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Old 01-03-2007, 10:17 PM
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The process of sounding a flute or a flue-type organ pipe employs an airstream directed at an edge. However, the process differs so much from the process of directing an airstream on an isolated edge that it is modeled in different ways and different language is used to describe it. Rossing's Science of Sound uses the term "air reed" to describe the situation in which the oscillating air stream is the means for excitation of the pipe sound. In using the "air reed" terminology, it is pointed out that the oscillation is controlled not by pressure, but by the air flow. The edge structure represents an "open end" for the pipe, and thus is a pressure node. The open end is an antinode for air motion (air displacement antinode) and this maximized air flow associated with the standing waves of the pipe can drive the oscillation of the airstream. By contrast, the mouthpiece end of a clarinet with its reed constitutes an acoustical "closed end", a pressure antinode which provides pressure feedback to help stabilize and control the reed oscillation.

The description of the excitation process as a "flow-controlled valve" or "flow-controlled oscillator" dates back to Helmholtz in his investigation of cavity resonance. The tone you produce by blowing over a coke bottle involves air oscillation in and out of the mouth of the bottle which can be described as "flow controlled". When the air is flowing out of the bottle, it directs the airstream outward, and when the airflow associated with the resonant vibration is inward, it directs the airstream inward to provide energy to sustain the oscillation. Helmholtz described the flute excitation in the same sort of way, with the air motion from a standing wave forcing the players airstream out of and into the flute periodically.

Benade comments on the shortcomings of Helmholtz's model, explaining that the player's airstream lags considerably behind these air motion changes from the pipes standing waves. He cites the work of Coltman and Fletcher as clarifying some of the details of the "flow-controlled valve" action at the flute's embouchure hole. Their work highlighted the importance of the transit time from the players lips to the side of the embouchure hole and its relationship to the periods of oscillation of the sound components (harmonics) present in the tone.

Hall discusses the nature of edges and the excitation of organ pipes. The role of the eddies or swirls in the air at the edge is explored and some illustration is given of feedback mechanisms which presumably contribute to the oscillation of the airstream and help with the sounding of the pipe at its resonant frequencies.

The difference between the "edgetone" as envisioned in the sounding of a flute, recorder, organ pipe, etc. and the tones produced by directing air over an edge which is not coupled to an air column has been a subject of considerable discussion and investigation. Benade comments "Until recently there has been a tendency ... to confuse the sounds produced by blowing a narrow air jet against a sharp edge when the edge forms part of a flute or an organ pipe (air reed behavior) with those produced when the system is run in isolation (edge-tone behavior). In the latter case a type of repetitive eddying called vortex shedding takes place on alternate sides of the air jet, and a sound is produced if a sharp edge is used to separate the two sets of vortices. Vortex phenomena have only a secondary influence on flute-type sound production; moreover, at ordinary musical blowing pressures the edge-tone frequencies are so high as to be nearly inaudible".
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