The Bassoon Reed as Wind Tunnel
In a previous post I've described the bassoon reed as a valve and a lever. In this post, I'd like to explore another simple model of fluid mechanics that relates to the bassoon reed.
Let's think of the space inside the bassoon reed as a wind tunnel. After all, that's the chamber through which our wind passes. We're all familiar with how airplane and auto designers test their models in wind tunnels. Designing the shape of the body of a car for the least amount of wind resistance is called streamlining.
When we adjust the wire roundness, ream, gouge, scrape the profile and shape we do things that effect the shape of the cavity inside the reed blank. These adjustments effect the flow of air through the reed. When testing a reed, we feel these adjustments make the reed more or less resistant or more or less vibrant.
Wire roundness -- Why does a reed with rounder wires have more resistance? The greater resistance is caused by a faster flair from the slit of the reed tip opening to round tube. A rounder first wire and completely round second wire cause the inside of the reed to approach roundness earlier in the journey from tip to tube. As the air rushes through the tip it is slowed by eddies created by the air fanning out into the larger space of the flair.
Think of a small creek that has a bottleneck caused by a branch or some rocks. A small, fast current of water rushes through the bottleneck. However, once past the bottleneck what happens? The current dissipates and fans out creating eddies and slowing and dispersing the direction of the water. This is what happens inside the reed as the slit-like opening of the tip gives way to the increasing roundness of the throat and tube of the reed.
A reed with more oval wire shapes prolongs the slit shape and delays the flair. This allows the air to move faster through the reed, causing fewer eddies and less resistance. That is why a reed with oval wires feels more free and less resistant to the player.
Ream -- Reaming the reed, especially in the throat area makes the reed more resistant because it enlarges the space inside the throat. Remember, air moves more quickly through a narrow passage than through a more open one.
Gouge -- A gouge that is elliptical (thinning towards the sides, thicker in the center) can help support the sides of the reed when pressure is applied by the lips. Gouging away the softer part of the cane (the pith) and leaving the harder cane nearer the bark makes the sides of the reed stiffer and less prone to collapsing under pressure.
Also, many players (myself included) sand the gouge before processing the cane. This has a direct effect on the sound wave inside the reed. Think of the acoustics of a room with a wood floor versus one with carpet. The carpeted room will be more acoustically dead because each carpet fiber catches and deadens sound waves. The wood floor, being more uniform in surface acts more as a reflector of the sound.
The sanded gouge has many of the microscopic peaks and valley of the cane's grain flattened out. The sandpaper flattens the peaks and distributes the cane "sawdust" into the valleys, leaving a flatter surface that is more reflective.
Shape -- The shape of the reed has a direct effect upon the shape of the "wind tunnel" for obvious reasons. Again, the theory here is that air moves faster through a space that has less change in its dimensions throughout.
I'll leave discussion of the way scrape effects the internal dimensions of the reed's "wind tunnel" for another post as this is a much more involved subject.