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Permalink Reply by Ron "Oily" Sprague on June 4, 2017 at 8:47am

Nice work, Rob, and welcome! Re sound holes: cut ONE, and only ONE, in the side of the box facing up towards you, since you seem to want to preserve the box face. Make it roughly the size of a US quarter, no larger (you may only have that much room, anyway).

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Permalink Reply by Rob Zamites on June 4, 2017 at 9:29am

Heya Oily,

Thanks for the suggestion, I will be very careful and only make *one* soundhole. I'm not gluing the top down yet, since I want to throw a piezo into this art some point very soon. #2's scarf joint failed, mostly because cutting with a handsaw isn't nearly as accurage with flat surfaces as it would be suing a scroll/band/miter saw to make the cut. I'm making due though :)

"Punch" sounds much more full than the "BBG" I bought on a whim from Elderly. Longer scale length and fatter strings sounds so damned good!

Thank again for the advice.

Rob

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Permalink Reply by Ron "Oily" Sprague on June 4, 2017 at 1:09pm

Rob, my pleasure. Now, lemme tell you my logic about WHY one quarter-sized sound hole. Because I can't help myself ;-)

A number of years back, a few of us did some experimenting, based on a fair amount of acoustic theory, plus some experimental work done at a couple universities, and some actual soundboard tests. We found that a quarter-sized soundhole, or smaller, gave the best bass response for a CBG. All CBG boxes have a smaller volume of air than nearly any acoustic guitar you can name. And most CBGs can be accused of sounding somewhat tinny, not growly (string choice, scale length, and a host of other details influence tone, too). We found that the larger the soundhole, the more treble frequencies were expressed, while the inverse was true for bass frequencies. Most players want to try to give their 9"x7"x1-1/2" cigar box that deeper growly tone (I have another way to do this acoustically, but it requires refinement and further experimentation); most won't build themselves a 13"x10-1/2"x3-1/2" box to get that deeper tone (wine boxes can get you there). Bottom line, you gotta push more air to get more bass, and let just a little bit of soundboard edge vibrate to enhance those frequencies.

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Permalink Reply by Rob Zamites on June 4, 2017 at 5:08pm

Thanks for the details - it's great to find out things like this without reinventing the wheel. as it were.

The $30 pre-made rig I go is louder than Punch, but I've determined it to be the f-holes in the front surface. I'm not married to Punch remaining pristine, however, I'm wondering of a sound hole in the top will provide that acoustic bass response similar to the f-holes on the cheap rig?

Any insight before I cut is greatly appreciated.

Rob

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Permalink Reply by Ron "Oily" Sprague on June 4, 2017 at 11:14pm

Punch boxes are usually renowned for their loud tone anyway. Lotsa people here swear by them.

About f-holes: turns out that the development of f-holes in violins followed a specific path to enhance tone generation. They started out as just plain holes. Remember also that a violin top is arched. As f-holes developed from holes, to c-holes, to narrower f-holes, it was noticed that you got better frequency response and projection (although not expressed that way at the time). Modern experiments looking at violin f-holes show that the offset due to the arching soundboard is also important. The uneven edges of the violin f-holes act like vocal cords, vibrating to specific frequencies. What does that have to do with CBGs? Well, CBG tops are not arched, and are usually quite a bit thicker than your average hand sanded violin arch top. But you can get a crude approximation of the violin f-hole response by placing the bridge between the f-holes (notice where the bridge sits on a violin...). The string vibration drives the bridge, which flexes the soundboard, which compresses the air inside the box, like a loudspeaker.

The difference in putting a soundhole in the side of the box facing up is that there is a lot less opportunity for flex, since the bridge is not driving that side of the box. In this respect, the soundhole in the side acts more like a bass reflex port, as seen on many loudspeaker designs. In this instance, the compressed air inside the box is following a somewhat longer path to the soundhole, emphasizing bass response. Try a test: play the Punch box without a soundhole for awhile. Hear it. If you have a smartphone, there are apps like AudioTools which can measure sound pressure in decibels (db). That gap in your neck slot will act like a soundhole. Measure the db from 1 foot away. Then cut your bass port. There will be a small, but measurable, difference, in both tone and db level. Realize, too, that you can always cover up that bass port if you don't like the tone.

If you're going to install a piezo in it, cover the piezo in silicone, and attach it to the bass string side immediately beneath the bridge. Coating it in silicone, or making a wooden or leather wafer to surround the piezo, will dampen unwanted higher frequency noise.

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Permalink Reply by Rob Zamites on June 5, 2017 at 3:10am

Oily,

I like your style. Applying science to a musical instrument is right up my alley. I'll leave Punch closed up for now (I need funding to get the piezo and other goodies), and will get some baseline measurements.

I'll also get measurements of the other instrument with and without the F-holes covered up, you know, just for science! Thanks so much for your guidance, I've build other instruments, but that required a whole freakin' wood shop, and it's not practical in a 3rd floor apartment (that landlord got test with me trying to sneak in a bandsaw).

I'll report findings later (after I play around more!)

Rob

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Permalink Reply by Ron "Oily" Sprague on June 6, 2017 at 12:33am

Rob,

I sensed the Force in you. ;-)

Don't let the scientific aspects of this overwhelm the fun of building and playing, however. Make instruments, make music, make people smile, laugh, and cry.

If you know WHY you're doing it, it just makes it that much better.

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Permalink Reply by JL on June 6, 2017 at 1:11am

There was a link here awhile back to a good article where a university measured the sound holes, round/C/F and it turned out the narrow slit properties of an F-hole also made the air move faster through them giving more volume.

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Permalink Reply by Ron "Oily" Sprague on June 6, 2017 at 12:12pm

Yeah, I tried finding it but couldn't. May have to look again...

Ha! Found it:

http://www.cigarboxnation.com/forum/topics/the-science-of-f-holes

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Permalink Reply by Rob Zamites on June 6, 2017 at 12:38pm

Wonderful! Thanks for finding that article - bookmarked!

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