Answer
Notes produced by the windpipe increase in pitch when a person breathes helium.
Work Step by Step
Equation 16.22 gives the frequencies of a stopped pipe, open at one end, $f_n=\frac{nv}{4L}$.
As seen in Table 16.1, the speed of sound is much greater in helium than in air. Because the speed of sound v increases, the pitch of the voice increases.