  The gravitational lensing of gravitational waves might cause beat patterns
detectable by interferometers. The feature of this kind of signal is the
existence of the beat pattern in the early inspiral phase, followed by a
seemingly randomly changing profile. After the strain peaks for the first time,
the signal takes the usual waveform and the strain peaks for the second time.
Once this signal is detected, the actual magnification factors can be obtained,
so the true luminosity distance of the binary system is known. If the lens can
be described by a point mass or a singular isothermal sphere, the functional
forms of the time delay and the magnification factors are simple enough, so we
can infer the mass of the lens or the cosmological parameters.
