Brains and Membranes
Bassoon Reed Making
by Christopher Millard
Chapter 5 – The Big Picture
Let’s think for a few minutes about different ways of producing sound.
Every possible fingering on the bassoon has natural resonating frequencies. There are certain fundamental frequencies that are eager to establish themselves for any given length of bore.
Often in nature, we see a strong inclination for objects – or the spaces they contain – to vibrate at certain frequencies. Musical instruments take those natural tendencies to the highest level.
It is a defining feature of successful musical instruments that predictable frequencies are efficiently achieved and maintained. Tuning forks like to vibrate at frequencies determined by their size and mass. Bells ring at the frequencies determined by their size and shape. Drums tend to ring at pitches relative to their size and the tension on their heads. Even the table you’re sitting at might have a resonating frequency that you can hear by whacking it. With enough beer, you can knock out the William Tell Overture on your head by modulating the size of your mouth!!
After you’re done with the Rossini keep the bottle. It’s a great way to demonstrate the natural resonating frequencies for the airspace inside the bottle. Just as hitting a tuning fork causes the mechanical energy of that impact to set up a specific and long lasting tone, so blowing across the open end of the bottle excites the natural resonating frequency of that particular volume of air.
If you had gargantuan lips, you might be able to get a pop bottle sound on the bassoon by fingering a low Bb and blowing across the open end of the bell. [You will need really long arms, too.] In fact, the large diameter of the open bassoon bell discourages this being a useful way of playing the bassoon, but you can imagine the process. A simpler way to test the natural resonating frequency of your bassoon is to finger that same low Bb, blow into the bocal and slap your tongue against the open bocal tip. Acoustically, this is pretty much like playing the violin pizzicato – it’s a single, unidirectional energy input and the bassoon resonance only lasts briefly. It works from both ends, too. You can slap the palm of your hand at the top of the bell and hear a natural resonance, the frequency of which will depend on how many keys you have closed.
The point is, there are different ways to excite vibrations in the bassoon bore, including the single impact, unidirectional slapping at one end or the other and the imaginary method of playing at the bell end like a pop bottle or flute. However, just as a violin needs the continual energy input of the bow on the string, the bassoon works best with an efficient interface between resonating chamber and the blowing energy of the player.
The hair of the violin bow interacts mechanically with the violin string, catching and releasing in a complex way that continually excites the natural frequencies of the string.
In most wind instruments [excluding the flute but including all the other brass and woodwinds] the interaction between player and bore resonance frequencies is achieved by means of a pressure controlled valve. I introduced this term in the previous chapter. Let’s see if you visualize it this way…
You use a pressure valve when you control the flow of water from a tap, releasing the pressure from your city water system and allowing the stream of water to flow freely. The lips of a trumpet player or the reed on the bassoon perform a similar function, in that they release the air pressure built up in your lungs. But unlike the faucet, which controls unidirectional water flow into the average atmospheric pressure in your kitchen, these musical instrument valves are bidirectional mechanisms that are able to respond to pressure variations in the bores of the instruments and allow the mechanisms to operate at variable frequencies.
Pressure control valves such as bassoon reeds [or trumpet players’ lips!] convert the potential energy within your compressed lungs into mechanical energy. By emitting pressure pulses into the bassoon they sustain the tone in somewhat the same way that a bow sustains the legato of the violin.
The catch and release process of the bow hair on the string has to happen at the same frequency as the primary resonance frequencies of the string. If the violinist is playing a middle C, the friction of the bow hair interacts with the string 262 times per second. That frequency is not determined by the pressure of the bow or speed of the stroke but by the natural frequency for the particular string at that particular length.
Similarly, the bassoon reed must convert steady blowing pressure into ‘pressure pulses’ at the frequency of the natural bore resonances of the bassoon. If we finger a middle C, the reed will be supplying pulses of air pressure 262 times per second, regardless of how loud we are playing. Pressure controlled valves like bassoon reeds have a feedback mechanism that controls the frequency of their energy conversion.
Let me say this again: the bassoon reed is a pressure-controlled valve.
That water tap is a valve that opens to allow water from our municipal systems to flow into our homes. That water is under pressure. When you blow into a bassoon the air in your lungs and mouth are also under pressure. But this is not what ‘pressure’ means in the definition ‘pressure-controlled valve’.
Your blowing pressure controls loudness – not pitch. The physicist would say your blowing pressure controls amplitude – not frequency. So, while its true that the blowing pressure used in playing an instrument controls the amplitude of vibration, the variable acoustic pressure inside the reed control the frequency of those vibrations. This is the meaning of ‘pressure’ in our definition.
I suppose, to be absolutely clear, we could define the bassoon reed as ‘a valve whose frequency of operation is controlled by the changing internal pressures at the tip of the bocal’, but it’s a bit of a mouthful, don’t you think?
In our next chapter we’ll look at how this frequency control works. Tune in next week!