Official Luthiers Forum!

I had my binding done and recorded some tap tones on my #3.
The resonance target weren't exactly to my liking (when I compare to my #2), the main air resonance was a bit too high.
So I just play around trying to lower the main air by sealing the sound port with tape.
To my amazement, the spectral analysis showed something interesting that the resonance peaks had significant changes:

1) The main air drop and main top shifted higher
2) The main back resonance virtually disappear

I think this gives some explanation on why my #2 had such a high main back resonance peak (somewhat higher than the main top initially).
I think it was due to the sound port on my #2, unfortunately I don't have #2 with me I can't really confirm this.
As I understand it, Trevor's analysis was done without a sound port at all.
Well the bridge isn't glued as yet, I will do another analysis once the bridge is glued.

_________________
-----
Sen CL Goh
http://senguitar.blogspot.com

Interesting. I wouldn't say the main back resonance disappeared. It seems to be the same relative strength compared to the main top. It just got swallowed up a bit in your plot by the top resonance. I would bet if you recorded some taps on the back instead of the top you would see that back resonance loud and clear both with and without the sound port.

On page 2-39 of Trevor's book he shows that the main air raises by .85 hz/mm as the sound hole diameter increases . So if I assume that you have a 3 7/8 inch sound hole. AND if I assume that adding a sound port is the same as increasing the sound hole diameter, I predict that it would take a 2 inch sound port to raise the main air from 101 hz to 112 hz. Is this close to what you have? The same graph shows that the Main Top will also raise a little but you saw a small drop.

So this raises the question : If I add a sound port, should I do a reverse calculation and reduce the sound hole diameter by a given amount to keep the Main Air at the same frequency?

_________________
_____________

EddieLee

You might want to try putting magnets that are the same weight as your bridge/saddle/pins in the bridge area to lower your top resonant frequency closer to it's final note. For me, this gives a better representation of both the top and back frequencies (and the air for that matter)

Opening a port does raise the pitch of the 'main air' resonance. It is not, however, a simple matter of adding the port area to the main hole area to compute the rise. Generally speaking, the further from the 'main' hole the port is the greater the pitch shift will be for a given size of port.

Alan - Do you recommend any design changes to compensate for the shift in the Main Air frequency?

_________________
_____________

EddieLee

You can make the main sound hole smaller; it's just hard to say by how much in a given situation. Start small, leave a wide margin between the hole and the rosette, and sharpen you knife well...

David,
That's an interesting technique. This get's you pretty close? I would of thought that maybe you'd come out too low, due to the lack of additional stiffness the bridge brings.
I'll have to try it.

_________________
Jim Watts
http://jameswattsguitars.com

Hi Jim ... adding the weight of your bridge/saddle/pins to the top gets the top closer to it's final tap tone (i.e. lowers the tap tone), but like you say, doesn't take into account the added stiffness the bridge contributes (which of course raises the tap tone). With my guitars, these two factors don't cancel each other out ... the mass lowers my tap tone more than the stiffness raises it. In my case, I have a few models that I make over and over, and have learned the relationship between the weighted top and the final tap tone. I suppose you could lessen the weights used, to compensate for these two phenomenons (mass versus stiffness). Either way, you can predict what that final note will be.

Sorry for the slow answer to your question ... I'm not on the forum that much