  There are regions in stars, such as ionization zones and the interface
between radiative and convective regions, that cause a localized sharp
variation in the sound speed. These are known as "acoustic glitches". Acoustic
glitches leave their signatures on the oscillation frequencies of stars, and
hence these signature can be used as diagnostics of these regions. In
particular, the signature of these glitches can be used as diagnostics of the
position of the second helium ionization zone and that of the base of the
envelope convection zone. With the help of stellar models we study the
properties of these acoustic glitches in main-sequence stars. We find that the
acoustic glitch due to the helium ionization zone does not correspond to the
dip in the adiabatic index \Gamma_1 caused by the ionization of HeII, but to
the peak in \Gamma_1 between the HeI and HeII ionization zones. We find that it
is easiest to study the acoustic glitch due to the helium ionization zone in
stars with masses in the range 0.9--1.2 M_\odot.
