# Wave file creation with Sine

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Karthik Brs on 21 Apr 2016
Commented: Karthik Brs on 25 Apr 2016
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

Arnab Sen on 25 Apr 2016
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:
##### 1 CommentShowHide None
Karthik Brs on 25 Apr 2016
Thank you for your time Arnab! I have tried with 'spline' and all the other interp1 methods, none works unfortunately!

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