Brains and Membranes
Bassoon Reed Making
by Christopher Millard
Chapter 9 – The Big Bounce
I want to introduce a new term to your reed making vocabulary,,,
A membrane is a pliable sheet or layer – especially of animal or plant origin.
It comes from the Latin membranus, meaning skin. Skin separates and contains what’s within the body – while remaining pliable, compliant and responsive to our external environment. Reed ‘membranes’ separate and contain what’s within the bore – remaining pliable and responsive to acoustical pressure variations, Bernoulli induced vibration, and the subtle manipulations of the embouchure.
The walls of the bassoon and bocal are stiff and only slightly responsive to bore oscillations. The top and bottom blades of our reeds, which are extensions of those walls, are extremely compliant. Wood, rubber linings and metal walls give way to the ‘living’ membranes of the pressure-controlled reed valve.
Timpani heads are stretched membranes originally made from calf skin. Their vibrational frequencies are determined by adjustable tension and to some extent by the natural frequencies of the drum they’re attached to. A timpanist can control the pitch of his drum by altering the membrane tension with her pedals. This process is at play when we are designing profiles.
Reed membranes share similarity to drum heads [higher tension = higher frequencies], but governed by longitudinal, radial and diagonal elasticity. Compared to the quite broad pitch flexibility available to the timpanist on a single drum, bassoonists deal with a much smaller range of frequency adjustments while fingering a single note. Our basic pitch is determined by the length of bore. But we also have a bit of control over pitch, the difference between a very relaxed membrane [a soft, flat reed] and a very stiff membrane [a hard, sharp reed] is usually less than a quarter-tone in pitch difference. Alas, in that small range of behaviour lies a universe of heartache.
Using the world membrane helps me to think of the behaviour of reeds in more holistic terms. I think we waste a lot of time thinking about how individual parts of the reed operate independently. You might recall my reference to Sol Schoenbach’s assertion that different notes came from different parts of the reed? This shift in language helps me think of each membrane – top blade or bottom blade – in a more interconnected way.
Have you ever jumped on a trampoline? Every part of it is interconnected. There’s no better way of imagining holistic, elastic behaviour.
Practice Milde while bouncing.
Reeds have a primary function – to vibrate, which means they modulate airflow while controlled and constrained by the oscillations in the bore. Their first task must be react to the input of air from our lungs. This require a profile – often thinner at the tip and the sides – in order maximize the efficient conversion of airflow into periodic vibration.
The idea of first blowing the reed closed is critical to understanding why we profile as we do. For the membranes to bend inwards they require the flexibility achieved by profiling. The Bernoulli effect is proportional to air velocity, and air moves fastest where the gap between the blades is minimal, so we profile the membranes to be more flexible in front and in the wings. Capitalizing on the Bernoulli sucking effect explains the preference to make tips and sides thinner than backs and centres.
We can’t make an attack if the tip is too thick or the sides inflexible. The membranes must come together willingly to begin the dialogue between airflow and vibrating bore.
The mechanics of input response are governed by the distribution of tension and flexibility across the surface of the membranes. From side to centre, front to back and diagonally, the entire top and bottom blades are connected at a cellular level.
The combination of profile and internal structure determines how efficiently the reed responds to the Bernoulli force, both initiating and maintaining the sound.
Jumping on trampolines is actually a pretty useless way to learn Milde. But, it’s a great model for visualizing some of what reed membranes actually do.
If bassoons produced simple sine wave sounds with no harmonics, the reed/membrane/trampoline/timpani analogy would be really easy to visualize.
But acoustic musical instruments all include sound profiles with rich harmonics.
Reed membranes that are happily compliant with a specific harmonic tend to be compromised in unexpected ways by other harmonic components. I’ll explain…
If you are still jumping on that trampoline, consider this: there is usually a very large area that gives you good lift. Trampolines become less efficient as you move to the edges, but they remain accommodating to all kinds of bouncing. But the minute a second person joins you the behavior of the trampoline becomes more complex. Imagine you have a dozen people bouncing: everyone’s bounce impacts the sweet spots for everybody else. If you all coordinate your jumps at a single frequency you can effectively manage the trampoline’s elasticity. But if you all have independent jumping frequencies, the sweet spots are not evenly distributed, creating chaotic uncertainty about each individual’s bouncing success.
Our trampoline bouncers include very slow jumpers with large mass and slower bouncing preferences, average size jumpers with average size preferences and little, itty-bitty, fussy tiny ones with very high frequencies. The trampoline membrane has to make sense of all this – slower fundamental harmonics all mixed up with faster overtone harmonics. All this jumping starts to assume a repetitive character.
The fattest bouncers are happy to jump at 55hz, their close friends choose to bounce twice as fast [110hz] and their relatives bounce three times as fast [165hz] and their children like to go four times faster [220hz].
So imagine each jumper represents a specific bouncing frequency in the complex sound waves within the bore. The compression-rarefaction waves that the bassoon naturally likes to utilize are all pushing and pulling on the internal membrane surface, vying for energy and reinforcement. Different frequencies ask for different modes of flexing within each membrane; slower and wider flexing for the fundamental and quicker, more narrow flexing for higher harmonics. This describes output response, i.e. the membrane complying with the acoustic needs of the bassoon itself.
The mechanics of output response are governed by the relative tensions of the membranes. The combination of profile, cane resiliency and internal structure determines how efficiently the membranes comply with the internal acoustical forces, both responding to and reinforcing the vibrational frequencies of fundamental and harmonics living in the bassoon.
There you go, a trampoline membrane operating as a musical instrument with a fundamental and a family of harmonics. Not exactly like a reed, but a useful analogy I hope.
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
Doodles & Design by Nadina