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Incorporating Control Motion

Let $ y_m(t)$ denote the vertical position of the mass in Fig.9.22. (We still assume $ m=\infty$ .) We can think of $ y_m$ as the position of the control point on the plectrum, e.g., the position of the ``pinch-point'' holding the plectrum while plucking the string. In a harpsichord, $ y_m$ can be considered the jack position [350].

Also denote by $ L$ the rest length of the spring $ k$ in Fig.9.22, and let $ y_e \isdeftext y_m+L$ denote the position of the ``end'' of the spring while not in contact with the string. Then the plectrum makes contact with the string when

$\displaystyle y_e(t) \ge y(t)
$

where $ y(t)$ denotes string vertical position at the plucking point $ x_p$ . This may be called the collision detection equation.

Let the subscripts $ 1$ and $ 2$ each denote one side of the scattering system, as indicated in Fig.9.23. Then, for example, $ y_1=y_1^-+y_1^+$ is the displacement of the string on the left (side $ 1$ ) of plucking point, and $ y_2$ is on the right side of $ x_p$ (but still located at point $ x_p$ ). By continuity of the string, we have

$\displaystyle y(t)\eqsp y_1(t)\eqsp y_2(t).
$

When the spring engages the string ($ y_e > y$ ) and begins to compress, the upward force on the string at the contact point is given by

$\displaystyle f_k \eqsp k\cdot (y_e-y)
$

where again $ y_e=y_m+L$ . The force $ f_k$ is applied given $ y_e\ge y$ (spring is in contact with string) and given $ f_k < f_{\mbox{\tiny max}}$ (the force at which the pluck releases in a simple max-force model).10.15 For $ y_e<
y$ or $ f_k \ge f_{\mbox{\tiny max}}$ the applied force is zero and the entire plucking system disappears to leave $ y_1^- = y_2^-$ and $ y_2^+=y_1^+$ , or equivalently, the force reflectance becomes $ \hat{\rho}_f=0$ and the transmittance becomes $ \hat{\tau}_f=1$ .

During contact, force equilibrium at the plucking point requires (cf. §9.3.1)

$\displaystyle 0 \eqsp f_1+f_k-f_2 \protect$ (10.25)

where $ f_i \isdeftext -Ky'_i \isdeftext -K\partial y_i/\partial x$ as usual (§6.1), with $ K$ denoting the string tension. Using Ohm's laws for traveling-wave components (p. [*]), we have

$\displaystyle f_i \eqsp f_i^++f_i^-
\eqsp Rv_i^+ - Rv_i^-,
$

where $ R=\sqrt{K\epsilon }$ denotes the string wave impedance (p. [*]). Solving Eq.$ \,$ (9.25) for the velocity at the plucking point yields

$\displaystyle v \eqsp v_1^+ + v_2^- + \frac{1}{2R} f_k,
$

or, for displacement waves,

$\displaystyle y \eqsp y_1^+ + y_2^- + \frac{1}{2R} \int_t f_k. \protect$ (10.26)

Substituting $ f_k = k\cdot (y_e-y)$ and taking the Laplace transform yields

$\displaystyle Y(s)
\eqsp Y_1^+(s) + Y_2^-(s) + \frac{1}{2R} \frac{F_k(s)}{s}
\eqsp Y_1^+(s) + Y_2^-(s) + \frac{k}{2Rs}\left[Y_e(s) - Y(s)\right].
$

Solving for $ Y(s)$ and recognizing the force reflectance $ \hat{\rho}_f(s)$ gives

\begin{eqnarray*}
Y(s) &=& \left[1-\hat{\rho}_f(s)\right]\cdot
\left[Y_1^+(s)+Y_2^-(s)\right]+\hat{\rho}_f(s)Y_e(s)\\
&=& \left[Y_1^+(s)+Y_2^-(s)\right]
+ \hat{\rho}_f(s)\cdot \left\{Y_e(s)
- \left[Y_1^+(s)+Y_2^-(s)\right]\right\},
\end{eqnarray*}

where, as first noted at Eq.$ \,$ (9.24) above,

$\displaystyle \hat{\rho}_f(s) \isdefs
\frac{\left(\frac{k}{s}+R\right) - R}{\left(\frac{k}{s}+R\right) + R}
\eqsp \frac{\frac{k}{s}}{2R+\frac{k}{s}}
\eqsp \frac{\frac{k}{2R}}{s+\frac{k}{2R}}.
$

We can thus formulate a one-filter scattering junction as follows:

\begin{eqnarray*}
Y_d^+ &=& Y_e - \left(Y_1^+ + Y_2^-\right)\\ [5pt]
Y_1^- &=& Y_2^- + \hat{\rho}_f Y_d^+\\ [5pt]
Y_2^+ &=& Y_1^+ + \hat{\rho}_f Y_d^+.
\end{eqnarray*}

This system is diagrammed in Fig.9.24. The manipulation of the minus signs relative to Fig.9.23 makes it convenient for restricting $ y_d^+(t)$ to positive values only (as shown in the figure), corresponding to the plectrum engaging the string going up. This uses the approximation $ y_1(t)=y_2(t)\approx y_1^+(t)+y_2^-(t)$ , which is exact when $ \hat{\rho}_f=0$ , i.e., when the plectrum does not affect the string displacement at the current time. It is therefore exact at the time of collision and also applicable just after release. Similarly, $ y_d^+(t)>f_{\mbox{\tiny max}}/k$ can be used to trigger a release of the string from the plectrum.

Figure 9.24: Instantaneous spring displacement-wave scattering model driven by the spring edge $ y_e(t)=y_m(t)+L$ .
\includegraphics{eps/uscatii}


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``Physical Audio Signal Processing'', by Julius O. Smith III, W3K Publishing, 2010, ISBN 978-0-9745607-2-4.
Copyright © 2014-03-23 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
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