Audio of a shepard tone on youtube.
So what is a Shepard tone?
A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower. (wikipedia).
A computer simulated Shepard tone goes on and on and on....... It never ends, literally. We feel (our brain perceives) that the amplitude or the frequency or whatever is increasing gradually but after some time we feel that that tone is repeating again, starting from same point. So, the frequency of the tone changes periodically like a sine wave.
But why we, the human voice cannot produce that tone? How hard we try, we cannot produce. This may be due to exhaustion or the capacity of lungs. Our voice seems to get saturated after a certain limit further which we cannot produce the sound. Why? If the frequency of the tone changes periodically like a sine wave, we should be able to continue producing the tone from where we started it. But no, this does not happen. Why?
PS- my terminology may be wrong. So, feel free to edit it.