Brains and Membranes

Bassoon Reed Making

Chapter 4 – The Physicist’s Viewpoint

by Christopher Millard

 

I was never very good at math or physics.  Equations don’t come easily to me, so I will refer to them sparingly when I can.  I’ll do my best using English…

If you ask a physicist to describe the basic mechanism of sound production on a bassoon, you might get the following:

  1.  The reed sits in your mouth in its resting state of equilibrium. As you consider the note you are about to play, you anticipate the aperture control this note will require, and make a small adjustment to its initial size using embouchure dampening.
  1.  When you increase pressure in your mouth and release the tongue – air starts flowing through the partly open reed.
  1. The accelerating movement of air through the reed causes the pressure on the interior surface of the reed to drop (!!!), drawing the blades together, reducing and eventually stopping the airflow momentarily. After the air flow stops, the natural resiliency of the cane causes the reed to spring open. Its resulting “outward” momentum causes it to overshoot its equilibrium state – thereby opening more widely than at its resting position. In all, this motion gets the reed oscillating at all its natural frequencies – including many that are not associated with the frequency of the note you are trying to play.
  1.  At the same time, the abrupt beginning of airflow causes a pulse of air to travel down bore of the bassoon. When it reaches the first open hole (the Grand Canyon in in my Chapter 3 metaphor) it encounters a change of impedance, which induces a partial reflection of the pulse back up the bassoon towards the reed end. Impedance is a prickly concept; you can think of it as the ratio of sound pressure in air to the velocity of the air particles.  Measuring the acoustic impedances of a bassoon bore is a way of determining the natural resonating frequencies for a bore.  But we’re going to kick the word ‘impedance’ out of the conversation.  Just know that when a pressure pulse travels down the bassoon bore and meets some open tone holes, it doesn’t just spill all its energy into the room.  Much of its energy reverses direction and heads back home.
  1. As this returning pulse reaches the bocal tip, it finds the reed is vibrating in a disorganized way.  The initial blowing energy got it vibrating with a very broad collection of frequencies, with unequal amplitudes. When this returning compression pulse reaches the vibrating reed, the pulse’s higher air pressure tries to open the reed, and it inhibits the reed from vibrating at many of its natural frequencies.  It particularly dampens those frequencies for which the reed is trying to close when the pulse arrives, while reinforcing the frequencies for which the reed is trying to open when the pulse arrives.
  1. In this manner, this first reflected pulses constrain the reed’s vibrations and encourages them to be consistent with the natural frequency and harmonics of the bassoon tube. Bassoon makers have expended huge efforts over the last 150 years to make these natural frequencies pleasantly related and in tune with each other. These natural frequencies are immensely complicated functions of:
  • the tube length to the first open hole,
  • lengths to closed holes below that first open one
  • all the details of the bore diameter, including cross-sectional-area “bumps” in the bore caused by closed holes,
  • the volume of the bore,
  • the tendency of the bore towards being either cylindrical or conical
  1. As more and more pulses travel up/down the bore, they even out and set up a standing wave in the bore for each fundamental and its participating harmonics. This equilibrium is reached quite quickly, because the pulse velocity in the bore is essentially very fast (about 1 foot per millisecond). Remember my description of the ‘peeping pitch’ – the simplest tone you can get on a bassoon reed?  Well, when you take a finished bassoon reed and peep on it lightly enough that no ‘crow’ starts, you are not hearing the cane’s natural frequencies. Instead, you are hearing a tiny standing wave set up inside the reed/tube and caused by impedance-discontinuity reflection at the lower, open end of the reed tube. The reeds natural frequencies are much higher and are very “broadband” because of the graininess of arundo donax.

As the next chapters unfold, I’ll try to explain the above in different ways.  Have a drink.

Viennese bassoon with beer

If you’re fingering a middle C [262 Hz], no matter what you do with your blowing pressure and embouchure, you’re not going to produce a D. The most you can do is play the C flatter or sharper – unless you build a reed that’s four inches long or the size of a toothpick.

It stands to reason that something about the behaviour of the reed is determined by how many keys you have open or closed.   If the reed was an independent sound generator – remember Skinner’s tuned oscillator? – how could it’s behaviour be restricted by the length of the air column?  If you finger C, the reed vibrates at 262 Hz? Why not 277 Hz?  Why not 246 Hz?

The answer is contained in paragraphs 5 – 7 of the Physicist’s Viewpoint above. The bassoon itself is exercising massive control over the behaviour of the reed.  The delivery of acoustical energy may start as a one-way street – blowing into the reed and into the bassoon – but sustained tone at the correct pitches turns out to be a two-way street.  You can even go so far as to describe it as a master/servant relationship.  And the reed is the servant!

And perhaps the best way to describe it is this – a reed is a pressure-controlled valve.  It’s not your blowing pressure that controls; rather it’s the acoustical pressure variations within the bore – the constant alternation between compression and rarefaction – that govern the reeds behaviour.

The bassoon itself is the real source of the sound; the reed just supplies the ongoing energy.   

This is likely to be a shift in how you imagine and visualize your reeds.  As these ideas are going to need reinforcement, I’m going to go back and explain them using different language.  Keep reading.

Chapter 5 – The Big Picture

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

Art by Hermann Armin von Kern