HFGW

Which is the definition of a high frequency gravitational wave? On a SETI science forum I have found a paper which cites a formula given by W. Israel and S.Hawking in 1979, For these authors a HFGW starts at 100 kHz and up. In another paper found in the LIGO site a HFGW goes from 1 kHz up. And this makes me think of a possible formula giving the energy of a graviton (if it exists) similar to the Einstein-Planck formula for the photon. Which would be the analogous of the Planck constant? Probably a much smaller quantity.
Tullio