I'm trying to produce a sampled piano note and I keep getting the error "Index exceeds matrix dimensions." I'm not sure where i've gone wrong :/

%% Part 1: Preparing the signal

[pianosig,Fs]=wavread('piano_A.wav'); %%Imports the piano signal
Length=length(pianosig); %%len = the length of the signal
Duration = Length/Fs; %%the duration of the signal is length divided by the sampling frequency
Ts=1/Fs; %%sampling time is 1 divided by the sampling frequency
Time=0:Ts:Duration-Ts; %%makes samples at set intervals
plot(Time,pianosig);xlabel('time (sec)'); %%plots the signal on a graph, x label is time in seconds
ylabel('Amplitude');title('Piano A time domain'); %%y label is amplitude
sptool %%View the spectrum information
sound(pianosig,Fs) %%play signal

%%Part 2: Harmonic Content

% Harmonics     Frequency(Hz) Magnitude(dB)
% 1st           441         -34
% 2nd           880         -38
% 3rd           1325        -47
% 4th           1770        -50
% 5th           2222        -61
% 6th           2680        -70
% 7th           3144        -72
% 8th           3609        -86
% 9th           4087        -94
% 10th          4587        -103
% 11th          5069        -99
% 12th          5999        -92
% 13th          6999        -94
% 14th          7999        -96
% 15th          9000        -96
% 16th          10000       -98
% 17th          11000       -98
% 18th          12000       -101

%%Part 2: Creating the envelope

Temp = pianosig.^2;
plot(Temp); 
d = fdesign.lowpass(40,100,1,80,44100);
Lowpass=design(d,'kaiserwin');
Envelope=filter(Lowpass,Temp);
plot(Envelope);

%%Part 3: Creating the sampled signal

Frequencies = [441, 880, 1325,1770, 2222, 2680, 3144, 3609, 4087, 4587, 5069, 5999, 6999, 7999, 9000, 10000, 11000, 12000];
Magnitude = [-34, -38, -47, -50, -61, -70, -72, -86, -94, -103, -99, -92, -94, -96, -96, -98, -98, -101];
Magnitude = db2mag(Magnitude); %%converts dB to magnitude values
Signal=0;
for i = 1:length(Frequencies) %%creating sine waves and adding them together

      Signal = Signal+Magnitude(i)*sin(Frequencies(i)*2*pi*Time);
  end
  SynthSignal = Signal(1:length(Envelope)).*Envelope; %%applies envolope as a guide

SynthSignal=SynthSignal./max(abs(SynthSignal)); %%Normalises the signal
plot(SynthSignal);
sound(SynthSignal,Fs);

Answer 1

[Wayne King]

Without commenting on your code, the following should fix your problem

 for i = 1:length(Frequencies) 
      Signal = Signal+Magnitude(i)*sin(Frequencies(i)*2*pi*Time);
  end
  Signal = Signal(:);
  SynthSignal = Signal(1:length(Envelope)).*Envelope; %%applies envolope

Note that I have inserted

Signal = Signal(:);

after your for loop exits.