Plot a sine function

I am having trouble with entering the following formula into matlab as it is giving me a variety of different error messages. The formula is y = sin(2πt)*sin(2π(20t)). Can anybody advise on this? Thanks.

[edward gibbons]

>> yup me too... Ive been at it for about six hours...

[Image Analyst]

edward, I presume once you saw the answer below, your 6 hour attempt ended successfully. Write back if it didn't work for you.

Answer 1

[Azzi Abdelmalek]

t=0:0.01:2; 
y = sin(2*pi*t).*sin(2*pi*20*t);
plot(t,y)

Answer 2

[Image Analyst]

Azzi's code should work. I can't imagine what trouble you encountered. Did you use pi instead of the symbol you used here? Did you use * in between terms to do the multiplication?

Anyway, you're making envelopes. If you're interested in having some fun with envelopes, then try this demo where I turn 3 envelopes into a wave file and play and plot the sound. Save as make_wav_file.m and run it.

% Program to create a warbling wave file with variable amplitude and pitch.
function make_wav_file()
% Initialization / clean-up code.
clc;    % Clear the command window.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

% Create the filename where we will save the waveform.
folder = pwd;
baseFileName = 'Test_Wave.wav';
fullFileName = fullfile(folder, baseFileName);
fprintf('Full File Name = %s\n', fullFileName);

% Set up the time axis:
t = 1:15000;

% Set up the period (pitch, frequency):
T = 13;
T = linspace(25, 8, length(t)); % Pitch changes.

% Create the maximum amplitude:
Amplitude = 32767;
% Add an exponential decay:
Amplitude = Amplitude .* exp(-0.0003*t);

% Add an ocillation on the amplitude:
% Amplitude = Amplitude .* rand(1, length(x)); % Makes a shushing/roaring sound.
Amplitude = Amplitude .* sin(2.*pi.*t./2000); % Decaying pulsing sound.

% Construct the waveform:
y = int16(Amplitude .* sin(2.*pi.*t./T));
% y = abs(int16(Amplitude .* sin(2.*pi.*x./T)));

% Plot the waveform:
plot(t, y, 'b-');
title('Waveform', 'FontSize', fontSize);
xlabel('Time', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
grid on;
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
fprintf('Writing file %s...\n', fullFileName);

% Write the waveform to a file:
wavwrite(y, fullFileName);

% Play the sound as many times as the user wants.
playAgain = true;
counter = 1;
while playAgain
  % Play the sound that we just created.
  fprintf('Playing file %s   %d times...\n', fullFileName, counter);
  PlaySoundFile(folder, baseFileName);
  % Ask user if they want to play the sound again.
  promptMessage = sprintf('You have played the sound %d times.\nDo you want to play the sound again?', counter);
  titleBarCaption = 'Continue?';
  button = questdlg(promptMessage, titleBarCaption, 'Yes', 'No', 'Yes');
  if strcmpi(button, 'No')
    playAgain = false;
    break;
  end
  counter = counter + 1;
end

% Alert user that we are done.
message = sprintf('Done playing %s.\n', fullFileName);
fprintf('%s\n', message);
promptMessage = sprintf('Done playing %s.\nClick OK to close the window\nor Cancel to leave it up.', fullFileName);
titleBarCaption = 'Continue?';
button = questdlg(promptMessage, titleBarCaption, 'OK', 'Cancel', 'OK');
if strcmpi(button, 'OK')
  close all;  % Close down the figure.
end

%================================================================================================
% Play a wav file.  You can pass in 'random' and it will pick one at random from the folder to play.
% PlaySoundFile(handles.soundFolder, 'chime.wav');
% PlaySoundFile(handles.soundFolder, 'random');
function PlaySoundFile(soundFolder, baseWavFileName)
  global waveFileData;
  global Fs;  % Wave file information.
  try        % Read the sound file into MATLAB, and play the audio.
%     soundFolder = fullfile(soundFolder, 'Sound Files');
    if ~exist(soundFolder, 'dir')
        warningMessage = sprintf('Warning: sound folder not found:\n%s', soundFolder);
        WarnUser(warningMessage);
        return;
    end
    if strcmpi(baseWavFileName, 'random')
      itWorked = false;
      tryCount = 1;
      while itWorked == false
        % Pick a file at random.
        filePattern = fullfile(soundFolder, '*.wav');
        waveFiles = dir(filePattern);
        numberOfFiles = length(waveFiles);
        % Get a random number
        fileToPlay = randi(numberOfFiles, 1);
        baseWavFileName = waveFiles(fileToPlay).name;
        fullWavFileName = fullfile(soundFolder, baseWavFileName);
        waveFileData = -1;
        try
          if exist(fullWavFileName, 'file')
            [waveFileData, Fs, nbits, readinfo] = wavread(fullWavFileName);
            sound(waveFileData, Fs);
            %     soundsc(y,Fs,bits,range);
          else
            warningMessage = sprintf('Warning: sound file not found:\n%s', fullWavFileName);
            WarnUser(warningMessage);
          end
          % It worked.  It played because the audio format was OK.
          itWorked = true;
        catch
          % Increment the try count and try again to find a file that plays.
          tryCount = tryCount + 1;
          if tryCount >= numberOfFiles
            break;
          end
        end
      end % of while()
    else
  %     baseWavFileName = 'Chime.wav';
      fullWavFileName = fullfile(soundFolder, baseWavFileName);
      waveFileData = -1;
      if exist(fullWavFileName, 'file')
        [waveFileData, Fs, nbits, readinfo] = wavread(fullWavFileName);
        sound(waveFileData, Fs);
        %     soundsc(y,Fs,bits,range);
      else
        warningMessage = sprintf('Warning: sound file not found:\n%s', fullWavFileName);
        WarnUser(warningMessage);
      end
    end
  catch ME
    if strfind(ME.message, '#85')
      % Unrecognized format.  Play chime instead.      
      fprintf('Error in PlaySoundFile(): %s.\nUnrecognized sound format in file:\n\n%s\n', ME.message, fullWavFileName);
       baseWavFileName = 'Chime.wav';
      fullWavFileName = fullfile(soundFolder, baseWavFileName);
      waveFileData = -1;
      if exist(fullWavFileName, 'file')
        [waveFileData, Fs, nbits, readinfo] = wavread(fullWavFileName);
        sound(waveFileData, Fs);
        %     soundsc(y,Fs,bits,range);
      end
    end
    errorMessage = sprintf('Error in PlaySoundFile().\nThe error reported by MATLAB is:\n\n%s', ME.message);
    fprintf('%s\n', errorMessage);
    WarnUser(errorMessage);
  end
  return; % from PlaySoundFile