  We use the three-cornered-hat method to evaluate the absolute frequency
stabilities of three different ultrastable reference cavities, one of which has
a vibration-insensitive design that does not even require vibration isolation.
An Nd:YAG laser and a diode laser are implemented as light sources. We observe
$\sim1$ Hz beat note linewidths between all three cavities. The measurement
demonstrates that the vibration-insensitive cavity has a good frequency
stability over the entire measurement time from 100 $\mu$s to 200 s. An
absolute, correlation-removed Allan deviation of $1.4\times10^{-15}$ at 1 s of
this cavity is obtained, giving a frequency uncertainty of only 0.44 Hz.
