Matlab 30 Khz still be heard

By using the Matlab code as below, I created 30 Khz signal and used sound function to hear the signal created. But, according to my knowledge, I, as a human, should not have heard anything above 20Khz. What is the part that i am missing below? Am I not a human?

clear
Fs = 96000;
toneFreq1 = 30000;   
nSeconds = 2;
f1 = 10*cos( linspace (0, nSeconds*toneFreq1*2*pi, round(nSeconds*Fs) ) );
freqz( f1, length(f1) )
sound(f1, Fs);