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Single-Reed Instruments

A simplified model for a single-reed woodwind instrument is shown in Fig. 9.38 [431].

Figure 9.38: A schematic model for woodwind instruments.
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If the bore is cylindrical, as in the clarinet, it can be modeled quite simply using a bidirectional delay line. If the bore is conical, such as in a saxophone, it can still be modeled as a bidirectional delay line, but interfacing to it is slightly more complex, especially at the mouthpiece [37,7,160,436,506,507,502,526,406,528], Because the main control variable for the instrument is air pressure in the mouth at the reed, it is convenient to choose pressure wave variables.

To first order, the bell passes high frequencies and reflects low frequencies, where ``high'' and ``low'' frequencies are divided by the wavelength which equals the bell's diameter. Thus, the bell can be regarded as a simple ``cross-over'' network, as is used to split signal energy between a woofer and tweeter in a loudspeaker cabinet. For a clarinet bore, the nominal ``cross-over frequency'' is around $ 1500$ Hz [38]. The flare of the bell lowers the cross-over frequency by decreasing the bore characteristic impedance toward the end in an approximately non-reflecting manner [51]. Bell flare can therefore be considered analogous to a transmission-line transformer.

Since the length of the clarinet bore is only a quarter wavelength at the fundamental frequency, (in the lowest, or ``chalumeau'' register), and since the bell diameter is much smaller than the bore length, most of the sound energy traveling into the bell reflects back into the bore. The low-frequency energy that makes it out of the bore radiates in a fairly omnidirectional pattern. Very high-frequency traveling waves do not ``see'' the enclosing bell and pass right through it, radiating in a more directional beam. The directionality of the beam is proportional to how many wavelengths fit along the bell diameter; in fact, many wavelengths away from the bell, the radiation pattern is proportional to the two-dimensional spatial Fourier transform of the exit aperture (a disk at the end of the bell) [308].

The theory of the single reed is described, e.g., in [102,249,308]. In the digital waveguide clarinet model described below [431], the reed is modeled as a signal- and embouchure-dependent nonlinear reflection coefficient terminating the bore. Such a model is possible because the reed mass is neglected. The player's embouchure controls damping of the reed, reed aperture width, and other parameters, and these can be implemented as parameters on the contents of the lookup table or nonlinear function.



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Next: Digital Waveguide Single-Reed Implementation

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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.


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