  We investigate gravitational waves from a dust disk around a Schwarzschild
black hole to focus on whether we can extract any of its physical properties
from a direct detection of gravitational waves. We adopt a black hole
perturbation approach in a time domain, which is a satisfactory approximation
to illustrate a dust disk in a supermassive black hole. We find that we can
determine the radius of the disk by using the power spectrum of gravitational
waves and that our method to extract the radius works for a disk of arbitrary
density distribution. Therefore we believe a possibility exists for determining
the radius of the disk from a direct observation of gravitational waves
detected by the Laser Interferometer Space Antenna.
