M. J. Moloney
CL-109
Office 812 877 8302
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Revised
**11/21/07**

Text:

Late Professor Emeritus

Rose-Hulman Institute of Technology

Course Materials (Information, Notes, Handouts) Winter 07-08

**Contents **Projected
order of chapters in Meeks,
*Acoustics.*

- 1 (Harmonic Motion)
- 2 (Waves on Strings)
- 3 (Waves in Membranes)
- 6 (Waves in Fluids)
- 7 (Reflection and Refraction of Plane Waves)
- 4 (Elements of Elasticity; stress, strain, Young's modulus, poisson's ratio)
- 5 (Waves in Rods and Bars)
- 8 (Piston Source)
- 9 (Resonators)
- 11 (Loudspeakers and Microphones)

Last 10 days to two weeks devoted to a project (or maybe a paper)

Tentative allocation of credit

- Homework: 40%
- Exams: 40%
- Project 20%

- musical acoustics (guitar, sax, flute, recorder etc.)
- acoustics of a tuning fork
- helmholtz resonances in a moving automobile
- build a torsional pendulum (clamped steel
or aluminum rod with rotational inertia on the end)

- transmission properties of a resonator in a transmission line
- absorption of room sounds by a resonator
- diffusion of one gas into another, measured
by sound waves

- slinky properties for sound velocity vs. frequency
- resonant frequencies of pipes with holes in them (spreadsheet theory, build and test)
- absorption coefficient measurement (long
pvc pipe and microphone moved inside it)

- horn theory vs. experiment (spreadsheet theory, build and test)
- Lab model of sound reflection (AJP Nov 87)
- Doppler measurements of moving objects (could use a sound card)
- Build a simple gadget to make quiet doors
noisy near blind corners

Animation of a pulse reflecting from a boundary

Instructions for adding a slider to a spreadsheet

Driven
damped oscillator spreadsheet

Animation of pulse on a string fixed at
both ends (maple code)

Animation of pulse on a string ; we are
not told the boundary conditions

Driven damped oscillator (graphs, maple
code, problem)

Animation and maple code for simple
vibrating membrane

Transverse waves on rods and bars
(chart, graphs, boundary conditions, maple code)

Elasticity and Bending of Beams

- Bending moment M of a beam in terms of derivatives of y with respect to x
- Equation of motion of a vibrating beam
- Transverse Vibrations of Bars (free-free, clamped-clamped, clamped-free)
- (Digression - critical load for buckling of long thin beam)

This animation shows a pulse traveling on a string fixed at both ends.

The contents of this maple file are :

**strpulse.mws pulse on a string via eigenfunction expansion**

**Use notation of Meeks, p. 47**

**An cos n pi ct/L + Bn sin n pi ct/L**

**An = 2/L integral 0 to L sin n pi x/L y(x,0)**

**Bn = 2/m pi c integral 0 to L v(x,0) cos n pi x/L**

**pulse centered at x=7/8 @t=0 pulse isn't really 0 at boundaries,
but close to it.**

**string from 0 to 2. 11/17/97**

**restart;with(plots):**

**c:=4; # pulse speed (which must be string
wave speed!)**

**y:=3/(2+450*(x-7/8-c*t)^2);**

**animate(y,x=0..2,t=0..1/2,frames=20,color=black);
# this pulse runs off end of the string!**

**vy:=subs(t=0,diff(y,t));**

**N:=30; # of terms in series**

**L:=2; # length of string**

**for m from 1 to N do**

**B[m]:=evalf(2/(m*Pi*c)*Int(vy*sin(m*Pi*x/L),x=0..L));**

**A[m]:=evalf(2/L* Int(subs(t=0,
y)*sin(m*Pi*x/L),x=0..L));
od:**

**str:=0:**

**for m from 1 to N do**

**str:=str+(A[m]*cos(m*Pi*c*t/L)+B[m]*sin(m*Pi*c*t/L))*sin(m*Pi*x/L);
od: animate(str,x=0..2,t=0..1,frames=30,color=black);**

What kind of string is this pulse travelling on? Can you modify the program above to give this behavior?

[This string is not fixed at both ends!]

The response
curve
for a **driven damped oscillator** is shown below, both for
amplitude
vs frequency, and phase vs frequency. The **phase** shown is the
amount
by which the **driver leads the response of
the driven
oscillator.**

Click here to download the maple file which generates these graphs. After it comes up, you will want to do a 'Save File', and later open the file in Maple.

The commands in this file are as follows:

drivnosc.mws 9/22/97

Plot the phase and amplitude response of a driven, damped oscillator as a function of frequency.

It is interesting that very heavy damping (b=10, k=10, m=1) crushes the response away from zero frequency.

**restart;with(plots):**

**assume(k,real);assume(m,real);assume(A,real);
assume(omega>0);assume(t,real);assume(b,real);assume(phi,real);
eqn:=A*exp(I*(omega*t+phi))-k*x(t)-b*diff(x(t),t)
= m*diff(x(t),t,t); > s:=dsolve(eqn,x(t));**

**x:=rhs(s);**

**values:={A=1,m=1,k=10,b=1/5};**

**xs:=subs(values,x);**

**steady:=op(1,xs); **#
keep only the oscillating terms for steady state response** ****ev:=evalc(subs(t=0,phi=0,steady));**

The conventional phase response is the driver phase - the responding phase, so the amplitude response will be found as exp(-I alph)

**assume(alph,real);**

**cosal:=op(1,ev); sinal:=-op(2,ev)/I;**

**alph:=arctan(sinal/cosal);**

**plot(alph+Pi*Heaviside(-alph),omega=2..4,title=`
phase [0->Pi] vs. omega`);**

The driver is in phase with the driven system at very low frequencies, and pulls ahead until it is Pi/2 ahead at resonance, and goes on to be Pi ahead (180 degrees out of phase) at very high frequencies.

The Heaviside function forces the inverse tangent to run between 0 and Pi, rather than between -Pi/2 and +Pi/2.

**xcc:=subs(I=-I,steady); # get complex
conjugate**

**xmag:=simplify(steady*xcc); # get magnitude
squared**

**xev:=simplify(evalc(xmag));**

**plot(sqrt(xev),omega=0..6,title=`Response
amplitude vs freq.`);**

**Homework problem P6**

**a) Figure out the theoretical Q value for
the
oscillator in the problem above.**

**b) Re-plot the amplitude response curve in
Maple from, say, 3 to 4 sec, and determine the Q 'experimentally' by
clicking
on the plot at points where the amplitude is 0.707 of the max
amplitude.
Show your calculations for Q.**

**c) Double the Q of the system (be sure to
tell
me how you did this). Re-plot and 'experimentally' check that the new
parameters
resulted in the correct value of Q.**

Here is an animated vibrating rectangular membrane, in its lowest mode

Click here to download the maple file which does this animation. Do a 'Save File', then open the file in Maple.

Here is the code in the maple file:

**restart;with(plots):**

**Lx:=2; Ly:=1;**

**g:=sin(Pi*x/Lx)*sin(Pi*y/Ly);**

**plot3d(g,x=0..Lx,y=0..Ly);**

**c:=100;** # wave speed on membrane

**omega:=c*sqrt((Pi/Lx)^2+(Pi/Ly)^2);**
# lowest mode angular freq

**Nframes:=10; tfac:=2*Pi/(omega*Nframes);**

**for i from 0 to Nframes-1 do**

**plt.(i):=plot3d(sin(omega*i*tfac)*g,x=0..Lx,y=0..Ly);od:**

**display([plt.(0..Nframes-1)],insequence=true);
**#
this is the animation

Transverse waves on bars and rods.

They are solutions of y'''' = k^4 y = w^2 /(kap^2 c^2),

where c = sqrt(Y/rho), and kap = sqrt(int r^2 dA/ int dA).

**frequencies: ** w = k^2 kap c.

same frequencies for free-free and clamped-clamped bars, but
different
functions!

free-free bar
y''( +/- L/2)=0 y'''( +/- L/2)=0 |
clamped-clamped
y( +/- L/2) = 0 y'( +/- L/2)=0 |
clamped-free
y(0)=0=y'(0) y''(L)=0=y'''(L) |

even:
solve tan kL/2 = -tanh kL/2 kL =. 3, 7, 11, etc. Pi/2 |
even:
solve tan kL/2 = -tanh kL/2 kL =. 3, 7, 11, etc Pi/2 |
all solutions : solve
cos kL cosh kL = -1 |

odd
solve tan kL/2 = +tanh kL/2 kL = . 5, 9, 13, etc. Pi/2 |
odd
solve tan kL/2 = +tanh kL/2 kL = . 5, 9, 13, etc. Pi/2 |
kL =. 1, 3, 5, 7, etc Pi/2 |

lowest even mode | lowest even mode | lowest mode |

lowest odd mode | lowest odd mode | next-lowest mode |

**Here
is the maple file which produced the graphs shown above. **
Do a 'Save File, then open later in Maple

Some theory on beam bending. .

**Examine the neglect of Ic alpha in the derivation of equation [2]
above, and then obtain an improved equation for the bar, which enables
us to make frequency corrections at high frequencies.**

Vibration frequencies and equations - free-free, clamped-clamped, and clamped-free.