Brains and Membranes

Bassoon Reed Making

Chapters 2 & 3

by Christopher Millard

Chapter 2 

Can you explain how a bassoon reed works?” 

 This is the first question I ask of a student reed maker. It’s an incredibly difficult question, as it implies some knowledge of acoustical physics.  Have you noticed how few music schools offer acoustical physics as required courses? The answer to this question reveals a whole lot about the student’s ability to visualize and verbalize.

The first response is always a look of consternation. After a few moments of helpless uncertainty  the tentative answer might include:

  • “It produces the sound, right?”
  • “It vibrates, but I’m not completely sure what that means.”
  • “Well, it makes a crow, so I guess the bassoon transforms that crow to produce the different frequencies and the quality of sound.”

From the very first day we hold a bassoon in our hands, we experience the reed as the primary connection between body and bassoon. We feel its vibration with our lips – it’s a very personal and interactive relationship.  We can even taste it!

As we try to master the basics of the instrument, we are painfully aware that reeds seem to determine response, intonation, articulation and sound quality – as well as our emotional state! We develop a largely subjective vocabulary about reeds:  resistant, unresponsive, hard, bright, dark, buzzy, muffled, tubby, flat, harsh, sharp or simply bad. Considering how much time we devote to reed making, it’s alarming how little we understand. Most remarkably, we can become quite expert at producing good reeds and still not understand them.

It’s logical to conclude that because the reed sits at the tip of the bocal [and is the source of so much grief] it must be the actual source of the sound itself. Bassoonists tend to think of their reeds as independent sound generators that are adjustable in all sorts of subtle ways to deliver either a beautiful or a horrible tone, good or bad intonation and flexible or inflexible response. We nurture a unidirectional view – where sound comes out of the reed into the bassoon – a one way flow of energy. The reed makes noise and the bassoon transforms this into tone and pitches.

Unfortunately, it’s not an accurate description. To find a better model, let’s consider sound production in other instruments.

The sound of a violin comes from the vibration of the string amplified and modified by the body.  Vibration occurs when the string is displaced from its resting position.  The energy of this motion is transferred to the violin body, which enriches the complex modes of the string’s vibration and excites the air molecules both inside and outside the violin.  This excitation occurs at specific frequencies, made sonorous by complex overtones.  The top and the bottom of the violin are actively vibrating, as is the air enclosed within the body.  These vibrations cause compression waves to move outward into the room and eventually engage our eardrums. 

Nature gives a stretched violin string a natural tendency to move back and forth at frequencies dependent on tensile strength, elasticity and length.  The most basic motion of a string is a simple displacement from end to end.  Strings also exhibit more complicated modes of displacement; while the string moves back and forth in its whole length, it also experiences motions in smaller segments.  These modes always act in a very predictable way, with the string dividing into halves, thirds, quarters, etc.  Each of these vibrations happens at different frequencies, all of which are related in a simple mathematical ratio.  We call the extra vibrational modes that reach our ears – harmonics.   It’s an extraordinary fact of nature that the string vibrates in multiple modes of displacement simultaneously.  We’ll come back to this subject in future chapters.  For now, just remember that musical instruments produce very complex vibrations. 

 The simple way to get a violin string to vibrate is to pluck it.  Pizzicato is a great musical tool, but because it involves a single act of energy input (one finger plucking), it can’t produce a sustained tone.  Guitars, with their very large bodies, extend the duration of their plucking significantly, but violinists need a better way of sustaining the sound.  By dragging stretched horsehairs across the string, the movement of the bow continually excites the natural frequencies of the string and we achieve a sustained tone.  It’s like thousands of pizzicato per second.

Violinists will pay a great deal of money for a good bow – and are meticulous about the condition and tension of the bow hairs – but I don’t imagine they ever think of the wood of the bow or even the horsehair itself as containing sound.  Rather, they understand a violin achieves a singing sound through the interaction of bow hair and string.  Notwithstanding the fact that well designed bows offer significant performance improvements, when a violinist says that “this bow sounds better”, she means “this bow produces a better sound” meaning “this bow gives me a more responsive interaction with the violin.”  Violinists implicitly understand that the basic tonal character comes from the violin; the bow is the means of supplying energy to those very expensive boxes.

Here is the big picture: the food that the violinist eats for breakfast is converted to stored potential energy in the body; the movement of the bow arm transfers this energy into the mechanical interaction of bow to violin, producing the acoustical energy we call tone.

 

Violin and bassoon

Chapter 3

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 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 tip of the bocal to the end of the instrument. What happens when that energy meets the first open tone holes or the end of the bell? Fortunately, a great deal of the energy is reflected back into the bore. Thank goodness.

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!

 

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