Sound hole placement...

[Darren Rahilly]

Hey guys, just wondering if there is a formula for placing sound hole, maybe it has to do with the scale length, or maybe nodes, like pickup placement? Also, since we're on the topic, sound hole size. Is that figured by volume of guitar body? Or maybe thickness of top?

[Alan Carruth]

This is one of those things that just 'falls out' as you satisfy other constraints, to some degree, and could well vary from one instrument to the next, depending on what you're after.

You need to have 19 full frets. Twenty won't hurt, but you need nineteen. That establishes the location for the upper edge of the hole. DAMHIKT

The hole size can vary a bit without too many consequences. The hole size is the primary determinant of the 'real' Helmholtz air resonant pitch, although the location has some effect. This, in turn, is the main thing that establishes the 'main air' resonance; sometimes referred to as the 'pitch' of the guitar. It's the note you hear when you tap on the bridge, and is the lowest pitched resonance that can produce sound effectively, so it has a lot to do with the low end response. The 'main air' pitch is always lower than the 'real' Helmholtz resonance, owing to the flexibility of the top (mostly) and the back (at least some). These influences vary a lot from one guitar to the next, so it's difficult to predict the 'main air' pitch in advance accurately. OTOH, for most purposes, it's not necessary to be really exact (except when it is...).

The hole size has effects in other tone ranges as well, which are not as well understood. It's a complex system.

Generally speaking, all else equal, and all of that jazz, the larger the sound hole the higher the 'air' pitch will be. The timbre tends more toward an 'open' or 'forward' sound with a larger hole, and may be a bit 'louder'; it's likely to be more powerful, but may not be perceived that way close up. A smaller hole will tend to sound 'fuller' but may also be 'covered'. Again, there's more going on here than meets the eye.

Traditionally there is not a lot of variation in sound hole size from what I can see. If you're using an established design, or one that's similar, stay with the hole size they used. If you feel like experimenting, you could leave a wide margin between the hole and the rosette, which would make it easier to enlarge the hole once the guitar is together. You could then do so by increments, and stop when the sound stops getting 'better'. If you want you could even put in veneer strips to reduce it and go back.

[Brian Evans]

I build archtops, and I find that when I build a traditional F-hole top I end up with a strong air resonance around Bb or C. If it's too strong I end up with a dead sounding note. I built an archtop with F-holes and a sound port on the upper shoulder, it had a stronger resonance around D (open 4th string) and almost a wolf tone - D notes on the fifth and sixth strings got avoided, they sounded quite dead. I tuned that guitar down a full tone to D-D, and for some reason the impact of the resonance kind of went away, no more wolf tones. Now to my point - I built an archtop with the sound hole in the upper shoulder - half in the top and half in the side (a-la Ken Parker's archtops). The main air resonance went down to low F (sixth string first fret), and that guitar has significantly better low end response than any other I've made. So, a definitive sample of one, but I found that fascinating. Some of Allan's writings seem to predict, or confirm, my experimental outcomes, I think.

[Alan Carruth]

There was a good article on 'The Basics of Air Resonance' by William Allen in American Lutherie #1; I'd strongly recommend it to anybody.

My most recent archtop was intended as a reasonable 'tonal copy' of a Loar L-5 that the customer had tried out. I did get to check it out, obtaining an 'impulse spectrum' at the shop where it was and talking over the measurements of the top grads that the shop owner had taken with a Hacklinger gauge. I noted that the F holes were in the usual place, and were the expected length, but were much narrower than most of what we see these days. The 'air' resonance was at 116 Hz; Bb. My 'sorta' copy came in with the air resnance at 115 Hz; close enough for government work. The customer was after a guitar for 20s and 30s style acoustic jazz, and it seems to work well for that. Sadly, we will porbably not be able to get the copy together with the original for a direct comparison.

[Darren Rahilly]

Huh... so bigger sound hole equals more bass, is that what I'm reading?

[Alain Lambert]

Huh... so bigger sound hole equals more bass, is that what I'm reading?

If you read Alan reply:

the larger the sound hole the higher the 'air' pitch will be

Higher pitch means less bass more treble

[Darren Rahilly]

Wow! So a smaller hole gives more bass... ok, got it. Thanks for the help on this
D

[Alan Carruth]

"So a smaller hole gives more bass... "

As usual, it's more complicated than that.

Using a smaller hole lowers the pitch of the 'Helmholtz' air resonance, but also makes it weaker. Usually we notice the pitch change more, but, especially in extreme cases, the lack of power can catch up to you. A lot of small holes together can act like one big one in terms of pitch, but like a small one in the energy output. Lute roses, or the pattern of holes on some Ovations, work like that. Moving a large hole out toward the end of the box will drop the 'main air' pitch a fair amount: this trick is used fairly often these days. It has other consequences, though. There are also what you might call 'audio illusions' that can make you 'hear' tones that are not even there. So, yeah, for the most part 'smaller=lower', but not always and in every circumstance.

[Darren Rahilly]

Hmmm... I know that when tuning a port on a speaker box. You pull the tube out until you're happy with the sound, and cut it there. Aside from cutting a small hole, stringing the guitar up, playing it, then unstringing it, enlarging hole, re-stringing etc until it sounds good, is there a way to formulate... I mean, it looks like most holes are about 4"... maybe thats a starting point?

[Alan Carruth]

Traditional designs have been worked out so that everything ends up about where it should. Note that we're not always certain as to why those particular values are 'correct' but departing very far from them may end up producing something that doesn't sound the way it should.

A lot of this is a matter of balance, in some ways. A very large hole will tend to produce a loud and 'forward' tone that could come in handy in some cases, but most some may find it unattractive. One reason there are so many different kinds of guitars is that they're used for different sorts of music in different circumstances that call for different sounds.

So, if you're looking to get the 'air' mode to come in at a particular pitch within a couple of Hz, I don't think there's a good way to do that. Trevor Gore came up with an Excel spreadsheet that he claims is pretty good at that, but he also said that it pushed the program to it's limits. This is not a simple problem, and often 'cut and try' ends up being the best way to a solution.

[Darren Rahilly]

I think i understand what you're saying. Ill use a j200 for reference in size and placement, so as to not deviate from what works. Thank you for all the tips and comments
D

[Alan Carruth]

A J200 Super Jumbo is a really large flat top in every respect. Is your guitar that big?

[Darren Rahilly]

Yes, even a bit deeper. Also, its a 30" baritone scale. Any tips?

[Alan Carruth]

For a baritone you may well want to go with a smaller sound hole, to drop the 'air' pitch in line with the lower tuning. I've got a baritone that a customer gave back to me when he was no longer able to play it. He knew that it is one on my favorite guitars, so now I actually have one of my own to play! It's a 16" small Jumbo, 4-1/2" deep, with a 3-3/4" sound hole. The main air resonance is at about 84 Hz, between E and F, which, since this is normally tuned to D, is at the third fret on the low string. The loss in power from the small hole is not really a problem, and the timbre in the low range is full.

[Tim Allen]

Darren, I have been following your and Alan's posts with interest. I built a D sized baritone with a 28 1/4" scale, and made the soundhole 3 7/8". I can't tell you if it would be better with a larger or smaller soundhole, however. After building this one, I guessed that if I built 8 or 10 more, varying the parameters a little bit, I might actually know something. If I had a team of elves poised to execute all my projects, I'd set them to it, but as a hobbyist builder, and a slow one at that, I'm not set up to develop numerous prototypes.

When I read "I know that when tuning a port on a speaker box. You pull the tube out until you're happy with the sound, and cut it there,'" I imagined Neville Thiele spinning in his grave. Yet in instrument building there's nothing wrong with experimenting and using your ear to guide you. A guitar soundhole with a tube behind it is the equivalent of a ducted port in a loudspeaker. Since you seem to have a bit of a yen for experimenting, you might consider adding a duct to your guitar's soundhole. If you search the Internet for "tornavoz" you can find out how this has been done previously. For example, https://www.classicalguitardelcamp.com/ ... hp?t=67245. I have experimented with temporarily taping thin cardboard tubes behind soundholes and confirmed it affects the sound of the bass. Further investigations were precluded by the elf shortage.

One other thing about baritone design--with a scale as long as 30" you probably are not going to be able to compensate the strings by carving a slanted 1/8" saddle, so you may want to consider alternatives, and do some checks with a mock-up, before you construct your bridge.

[Darren Rahilly]

Wow, thanks guys! I didn't expect so much info/tips, but am happy to get them. The elf shortage is due, I'm sure, to covid. The ported sound hole sounds intriguing. At the end of this one I may look into it. It sounds REALLY intriguing!
This baritone is a little over 16" at the lower bout, and a bit thicker than Alan's, so there is my starting point. I know compensation is going to be tricky. I see what you did and I like it. Looks like something I may try!
Thanks again, guys!
D

[Darren Rahilly]

Tim. My next build is going to be a 26" scale. Not a bari but a deep-voiced jumbo.this is the guitar i will be looking into "porting". Ill be in touch...
D