matlab words no help needed

I need to plot the sum of 2 sine curves. I can successfully plot (x,y1) and plot(x,y2), they are smooth curves, however when i try to plot(x,y) where y = y1+y2 the curve comes out like this? how do i fix this i dont know what i am doing wrong? Thanks

[Stephen23]

"...however when i try to plot... the curve comes out like this"

You are plotting lots of zeros (to within floating point precision), because that is exactly what you calculated.

"how do i fix this i dont know what i am doing wrong?"

You need to consider the B factor. Note that for a B which is an integer multiple of 2*pi (and you defined B as 4*pi, which is clearly a multiple of 2*pi), the following identity holds true:

sin(B*X) = -sin(B*(1-X))

For example:

B = 2*pi;
X = rand(1,9);
sin(B*X)

ans = 1×9

-0.6044 -0.8824 -0.4910 -0.9397 -0.7891 0.9338 0.8413 0.9918 -0.9736

sin(B*(1-X))

ans = 1×9

0.6044 0.8824 0.4910 0.9397 0.7891 -0.9338 -0.8413 -0.9918 0.9736

That is why you are plotting zeros (or at least values very very very close to zero).

[Stephen23]

Original question by Tyler Daines retrieved from Google Cache (unfortunately their deleted comments are not included):

https://web.archive.org/web/20210516175443/https://webcache.googleusercontent.com/search?q=cache%3A6xOVMwrjOj8J%3Ahttps%3A%2F%2Fwww.mathworks.com%2Fmatlabcentral%2Fanswers%2F827265-trying-to-plot-a-sine-sine-function-but-not-getting-a-smooth-curve

https://web.archive.org/web/20210516175823/https://www.mathworks.com/matlabcentral/answers/827265-matlab-words-no-help-needed

Trying to plot a sine+sine function but not getting a smooth curve

I need to plot the sum of 2 sine curves. I can successfully plot (x,y1) and plot(x,y2), they are smooth curves, however when i try to plot(x,y) where y = y1+y2 the curve comes out like this? how do i fix this i dont know what i am doing wrong? Thanks

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[DGM]

What you're looking at isn't meaningful. It's all just rounding error. You're essentially doing this:

y = sin(x) - sin(x);

I'm guessing there's some phase component that's not defined right yet, but I can't really guess at what you need it to be.

f = 2;
A = 1;
v = 1;
L = 1;
l = v/f;
B = (2*pi)/l;
w = 2*pi*f;
t = 0;
x = 0:0.0001:L;
z1 = A*sin(B*x - w*t);
z2 = A*sin(B*(L-x) - w*t);
y = z1+z2;

plot(x,z2); hold on
plot(x,z1);

For the future, please just paste your code using the code formatting tools. I had to retype all that and deal with the sub-pixel ambiguity differentiating the character '1' and 'l'.

[Tyler Daines]

Sorry mate i havent used this forum before will do in the future.

[DGM]

No worries. It was short enough.

Answer 2

[RAHUL MAURYA]

clear all;

clc;

f = 2;

A = 1;

v = 1;

L = 1*(180/(pi));

l = v/f;

B = ((2*pi)/l)*(180/pi);

w = 2*pi*f;

t = 0;

x=0:0.0001:L;

z1 = (A*sind(B*x - w*t));

z2 = A*sind(B*(L-x) - w*t);

y=(z1+z2);

subplot(3,1,1);

plot(x,z1)

title('X,Z1')

axis([0 10 -1 1])

subplot(3,1,2);

plot(x,z2)

title('X,Z2')

axis([0 10 -1 1])

subplot(3,1,3);

plot(x,y)

title('X,Z1+Z2')

axis([0 10 -2 2])

Convert B and L in radian to degree.

[RAHUL MAURYA]

Convert B and L in degree from Radian