  We estimate the gravitational wave spectra generated in strongly supercooled
phase transitions by bubble collisions and fluid motion. We derive analytically
in the thin-wall approximation the efficiency factor that determines the share
of the energy released in the transition between the scalar field and the
fluid. We perform numerical simulations including the efficiency factor as a
function of bubble radius separately for all points on the bubble surfaces to
take into account their different collision times. We find that the efficiency
factor does not significantly change the gravitational wave spectra and show
that the result can be approximated by multiplying the spectrum obtained
without the efficiency factor by its value at the radius $R_{\rm eff} \simeq
5/\beta$, where $\beta$ is the approximate inverse duration of the transition.
We also provide updated fits for the gravitational wave spectra produced in
strongly supercooled transitions from both bubble collisions and fluid motion
depending on the behaviour of the sources after the collision.
