Why Do Bowed Strings Have F-Holes?
We’ve covered a bit on the history of bows, and the wild route that lead to pernambuco becoming the wood of choice, we’ve covered a bit on scrolls, and also general violin making… so it stands to reason that we might explore the history of the f-hole! So to the query of “why do bowed strings have f- shaped sound holes”, the true answer to the questions is “we don’t know"!
So there you go. End of article.
Just kidding— in order to really dive into the question, let’s separate it into its two facets:
1.) Why do instruments need sound holes?
2.) Why are the sound holes on modern bowed strings shaped like “f’s?
The first question is a little more straightforward to answer: acoustic instruments need sound holes to let air move into and out of the instrument.
As the strings vibrate (or other sound-producing elements like hitting a drum head, or strumming a kalimba), they cause the instrument’s body and the enclosed air to vibrate as well. The sound holes allow this trapped air to move in and out of the body, creating changes in air pressure that radiate outward as sound waves. The size, shape, and placement of the openings all affect how air moves.
This is where we start getting into the interesting territory of sound hole shapes— as it turns out, not all sound holes are created equal!
The shape and placement of the sound hole very much affects how much air can move in and out of a chamber, and it also affects the amplification of the sound. Even more interestingly, the placement and shape of the sound holes significantly changes the way the top plate of an instrument can vibrate and flex.
There was a somewhat steady progression in the shapes of sound holes from the beginning of bowed strings to the birth of the violin, that looked somewhat like the picture below (this, of course, varied greatly over the vast swathes of the world where bowed strings existed):
And, as you can see in the diagram below (from an MIT study), it turns out that the acoustic airflow is far greater along the sides of the opening of a sound hole than in the center:
This means that in the case of a round sound hole, there is a large center region that is basically acoustically inactive. As maker progressively narrowed the sound holes and placed two on the top of the instrument-- one on either side of a bridge, they were increasing how effectively the sound hole was converting internal air vibrations into radiated sound power (air-resonance power efficiency).
By elongating the opening into the current f-shape, instrument makers managed to maximize the perimeter length to roughly double the instruments' air-resonance power efficiency.
Additionally, the elegant f-shaped hole does and astounding job of giving hte top plate of an instrument tremendous flexibility (which is very helpful for maintaining resonance at different pitches) by decoupling the center section from the outer edges.
If you would like to do a deep dive into how the plates of a violin resonate differently depending on different pitches, Strad 3D is a very interesting resource! You can see a quick snapshot below of the Jackson Stradivari Violin vibrating at about A440 and about A1760.
At the snapshot in time, the red would be moving away from the observer, and teh blue would be moving toward the observer (and the level of saturation in the color illustrates how far the plate is moving).
As to the question of when did the “standard” sound hole become “f” shaped:
By sometime between about 1530 and 1550, the violin was taking the form that we recognize today, and it would appear that sound holes for these instruments evolved into the f- (or s-) shape at the same time.
The earliest known work featuring clear “f” shaped holes on an instrument can actually be found on a fresco by Gaudenzio Ferrari painted in about 1535 on the cupola of the Cathedral of Saronno (about a two-day walk from Amati’s workshop in Cremona). As you can see, one instrument, a Viola da Braccio, is shown with fairly modern-looking f-holes. Another, a bass viol of some sort, is shown with f-holes that are inverted from what we would consider a modern f-hole.
Once Andrea Amati came around and invented the violin, he made the instrument with “f” holes, and the rest is history.
As to the question of why did the f-shape become truly standard rather than the inverted f on the viol above, or a decorative “I”, or some other similarly resonant shape, there is no clear answer!
Some people think instrument makers decided on an “S” for the word “Suono” or “Sonus” or “Sound” (and then added the bridge notches and accidentally formed an “f” instead), or that they decided on “f” for “forte” or “loud”, but there is no currently known historic record that shows exactly why this shape became standard.
One thing is for certain though: they sure are pretty, and they give makers a wonderful opportunity for artistic expression!
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