I have a customer request for an archtop with no f-holes. I have never made an archtop without f-holes and I'm curious what the input from this group is on this topic. I have found a number of archtops out there on the market that have no f-holes, some have side ports some nothing at all.
If you have any input I'd like to hear it.
No f-holes?
-
Alan Carruth
- Posts: 1266
- Joined: Sun Jan 15, 2012 1:11 pm
Re: No f-holes?
The holes are there mostly to provide a low pitched 'Helmholtz' type of air resonance, which reinforces the fundamentals of the note in it's vicinity acoustically. Depending on the size and location of the holes this could be pitched anywhere from about F on the low E to A# on the A string, with bigger holes producing a higher pitch. A lower pitch tends to sound 'warmer' or 'darker', and a higher pitch is often heard as more 'forward' or 'bright'. Adding a port raises this pitch, and can direct higher frequency sounds toward the player if they can see into it. If there's no 'normal' hole the port just takes it's place.
That low 'main air' resonance is a help acoustically, but can be a problem for a guitar that's mostly played through an amp. It's a major source of feedback: "if the room can hear the guitar the guitar can hear the room", and with enough gain in the loop you'll get feedback (think 'Jimi').
The rational behind a hollow body with no holes is that the 'wood' resonances, and the internal 'air' resonances that don't normally communicate with the room, can still 'color' the sound through the amp. Top motion due to those resonances affects the way the strings vibrate, and drive the pickup. With less energy being 'lost' to the 'Helmholtz' resonance the low strings can actually have more sustain through the amp than they would on a guitar where the air resonance is stronger. Bridge motion in that range is less than on a full acoustic (but more than a solid body, of course), and that contributes to better intonation.
If I were making one like that I'd pretty much do everything else in the 'normal' way, but just not cut the holes. I got fairly close to that once. I made one with a small outline and shallow ribs, with a maple top and only a single hole. The magnetic pickup was mounted on a heavy wood bar that ran from the neck block to the tail block. It went through a hole cut in the top that was fairly tightly fitted to the PU, but it didn't touch the top or add any weight to it. It worked pretty well until UPS 'modified' it for me when it was being shipped back from a show.
That low 'main air' resonance is a help acoustically, but can be a problem for a guitar that's mostly played through an amp. It's a major source of feedback: "if the room can hear the guitar the guitar can hear the room", and with enough gain in the loop you'll get feedback (think 'Jimi').
The rational behind a hollow body with no holes is that the 'wood' resonances, and the internal 'air' resonances that don't normally communicate with the room, can still 'color' the sound through the amp. Top motion due to those resonances affects the way the strings vibrate, and drive the pickup. With less energy being 'lost' to the 'Helmholtz' resonance the low strings can actually have more sustain through the amp than they would on a guitar where the air resonance is stronger. Bridge motion in that range is less than on a full acoustic (but more than a solid body, of course), and that contributes to better intonation.
If I were making one like that I'd pretty much do everything else in the 'normal' way, but just not cut the holes. I got fairly close to that once. I made one with a small outline and shallow ribs, with a maple top and only a single hole. The magnetic pickup was mounted on a heavy wood bar that ran from the neck block to the tail block. It went through a hole cut in the top that was fairly tightly fitted to the PU, but it didn't touch the top or add any weight to it. It worked pretty well until UPS 'modified' it for me when it was being shipped back from a show.
-
Freeman Keller
- Posts: 494
- Joined: Mon Feb 27, 2012 11:34 am
Re: No f-holes?
Along with the f-holes functioning as Alan discusses above it is my understanding that they have another function - weakening the top along the recurve on each side of the bridge. I recently built an acoustic archtop and did a lot of tapping with some spectrum analyzing software, when the f-holes were cut into the top it definitely became less stiff.
I guess you could always built the guitar without holes and string it up in the white - if you (and your customer) aren't happy with it add the holes.
By the way, one thing that was really interesting on mine was that when I closed the box (f holes were cut) I suddenly got peaks at 117 and 218 hz that weren't therre before - those have to be the air resonance that Alan talks about. Mine is a 16 incher so relatively small as archtops go.
I guess you could always built the guitar without holes and string it up in the white - if you (and your customer) aren't happy with it add the holes.
By the way, one thing that was really interesting on mine was that when I closed the box (f holes were cut) I suddenly got peaks at 117 and 218 hz that weren't therre before - those have to be the air resonance that Alan talks about. Mine is a 16 incher so relatively small as archtops go.
-
Alan Carruth
- Posts: 1266
- Joined: Sun Jan 15, 2012 1:11 pm
Re: No f-holes?
Those two peaks, at 117 and 218 Hz are most likely the 'air' and 'top' parts of the 'bass reflex couple' between the top and the air in the box. If you tape over the holes and try again the 'air' resonance should go away, and the 'top' resonance should drop a bit in pitch.
What I've found, using Chladni testing (the 'tech' version of 'tap tones') is that I'll start out by tuning the top without the holes. When they're cut the top does indeed become less stiff, with the 'ring' mode in particular dropping a lot in pitch and losing it's shape. I then glue in the rough bars, either 'parallel' or 'X' bracing, and the 'ring' mode jumps to a higher pitch, and the shape is still not right. Then I trim the bars, and the shape and pitch of the 'ring' mode both come back. So it seems as though the purpose of the bars is to replace the stiffness you lose when you cut the holes.
This brings up the question of whether a top with no holes even needs bracing. I can't be sure about that, since I've never tried it. On the one hand it seems logical that you could get by without the braces, but on the other I'd hesitate to go without them. Maybe use small ones? I suspect a lot depends on just how much break angle you build in, and what the arch shape and graduations are.
What I've found, using Chladni testing (the 'tech' version of 'tap tones') is that I'll start out by tuning the top without the holes. When they're cut the top does indeed become less stiff, with the 'ring' mode in particular dropping a lot in pitch and losing it's shape. I then glue in the rough bars, either 'parallel' or 'X' bracing, and the 'ring' mode jumps to a higher pitch, and the shape is still not right. Then I trim the bars, and the shape and pitch of the 'ring' mode both come back. So it seems as though the purpose of the bars is to replace the stiffness you lose when you cut the holes.
This brings up the question of whether a top with no holes even needs bracing. I can't be sure about that, since I've never tried it. On the one hand it seems logical that you could get by without the braces, but on the other I'd hesitate to go without them. Maybe use small ones? I suspect a lot depends on just how much break angle you build in, and what the arch shape and graduations are.