affected, but the nature of the change was unexpected. I have not had much of a chance to look into that further.

The bottom line is that and differences you see in the normal run of guitars are probably not going to affect the tone to speak of. They are usually done for structural reasons, and could well make a difference in the durability of the guitars.