Let’s step back again and look at what happens when we place a reed on the end of the bocal. The ideas I’ve presented in the last chapters allow a fresh way of visualizing this extraordinary partnership between bassoon and reed.

During tone production there’s a whole regime of organized acoustical activity going on up and down the bore.  Conga Lines for sure, but also ‘secondary’ Conga Lines, representing all the harmonic richness that nature prefers.  Pressure waves moving back and forth, their complex frequencies stacked onto their simplest frequencies – a kind of bubbling cauldron full of acoustical information.  Acousticians use the term ‘regimes of oscillation’.  These natural behaviours exert a dominating influence on the pressure-controlled valve which supplies energy to the system and sustains the tone.

Let’s look again at this acoustical ‘information’: natural resonances take the form of compression waves travelling back and forth from the reed to the open holes. Regions of high pressure [compression] are followed in regular sequence by regions of lower pressure [rarefaction].  These alternations happen at frequencies determined by the natural bore resonances. At various points of the bore, including the bocal tip, we have maximal alternation of compression and rarefaction.  So, if the fundamental bore resonance is A 220 Hz, the pressure inside the small end of the bore alternates compression and rarefaction 220 times a second.  In order for these natural resonating frequencies to be sustained, energy has to be constantly reinforced by the reed valve. It converts your steady air delivery into timed pressure pulses precisely matching the frequency of those natural bore resonances.  The pressure alterations inside the bore constrain the reed valve to supply pressure pulses 220 times a second.  You might take moment and review The Physicist’s Viewpoint.

Imagine the reed sitting at the front end of this process.

Your reed’s primary responsibility is to convert blowing pressure into precisely timed ‘pulses’.  Those pulses have to be delivered at the rate of the fundamental resonance frequency of the note being fingered.

We have been entirely focused on connection between bassoon and reed.  Let’s shift our attention now and take look at the ‘front end’, the basic physics of airflow from your mouth into the reed. 

Stop and consider your physical relationship to the bassoon.  It involves your lungs blowing steadily through the two blades of the reed and your lips controlling both the aperture opening and the inherent tension of the cane.  But how does blowing air into the reed result in motion of the blades?

We usually just state the obvious; blowing causes vibration.  But how? It’s an interesting question.

What happens when you first blow air into the aperture?  Does the reed close or open?

The answer will depend on whom you ask and how they classify the nature of this particular kind of pressure-controlled valve. [*see footnote]  Surely, once a tone starts the blades will open and close ad infinitum.  Does it matter what the initial physical response of the blades is?  You would intuitively think the introduction of blowing pressure would force the blades apart, but the primary reaction to airflow is the opposite: the flow of air draws the two blades inward towards each other.

This is due to the fact that pushing air thru a restricted passage first increases its velocity, which then reduces the pressure on the inside surfaces of the two blades.   That seems a bit counterintuitive, doesn’t it?  But you can easily get a picture of this process by holding two pieces of paper, bent to simulate the aperture of a bassoon reed.

If you direct the airstream into the aperture you’ll be surprised to find that the two pieces of paper quickly come together, despite the fact that you’re delivering a lot of air pressure.  

Counterintuitive, right?  But remember, you’re not blowing directly AT a surface, you’re blowing BETWEEN two surfaces.  In 1738 the physicist Daniel Bernoulli observed this fact and came up with this important principle of fluid dynamics:

P + 1/2pv2 + pgh = constant

It states that for a non-viscous flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure in the fluid’s potential energy.

Sorry, I said I would keep the equations at a minimum, but this one is kind of important.  

Don’t worry about the math or how to decode the equation.  You just to need absorb what Bernoulli is describing:  the two sheets of paper want to come together because the pressure on their inside surfaces becomes lower.  Lower than the average atmospheric pressure on the outside surfaces and lower than the embouchure pressure you are using to seal and control the reed.

I’ll say it slightly differently: when air begins flowing through the reed – and begins its journey into an ever-increasing bore diameter – it accelerates.   The measurable pressures on the inside surfaces of both top and bottom blades are reduced.  This causes the reed blades to be ‘sucked’ together.   This is the ‘Bernoulli’ effect and almost every part of reed design is a reaction to this phenomenon.

Keep this key thought: blowing into a reed draws the blades together.  Everything in the shape and profiles of your reeds reflect this principle.

Chapter 7 – Look Both Ways

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

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* See Fletcher and Rossing, The Physics of Musical Instruments, 1991.  Chapter 13: Section 13.1: Basic Reed Generators