Brains and Membranes
Bassoon Reed Making
Chapter 4 – The Physicist’s Viewpoint
by Christopher Millard
Take a deep breath now. There are no equations here but some of the language will be challenging. I promise to translate these ideas in later chapters.
If you ask a physicist to describe the basic mechanism of sound production on a bassoon, you might get the following:
- 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 inertial size and dampening.
- When you increase pressure in your mouth and release the tongue – air starts flowing through the partly open reed.
- 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.
- 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.Unless you have studied physics or electric currents, impedance is a prickly concept. You can think of acoustical impedance 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, and accept the fact that when a pressure pulse travelling down the bassoon bore meets some open tone holes, it doesn’t just drain out into the room, but reverses direction and heads back home.
- Before this returning pulse reaches the bocal tip, the reed is vibrating at multiple frequencies. The initial blowing impulse got it vibrating with a very broad collection of frequencies (not at all-equal amplitudes, however).When the reflected compression pulse reaches this vibrating reed, the pulse’s higher air pressure tries to open the reed. This inhibits the reed from vibrating at many of its natural frequencies—in particular, those frequencies for which the reed is trying to close when the pulse arrives). In contrast, the reflected pulse reinforces the reed motion for many other frequencies—those for which the reed is trying to opening when the pulse arrives.
- In this manner, this first reflected pulse constrains 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
- 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 reed’s natural frequencies. Instead, you are hearing the 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.
So, that’s the physicist’s viewpoint. Let’s get back to the bassoonist’s viewpoint.
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. But if the reed is an independent sound generator (remember Skinner’s tuned oscillator?), how can it be so restricted by the choice of fingering and the length of the air column? If you finger C, the reed vibrates at 261 Hz? Why not 277 Hz? Why not 246 Hz?
The obvious answer is contained in paragraphs 5 – 7 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, 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.
The preceding ideas are going to need reinforcement, so I’m going to go back and explain them using different language.
Tune in next Friday for Chapter 5!