The cover of my high school calculus text featured a close-up of a string instrument, with a particular focus on the sound hole. It was pretty slick cover art — although they’re unrelated, the integral sign and the f-hole look like each other, so that symbolism worked perfectly as shorthand for Calc I.
But what do those sound holes on violins actually do, and why are they shaped that way?
Luckily for us, MIT acoustician Nicholas Makris has devoted a considerable amount of time to investigating the history and function of the f-hole. Ironically, he became interested in the subject when he was trying to take a break from work. Makris took up the lute as a hobby, wanting to concentrate on nothing but making music. But then a fellow lutenist approached him with an interesting question: Did the intricate rose-shaped carvings (rosettes) on the instrument’s body have any meaningful impact on sound, or were they just there for decoration?
The curious Makris decided to look into it, and spent seven years leading a study examining the design changes in the sound holes of string instruments, looking at 470 Cremonese (think Amati, Stradivari and Guarneri) violins. There were two interesting takeaways: The standard f-holes we know today are the product of a process strikingly analogous to biological evolution, and they are far superior in shape to other versions.
When you look at much older visual representations of bowed string instruments, you might notice that, apart from the general shape, the holes on the front look a bit different from what you may see in the concert hall today.
For example, there are decorated c-shapes, as on this 1745 viola de gamba …
… or plain semi-circles, which you can hear in action on this medieval fiddle.
What’s cool about all of these shapes, though, is that they’re effectively serving the same function: to increase volume. According to Makris, it helps if you think about the air in the hollow body of a violin like a spring or a body of water. When you exert force on it, it bobs right back up, oscillating with successively weaker energy. But sound holes give that vibrating air a means of escape, allowing it to flow gracefully into your ear.
As far as the f-shape is concerned, it turns out that the more slender and curved the hole is, the more air can escape. This might seem a bit counterintuitive at first — after all, wouldn’t an open circle allow maximum release? Not necessarily, because we have to account for something called Helmholtz resonance, which is the vibration of air moving across an open cavity. A great example of this is the classic glass bottle, a chamber of gas with an opening at the top. Blow across it, and you produce sound. But since all of that airflow is at the edges and not in the middle, Makris found that when the hole is too open, a lot of the flow in that resonance isn’t doing much of anything it all — rendering it dead and useless. But when it’s streamlined, air is released from the body more efficiently and at a higher volume.
As you can see in the images above, the evolution of these “streamlined” holes is a long one, progressing over centuries from a semi-circle to the recognizable f-shape on the violins made by the Amatis, the first major luthier family in Cremona, Italy, who worked from the 16th to the 18th centuries. This is where that second idea — that the current version is the product of evolution — comes into play. As the team examined the 470 violins from the Amati, Stradivari and Guarneri families, they found that the f-holes got increasingly larger … as if by mistake. Makris’s explanation is simple, but interesting: They mutated.
Watch: The Incredible Sound of Making a Violin
Imagine you’re an Amati luthier, and you’re setting out to make a copy of a violin that sounds absolutely amazing. Everything is done by hand, so there are inevitably going to be some variations in each instrument you produce. Makris puts it this way: “You make four copies of the violin, each a little bit different, and one of those stands out and is actually a little better than the original. So you use that as your model for the next round, and so on.” From that idea, it’s not difficult to surmise that such a process led to, over centuries, the f-holes growing in size, landing us where we are today.
But that leads us to another question: Why did they (ostensibly) cease to evolve? It’s possible that these three particular families never revealed the specifics of how they made their instruments, so it’s likely the secrets of their craftmanship died out with the last of them. “There was just no transfer of know-how,” said Makris. “Violin manufacturing restarted in the area, but there was no direct link.” Effectively, he says, they became a sort of gold standard, and luthiers spent time trying to reverse-engineer the old violins they loved so much, standardizing it all in the process. Although the luthiers of old improved on their designs constantly, “It’s not clear how much they knew about the ‘why.’”
Those families in Cremona were chasing perfection, and in the process, it’s possible they gave us optimal sound holes … by accident. Ah, the mutations of evolution.
