Soundhole size and effect on body resonance

[Dave Bagwill]

Well I'll be danged. That is very interesting. Just speculating here - but could that feature be built into an acoustic guitar without sacrificing any sound or playability? I'll have to find a pic of a ported speaker to see what you are talking about.

[Bob Matthews]

I believe it was a German guy called Hermann Helmholtz who first looked at the theory of chambers and their resonances in the mid nineteenth century. We all know that if you blow across the top of a bottle, it will produce a certain note, and if we reduce the volume of the bottle by adding a substance like water, and blow again, the pitch of the note rises, but what is really interesting is that at any set internal volume, the pitch of the note produced can be made much louder when the area of the opening size is ideal. When this ideal size of opening is achieved, the note produced will dramatically increase in volume, this is why on small guitars we have small soundholes and on larger guitars we have larger soundholes and why thin guitars with large soundholes don't sound too great.

Now don't ask me how to accurately calculate the volume of a guitar cos I'd have to use graph paper and count the squares, but it is roughly the internal area of the back or top multiplied by the average internal thickness of the instrument.

If I remember correctly, the ideal diameter of the hole in a resonant chamber is one quarter of the radius of a sphere of the same volume as the chamber.
Hope this makes sense, but don't quote me on the quarter of the radius bit!

Bob

[Bob Matthews]

As a P.S. :)

I also believe that exhausts for performance and racing engines use the Helmholtz theory to tune the exhaust system to the engine!
Anyone for building a sports guitar?

[Dave Bagwill]

Good stuff Bob. I have heard of Helmholtz a bit.

So far, I need a built-in humidor, an access panel to allow for brace shaving the completed guitar, and now an adjustable diaphragm for the soundhole.

If there are any noob's reading this, I'm joking! :-)

[Tim Benware]

If I remember correctly, the duct is tuned to the cone and the box area is part of the formula. In a guitar the cone is the top and the hole is the port. What complicates things is the shape of the guitar (speaker volume is generally H x W x D) and determining the volume accurately, although I wouldn't be surprised if you couldn't find something on the internet to figure it out. The "port or vent" in the picture is a tube of a certain diameter and length. Tuned properly the vibrations from the driver on the back stroke bounce or reflect off the back wall and exit the port in phase with the vibrations on the forward stroke making use of the overall energy of driver movement and increasing the volume in theory by 2.

[Ray Ussery]

Lot's of folks would tell you that the over sized sound hole in Clarence White's D28 Brazilian made all the difference in the world....and I think it DID! In Clarence's hands.

Tony Rice, who now owns the guitar and plays it regularly, says he can't tell any difference...and I Believe him! He isn't Clarence white.
Everybody holds and plays the guitar differently.... because of that, it really does sound different.

Proof? Take ANY guitar and hold it in your lap, pull it tightly to your belly and play it....then move it away from your body so there is no contact with the back of the guitar and play it...the tone and the volume will differ greatly.

There have been thousands try to duplicate the sound of Steve Ray Vaughan and his Strat...isn't going to happen, even with all the electronics we have today, even with the same guitar...nope...his own brother admits he can't do it!

So many variables..and yet, there are SOME who can build with repeated success...why? what IS the secret?...the tech builder will tell you it's testing grain strength, thickness, bracing... the other end of the spectrum will tell you they can Just "HEAR it" they don't measure anything..they will all tell you to some degree anyway...it's the type of FINISH or lack of it...thickness...right, I've never seen a ruler on a spray gun...then there's the wood...I've seen and played one of Taylor's "Pallet" guitars (Made from an oak pallet he picked up in the lot out back) and I can tell you it was superb!

A couple of years ago in Denver, Wayne Henderson told me it was quality of workmanship and paying close attention to dimensions, properly glues joints and repeatability. From the guitars of his that I've played, including #400, he's doing something right!

Who really knows? :)

[Dave Bagwill]

So the same amount of energy results in 2x the volume; interesting. The obvious question is: why not a second port, or a third, each one tuned to the rest of the box? I must Google.