The gravitational wave spectrum (GW spectrum) is the range of frequencies or wavelengths of gravitational waves. The waves travel at the speed of light in a vacuum (c) so the same frequency/wavelength relation holds as for electromagnetic radiation. The waves are caused by reconfiguration of masses, such as when massive objects move between positions closer and further from you. Orbiting bodies produce such waves, on the order of a wave per orbit (for circular orbits of equal sized bodies, actually two waves per orbit: there are two times within the orbit when one of the identically-massed bodies is closer to you). In consequence, such waves, like orbits, can have periods ranging from seconds to centuries and beyond, and you hear of GW frequencies such as 10-10 Hz (which corresponds to a period of about three centuries).
A specific wave can be distinguished (e.g., by an instrument) if it has a recognizable frequency pattern and has sufficient amplitude to stand out among the background of waves from every orbit in the universe, the gravitational wave background (GWB). The amplitude (gravitational wave strain) is related to the masses of the orbiting objects, and orbiting black holes/neutron stars produce waves more likely to show above the noise. Orbiting binary SMBHs have the most mass but there may be few or none to observe, given their numbers and the final parsec problem, and many would have the very long periods, a challenge to identify.
The GW frequency regimes:
|period range||general frequency range||source||detector type|
|tiny fractions of a second||>1000 Hz|
|1/1000 to 10 seconds||0.1-1000 Hz||Final (smallest) orbits of merging stellar black holes and neutron stars||ground Michelson interferometers|
|1 second to years||10-12-1 Hz||Final orbits of merging supermassive black holes||pulsar timing arrays and space interferometers|
|up to billions of years||down to 10-18 Hz|
Some GWs are expected to be left from early universe activities (primordial gravitational waves or primordial GWs), which potentially span all these regimes. Supernovae under certain conditions may create detectable waves similar to that of merging stellar mass black holes. Ground GW detectors include LIGO and Virgo, and LISA is a planned detector in space.