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 [347].

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

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

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

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

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

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$.

Next Section:
Successive Pluck Collision Detection
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Digital Waveguide Plucked-String Model