Wave file creation with Sine

Hello everyone, I have produced a wave sound file for a given data which contains information about the sound pressure and frequency. I am using the equation y = A sin(2*pi*f*t) to produce sound. I am using sound pressure as amplitude. What's strange is, when I produce the sound using this method, I am getting frequency components which are exactly double the original frequency values. When I remove the 2 from the Sin(2*pi*f*t), its working! Also, I observed that, when I use interpolated values for frequency(from 'interp1'), I am getting this 'double frequency' problem, without the interpolation, Sin(2*pi*f*t) works fine. Do any of you have an explanation for this? I am including my script here:

 fs = 44100;  % Sampling frequency
Paco = 10^-12; % Reference sound power 
Po = 2*10^-5;  % Reference sound pressure of '20µPa' (at 1000 Hz.)
r = 1;         % Distance to the sound source from the listener
Q = 4;         % Directivity factor for a point source  
filename_audiowrite = sprintf('SSD_audiowrite_%d.wav',ii);
t_ramp = 20;   %[secs]
t = 0:1/fs:t_ramp-1/fs;
RPM_max = max(cell2mat(RPM));
Frq = cell2mat(frq);
Wav_sum = 0;
for tv = 1:ordn    
    t_v{tv} = RPM(tv)/RPM_max*t_ramp;                 %Creation of individual Time Vectors
    TLw_intrp{tv} = interp1(cell2mat(t_v(tv)),ord(tv).total_lw,t,'linear','extrap'); % Interpolating the values of sound power at different times for linear ramp
    f_intrp{tv} = interp1(cell2mat(t_v(tv)),ord(tv).freq,t,'linear','extrap');
    SPrL{tv} = cell2mat(TLw_intrp(tv))+10*log10(Q/(4*pi*r*r));  % Conversion of Sound power(TLw dB) to Sound Pressure Level(SprLdB)
    Spr{tv} = sqrt(2)*(Po*(power(10,cell2mat(SPrL(tv))/20)));   % Conversion of Sound Pressure Level(SprLdB)to Sound Pressure(Spr Pa)
    Wav{tv} = cell2mat(Spr(tv)).*sin(pi*cell2mat(f_intrp(tv)).*t);    % Data for the Wave file
    Wav_sum = Wav_sum + cell2mat(Wav((tv)));                    % Summation of wav data for all time orders
end
audiowrite(filename_audiowrite,Wav_sum,fs,'BitsPerSample',32)
  end
  end

Answer 1

[Arnab Sen]

Hi Karthik,

In the script linear interpolation is done which might create the issue. Try interp1 with spline option as below,

figure
vq2 = interp1(x,v,xq,'spline');
plot(x,v,'o',xq,vq2,':.');
xlim([0 2*pi]);
title('Spline Interpolation');

For more details, refer to the following link:

http://www.mathworks.com/help/matlab/ref/interp1.html

[Karthik Brs]

Thank you for your time Arnab! I have tried with 'spline' and all the other interp1 methods, none works unfortunately!