Brains and Membranes

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:

5. 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.

6. 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

7. 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.

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 262 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!