Generally, gravitational radiation (GR) is divided into three classes according to its nature: burst, continuous and stochastic waves. Continuous waves can be described as a sinusoidal stationary train of metric perturbation for a sufficiently long time, in contrast with burst waves, which are characterized by their short duration. The third type of GR, stochastic waves, is characterized by its random nature of statistics of arrival, regardless of wave form.

In searching for continuous waves with resonant antennas, various kinds of detecting methods and signal analyses are employed which are different from those used in the detection of burst waves. For example, a resonant antenna should be tuned precisely to the frequency of the source in order to obtain the best sensitivity. Also, long-time integration of the signal output from the detector is a necessary technique for distinguishing a coherent signal buried in a noise. Under these circumstances, usually, the sensitivity of a detector for continuous waves is determined by the level of Brownian motion of the antenna. These features are not common with the case of burst events.

Continuous sources, such as pulsars and binaries, have rather low frequencies, except for rapid pulsars (Backer et al., 1982) or new-born pulsars. Since the pioneering work of J. Weber (1969) bars have been widely used as resonant antennas in detecting burst waves.