Brains and Membranes
Bassoon Reed Making
Chapter 3 -Surf’s Up!
by Christopher Millard
Surf’s Up!
A bassoon without a reed is like a violin without a bow.
Just as a bow serves as the energy conduit from bow arm to instrument, the reed connects the blowing energy of the bassoonist to the sustained acoustical energy within the bore of the bassoon. Sound in the bassoon is produced by the motion of compression waves within the bore of the instrument. Comparing violins and bassoons can help us understand…
Sound pressure waves on a violin string act transversely; when you pluck a string it vibrates back and forth, while remaining fixed at both ends. We are used to seeing various kinds of transverse waves – at the beach on or two-dimensional diagrams. When we watch ocean waves moving towards land we know that the water molecules themselves are not travelling very far; it is the travelling energy of the wave that we see moving forward. Water is essentially non-compressible, so the waves must assume peaks and troughs. Because air is compressible, wind instruments function using longitudinal waves. We are so used to seeing transverse waves – at beaches and on violins – it’s initially difficult to visualize longitudinal waves.
So, here is a little thought experiment to help you understand.
Imagine taking a group of eager bassoonists and lining them up in a row – all facing one direction. Each puts their hands and the shoulders of the person in front, like a Conga line. Now, imagine that someone bumps the person at the back of the line and nudges them forward a bit. This would cause that person push into the guy in front of him, who would then push into the lady ahead of him, and the initial bump energy would transfer from that first bump all the way to the front of the line. This is how compression waves travel in an instrument, each molecule being pushed and itself pushing, until the initial input of energy comes out at the end of the bore.
Imagine a compression wave moving from the inside of the reed towards expanding bore of the instrument. What happens when its energy meets the first open tone holes or the end of the bell? Surprisingly, a great deal of the energy is reflected back up the bore. Thank goodness.
Imagine that you are all lined up as before, but this time the guy at the front of the conga line is standing at the cliff edge of the Grand Canyon. When the girl behind him pushes, he is going to yell really loud and try not to fall. He is going to try and resist the transition from the contained line of the people pushing him forward into the vast open space ahead of him. So, he screams and releases some of his energy. Then he leans back – relieved that he didn’t fall – and starts the whole pushing process in reverse.
This longitudinal back and forth is the way sound waves act in a bassoon.
The guy at the front of the line emits some energy when he screams, but he doesn’t quite make the big jump. In our conga line analogy, the first open tone holes are the cliff. So, only a portion of the energy in that compression wave escapes the first open tone holes. The bulk of the energy is reflected, reversing direction and moving all the way back to the reed, where the initial ‘push’ began!
If wind instrument bores gave up all their energy to those first available openings, we would not have wind instruments. Let me say this in Conga Line Language: when that longitudinal compression wave meets the Grand Canyon of the first open tone holes, most of its energy reverses direction and heads back to the reed, where the process will begin again. Compression waves are always followed by rarefaction waves, travelling forward and backward in the instrument. Because this all occurs at the speed of sound, the alternation of direction happens many times a second. The frequency of that directional alternation determines what we call pitch.
Violin strings have natural frequencies determined by their diameter, tension and length. Bassoons have natural frequencies determined by the length, internal diameter and taper of the bore. Violinists control pitch by shortening strings with the fingers of their left hand. As bassoonists, we have control over pitch by modifying the length of the bore according to tone holes and keys we open and close. Longer bores produce longer wavelengths [more people in the conga line] and lower pitches. Shortening the bore produces shorter wavelengths and higher pitches.
In a violin, the tension of the four strings and the placement of the fingers determine the pitch. In a bassoon, frequency is determined by the length of the air column.
But as we will see, this is all a bit more complicated; those violin strings vibrate in multiple modes simultaneously in order to produce harmonics. Wind instruments do this as well. But first, let’s talk about what happens when you blow into the damn reed…
Chapter 4 – The Physicist’s Viewpoint
If you want to understand more about the bassoon, its tone holes and its construction please follow this link… A Cruise Down the Bore River
Chapter 1 – Starting Down The Path Chapter 2 – It’s A Simple Question? Chapter 3 – Surf’s up! Chapter 4 – The Physicist’s Viewpoint Chapter 5 – The Big :Picture Chapter 6 – We’ll huff and we’ll puff… Chapter 7 – Look Both Ways Chapter 8 – Dialogue Chapter 9 – The Big Bounce Chapter 10 – The Incredible Shrinking Bassoonist Chapter 11 – A Useful Equation Chapter 12 – Goldilocks’ Dilemma Chapter 13 – Stairway to Heaven Chapter 14 – Reed MyLipsChapter 15 – Resonance Chapter 16 – Corvids & Cacks Chapter 17 – Chickens and Eggs Chapter 18 – Chiaroscuro Chapter 19 – Donuts Chapter 20 – Balance