Author: Luuk ★2017
Date: 2020-11-10 13:41
This is basic physics. Any mass-spring system will develop a periodic movement when fed with energy. The key question is which frequency will be developed, and that depends on, for instance, spring characteristics, tension, dampening, mass etc.
Musical instruments are designed in such a way that a merely random input of energy (noise) is used to develop the desired frequencies.
Violin: the microscopic action of the bow on the string is noisy (just rubbing sticky hairs against the string), but string length, tension and mass quickly promote a series of discrete frequencies. The same with trumpet: the buzzing lips produce a broad spectrum of frequencies, but the tube only resonates with certain frequencies. Thus, the input energy is used to create a discrete spectrum of sound.
Radio receivers work the same way: noisy signal from an antenna is fed to a circuit which resonates at a certain frequency. This resonating frequency can be tuned to a desired station. Another example: the collapsing Tacoma Narrows Bridge (look it up on YouTube!). This bridge fails because it develops a resonance under the merely random energy fed by the blowing wind.
The volume of air inside tube of the clarinet only resonates at certain frequencies. The source of sound energy, the flapping reed, is acoustically coupled to the tube. The two parts influence each other. The tube is very efficient in putting all energy in a discrete spectrum. This is the sound that will be heard.
In response of the waves developed in the tube, the reed will move 'in tune'. Not because the spectrum as developed by the tube fits the reed, but because it is forced to do so by the standing wave. Remember, almost all energy is put into that standing wave. This 'kicks' the reed every time it reaches it. Since the reed is small and light, it follows the tube.
Of course, reed mass or strength will have some influence on the spectrum in the tube. The reed is small, but not neglectable. This is the essence of acoustical coupling: both parts influence each other. However, some factors (tube length) are much more important than others (reed mass).
For a thorough but light introduction: see Arthur Benade, Fundamentals of Musical Acoustics.
Regards,
Luuk
Philips Symphonic Band
The Netherlands
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The Clarinet BBoard
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