how can i add noise in particular place of the sine wave?

i want to add noise in the middle of the sine wave. but if i use the rand() function it adds noise to the total signal.how to add noise in some parts of the signal only.kindly help me.......

Answer 1

[Rick Rosson]

   noise = [ zeros(N,1) ; rand(M,1) ];
   distorted = source + noise;

[suganya saravanan]

thank you for your answer. but it shows me error.its saying matrix dimension must agree. i attached you my coding along this. kindly help me in getting output.

if true
  % clc;
clear all;
close all;
noiseAmplitude=2;
t=[0:0.1:10];
freq=1500;
samplefreq=2500;
source= sin(2*pi*(freq/samplefreq)*t);
figure(1);
plot(source);
grid on;
hold on;
noise = [ zeros(N,1) ; rand(M,1) ];
distorted = source + noise;
figure(2);
plot(distorted);code
end
if i add rand() then noise is added to full signal. i need in particular places only. kindly help me.

[Zikobrelli]

you need to specify the values of M and N. For example, if you want to add noise to the first 10 points of your signal you can use,

N=10

M=numel(t)

changing the values of M and N will enable you to choose the signal indexes where the noise is applied