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u x x u ( x + d x , t ) - 2 u ( x , t ) + u ( x - d x , t ) d x 2

These approximations may be substituted into our original partial differential equation in order to solve for u ( x , t ) .

The second finite difference method used to solve the wave equation is the trapezoidal approximation method, where we have the system of equations

u u t t = 0 I x x - c ( t ) δ ( x - x c ) u u t + 0 b ( t ) δ ( x - x b )

which we will denote as

V ' ( t ) = A ( t ) V ( t ) + B ( t )

where I represents the identity matrix. The x x operator is a tridiagonal matrix with -2 along the diagonal and ones along the superdiagonal and subdiagonal. The approximating integral equation

( j - 1 ) d t j d t V ' ( t ) d t = ( j - 1 ) d t j d t ( A ( t ) V ( t ) + B ( t ) ) d t

is solved to reveal a solution for u ( j d t ) . The trapezoidal integration method turns out to be the more accurate of the two solution methods, since its error is less than the error of the forward Euler method. Real values for the string's tension, density, and length were used to evaluate the solution for the trapezoidal method, but gave us unintelligible results when used for the forward Euler method. Both methods were used to evaluate the solution using arbitrary values.


With all the preliminary work established, we can move on to the optimization problem. We investigated two objective functions for optimization. The first objective function considered was the following

J ( c ( t ) ) = 0 ( u ( x , T ; c ( t ) ) - u 0 ( x , T ) ) 2 d x .

Here a suitable time T is preordained. It is legitimate to do this since damping should not affect the periodic details of the waveform. The u ( x , T ) is solved with the given c ( t ) and fitted to u 0 ( x , T ) = sin ( 4 π x ) . We parameterize

c ( t ) = e - c 1 t - e - c 2 t

which acts at one point on the string with a shape similar to the following.

It is important also that c 1 < c 2 to guarantee the above shape. Our control problem then, is to

min c 1 , c 2 0 ( u ( x , T ; c ( t ) ) - u 0 ( x , T ) ) 2 d x

subject to u ( x , T ; c ( t ) ) solving our wave equation with the same initial and boundary conditions. There is a scaling issue since u ( x , T ) is small (on the order of 10 - 5 ) at times. To correct this we scale our target u 0 so that the two waveforms are comparable before we run the optimization. Normalizing u instead, would be cumbersome since the maximum amplitude depends on time. To expedite the optimizer, we supply the gradient of our objective

J ( c ( t ) ) = ( J ( c ( t ) ) c 1 , J ( c ( t ) ) c 2 ) .

The equations for the partial derivatives are as follows.

J ( c ( t ) ) c 1 = 2 0 ( u ( x , T ; c ( t ) ) - u 0 ( x , T ) ) ( u ( x , T , c ( t ) ) c 1 ) d x ,
J ( c ( t ) ) c 2 = 2 0 ( u ( x , T ; c ( t ) ) - u 0 ( x , T ) ) ( u ( x , T , c ( t ) ) c 2 ) d x

The inner partial derivatives will be approximated by the same finite difference method we used above.

One of the main difficulties with this objective function is that it requires a T to be found beforehand and thus we can only optimize with respect to our spacial dimension. Optimizing over both space and time would rid us of needing to find a good T but complicates our objective function and retards our optimizer. Another concern with this objective is that it takes into account the sign of the target waveform, but whether the waveform is sin ( x ) or - sin ( x ) is no matter to the musician. Along the same lines we have the scaling difficulty. Another objective function we have explored is the following energy minimization problem. We note that u ( x , t ) can be represented as a combination of sinusoids. For a given T we have

u ( x , T ) = n = 1 N u n sin ( 2 n π x ) .

Here each u n represents the nth Fourier coefficient that corresponds to the expression of the nth mode in the total wave. Since we are interested in expressing only the fourth mode, our optimization problem will try to minimize all other modes

min c 1 , c 2 F ( c ( t ) ) = 0 T f i n n = 1 , n 4 10 0 ( u ( x , t ; c ( t ) ) sin n π x d x 2 d t

subject to our wave equation with the same conditions. We decide on clearing up the first ten modes (except the fourth) to ensure that we are left with a waveform closest to our target. Like before we supply the gradient.

F ( c ( t ) ) c 1 = 2 0 T f i n n = 1 , n 4 10 0 ( u ( x , t ; c ( t ) ) sin n π x u ( x , t , c ( t ) ) c 1 d x d t ,
F ( c ( t ) ) c 2 = 2 0 T f i n n = 1 , n 4 10 0 ( u ( x , t ; c ( t ) ) sin n π x u ( x , t , c ( t ) ) c 2 d x d t .

This objective function solves the sign and amplitude problems of the first. Additionally we are now optimizing over time so we need not specify a T . One may wish to reset the bounds of the time integral through a different interval.


This is the result of our optimizer given a certain driving force b ( t ) using our first objective function.

This is the result of our optimizer at a certain time using the energy minimization objective. Since we optimized over space and time, we express this as a three dimensional plot.

For the energy minimization objective function using multiple dampings, we have this result.

Comparatively, the values of the objective function for single and multiple dampings are on the same order of magnitude, with the value for the single dampings being slightly smaller. Therefore the optimization process for a single damping is more effective, but not by much.


We would like to give a big thanks to Dr. Steve Cox for his guidance and support throughout the course of the project. This paper describes work completed with the support of the NSF.


All of the codes used in this project are available on our website at

http://www.owlnet.rice.edu/ mlg6/strings/.


  1. Bamerger, A., J. Rauch and M. Taylor. A model for harmonics on stringed instruments . Arch. Rational Mech. Anal. 79(1982) 267-290.
  2. Cheney, E. W., and David Kincaid. Numerical Mathematics and Computing . Pacific Grove, CA: Brooks/Cole Pub., 1994. Print.
  3. Cox, S., and Antoine Hernot. Eliciting Harmonics on Strings . ESAIM: COCV 14(2008) 657-677.
  4. Fletcher, Neville H., and Thomas D. Rossing. The Physics of Musical Instruments . New York: Springer-Verlag, 1991. Print.
  5. Knobel, Roger. An Introduction to the Mathematical Theory of Waves . Providence, RI: American Mathematical Society, 2000. Print.
  6. Rayleigh, John William Strutt, and Robert Bruce Lindsay. The Theory of Sound . New York: Dover, 1945. Print.

Questions & Answers

what is the stm
Brian Reply
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industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
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Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
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Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
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Akash Reply
it is a goid question and i want to know the answer as well
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s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
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Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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