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The dreadnought plans I have indicate a sound hole size of four inches. That is also the size on my Martin D35. I received
a dreadnought top with a kit that has a precut sound hole that is 3.75 inches in diameter. How will this affect the sound of
the guitar?

Thanks

Dave

The size of the hole will affect the main air frequency. Making the hole smaller will lowers the main air frequency, larger hole will raise it.

I have big hands and have found I better make it big enough to get my hand into

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Steve Smith
"Music is what feelings sound like"

That's a good point Steve.

Michael. Does that mean that with a 2.75" sound hole instead of 4" the bass will be increased? I don't
think you would want any more bass than normal on a dread. I have some tops I bought from Shane so
I will probably try to use one of them. I'm just not sure I'm quite ready to try a rosette.

Thanks for your replies.

Dave

In my mind, and to my ears Soundhole size is like the treble control on a stereo. I don't think any bass is gained by decreasing the size, but treble is decreased. Same goes for enlarging it. I don't think it decreases bass, but increases treble. Maybe Alan will see this thread and explain it.

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woody b,

I'm not sure there is a magic point where treble/bass become independent......would like to hear more explanation if someone thinks this is true. Per Roger Siminoff, if you size the hole optimally for the volume of air in the guitar, volume will increase. Otherwise, I think changing the hole size raises or lowers the main air resonance frequency......which is a dominate sound of the guitar as I understand things.

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Formerly known as Adaboy.......

A guitar with too big a sound hole will have more volume, shorter sustain. The notes are more open or less focused. Take this Gerard Audirac for example, it sounded more like a normal guitar when 3/5 of the hole is blocked.

Another point is that larger holes are more efficient as 'resonators' than smaller holes. I suspect that although making the hole smaller lowers the main air pitch, as the hole gets too small, the volume (i.e. amplitude) of the main air pitch drops because it's becoming less and less efficient. This is pure educated speculation on my part though.

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Here is an article from the University of New South Wales that explains is nicely. Scroll down a bit on the page until you get to "Helmholtz Resonance and Guitars." The second paragraph deals directly with the size of the soundhole. (Thanks again to Google... )

http://www.phys.unsw.edu.au/jw/Helmholtz.html

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Peter

William Allen wrote a good article on air resonances in the first issue of 'American Lutherie', and, of course, it's in the first 'Big Red Book'.

The short answer is that I would not expect much difference from a 1/4" change in the diameter. If you complete the guitar with the small hole, you can always enlarge it later to see if there's a change, but then it's hard to go back if you don't like it.

In theory making the sound hole smaller does drop the pitch of the 'Helmholtz' air resonance. In practice, as the site said, what you get on a guitar is not a 'pure' Helmholtz resonance anyway, and the pitch is probably a lot lower than the standard equations would lead you to believe. This happens because of coupling with the top (mostly), so the looser the top the lower the 'main air' resonance pitch is likely to be.

Making the hole smaller also increases the ratio of edge to area of the hole. Air flowing in and out of the box has to go around the corner, and there's a fair amount of drag which cuts down on the power of the 'main air' mode. Those parchment or carved roses on the old guitars and lutes, and the 'feedback busters' that look like them, reduce the strength of the 'main air' mode to practical insignificance in some cases. Again, I would not expect a 1/4" difference in hole size to make a major change in this regard, but you never know.

The interesting thing to think about is how things balance out. When you reduce the hole size the 'main air' resonance drops in pitch, but it gets a little weaker. Does the guitar sound bassier because of the lower pitch, or less bassy because of the reduced power? Even if you couls calculate the exact magnitude of tghe different effects it would be hard to say what it would sound like: there is no exact way to correlate measurments and perception in these cases. That's why we have to do experiments.

There is one other thing that should happen, but I have not had the chance to do the experiment yet. A hole in a baffle has what's called an 'upper cuttoff frequency'. Basically, a small hole is not very efficient at radiating sound at low frequencies. As the wave length of sound in the air approaches the diameter of the hole the efficiency approaches 100%. The larger the hole the lower the cuttoff frequency, and the more rapidly the efficiency rises with pitch. Guitar soundholes should have a cuttoff frequency in the range of 3000-5000 Hz, if I did the math right; up where your ears are pretty sensitive. So soundhole size might have an effect on the 'clarity' or 'detail' of the sound through this mechanism. There would be other effects as well. At any rate, Art Benade, back in the '70s, showed that the cuttoff frequency of wind instruments made a big difference in how they sounded to players. Changing the cuttoff by 6% (a semitone) made all the difference between 'dark'/'dull' and 'harsh'/bright', depending on who you asked. The soundhole on a guitar is only one part of the sound producing mechanism, and might not make or break the tone, but it's certain to have an effect.

Thanks for the replies guys. Considering that I like the sound of my first build and of my Martin both of which have 4"
holes I think I will attempt my first top and rosette. I don't think the vendor (one of the OLF sponsors) will take back
a top that has been thinned a little and has pencil marks for braces on it.

Thanks again.

Dave