Brains and Membranes

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 converts the blowing energy of the bassoonist into the sustained acoustical energy within the bore of the bassoon. Sound in the bassoon is produced by the complex motion of compression waves within the bore of the instrument.

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.

This is a bit hard to visualize, so here is a little thought experiment to help you understand.

Imagine that you are all lined up as before, but this time the guy at the front of the 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 that occurs from the contained line of compressed people 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. Only a portion of the transferred energy escapes the first open tone holes, the reset start pushing in the opposite direction, all the way back to the reed where the first push started.

If wind instrument bores gave up all their energy to those first available openings, we would not have wind instruments. Let me repeat this in slightly different language: when the compression energy of the longitudinal wave meets the Grand Canyon of the open tone holes, most of that energy reverses direction and heads back to the reed, where the process will begin again. Compression waves are always followed by rarefaction waves, travelling back and forth 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.

This is pretty easy: in a violin, the tension of the four strings and the placement of the fingers determine the pitch In a bassoon, the length of the air column determines the note you play.

This is not so easy: just as a violin string operates with simultaneous modes – harmonics – so too does the bassoon.

We’ll get to that and much more next week!