Audio Filter with pulse function

I am trying to take a .wav file, read it using wavread and add a sine wave of 12khz to the file to create a noisy signal. I am then trying to create a pulse that when multiplied with the frequency components of the noisy signal will zero out unwanted frequencies. In this example i am trying to make a filter with a cutoff frequency of 10000 Hz so I am assigning ones in the range of -10000 to 10000 hz and assigning zeros for the rest of the length of the signal. However, when I multiply the filter and the noisy signal it doesnt produce the correct results. My code is below, I was wondering where I went wrong.

[y,fs]=wavread('one.wav');    % fs=44101
f=12000;
fnyquist=fs/2
L=length(y)
nfft=1024;
ts=1/fs;

%t=0:ts:(L-1)/fs; %lengthy /fs = duration of audiofile 
t = (0:L-1)/fs;
x=sin(2*pi*f*t);
%sound(x,fs)
figure('Name','Time Domain Audio','NumberTitle','off');
plot(t,y)

left = y(:,1);         %y was 1654400x2 and now is 1654400x1  

wavwrite(x,fs,'sound.wav');
a=wavread('sound.wav');

Z=left+a;
%sound(Z,fs);

Y = fftshift(fft(Z));
f = (-(L-1)/2:L/2)/fs;
w = 2*pi*f;

MagY = abs(Y);
PhY = angle(Y)*180/pi;  
figure('Name','freq ');
plot(f,MagY)
figure('Name','freq domain'); 
plot(w,MagY/L)

idx_p = find(f <= 10000,1);              
idx_n = find(f >= -10000, 1); 

lpf = zeros(1,L);
lpf (idx_n:idx_p) = 1; 
figure('Name','lpf');
plot(f,lpf)
lpf2= lpf.';                    %transposed from 1x1654400 to 1654400x1  

filteredsignal=lpf2.*Y;
filteredsignal2=ifft(filteredsignal);
sound(filteredsignal2,fs);