Official Luthiers Forum!

We have our first request for a sound port.

I'd like to know how punching a hole in the side of the guitar affects the main top and back freqs. Obviously the main air resonance will go way up, but what about the other two?

I plan on stringing up the guitar with no sound port at all, then punching through and enlarging it so that it sits where I want it freq wise, wherever that ends up.

As an aside, does anyone have any data in regards to how the size and freq are related? Like maybe a 1" port drives it up 10hz, 1 1/4" 15hz, just as random examples? I think Allan Carruth did a study of it at some point...

Ed, probably not exactly what you're looking for, but Allan and others shared some thoughts on sound ports back in May.

viewtopic.php?f=10101&t=43473&p=575053&hilit=sound+port#p575053

Thanks, a good read...

I always voice as if the port is not there. It will effect the main top and main back slightly, but not more than a few hz.

Thanks Sam,

Does it tend to nudge the plates up or down?

My impression without really crunching the numbers is that it follows Gore's formulas for the sound hole size pretty closely. Al has stated that the coupling between the sound port and the Sound hole may not be good (as I understood what he said). That was not what I found. Here are some measurements I took.

Sound Port Open Close
Main Air 102 96
Top 182 182
Back 225 250

That is with the box closed and no bridge.

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EddieLee

Interesting that the top stayed the same while everything else changed...

We need a member of the brains to chime in again.

Steve

I'm thinking that with less resistance from the air in the box, the plates will be able to vibrate more freely, thus lower the freqs a smidge. I'll plan for that anyway and see what happens...

Obviously, the 'main air' resonant pitch goes up when you open a port. The larger the port and the further it is from the main soundhole, the greater the A-0 pitch rise.

The 'main air' and 'main top' resonances are parts of a 'bass reflex couple', and influence each other. On my most recent 'test mule', which I used for a series of blind player tests, I got the following:

port closed port open

A-0 95 105
'main top' 192 194
top cross di 233 233
top long di 388 391
top cross tri 490 498
top quad 542 547

main back 216 221
back long di 256 253
back cross di 404 408

This is a fairly typical Classical guitar in most respects, with a fan-braced redwood top, Indian rosewood back, and a 50mm diameter port at the wide part of the upper bout on the bass side. The amount that any particular 'wood' mode changes depends on the details of how it couples with an interior 'air' resonance. The top 'long dipole', for example, couples fairly well with the 'A-1' air resonance, and the port alters the 'air' pitch since it is in a location where it can 'hear' that mode.

Thanks Alan,

Interesting that nearly everything went up. I would have expected theair to go up but main plates to go down. We'll see what happens...

Al, how do you identify the resonances? (Fft, chladni?)

Sure enough, plates went up about 4hz each.

I use Chladni patterns to get plate resonances, and find air resonances with a dB meter at the hole/port, or, in some cases, a mic-on-a-stick inside. There are pitfalls no matter how you do it. Changing the way the guitar is supported will alter some resonances, and, of course, the patterns/pressures you're seeing are the behavior of a coupled system. Thus, to give the simplest example, the 'main air' resonance the mic picks up is not the same thing as the 'Helmholtz' air mode that you'd see if the walls of the box were immobilized. You can do things to get closer to the 'real' resonances, but then what you're seeing is not what the guitar is going to do in performance. And so it goes.

FWIW,

The change in sound is so subtle as to not be worth the effort of doing it at all IMO...just adds 100$ to the guitar and looks weird.

What size hole did you cut?

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EddieLee

About like so. I'm going to expand it a bit, as it is causing a wolf note on the g on the low e string, as the main air is now g + 13 cents...too close for comfort...

Had to go from that to this...

Using this...

To clean up this...

To end up with this...

This...

And this...