We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-dimensional electron system, in strong magnetic fields, by measuring its change in resistance. At the even denominator Landau level filling factors, ν, of 1/2 and 3/2 the electron–phonon interaction is observed to be cut-off above a particular value of the component of the wavevector perpendicular to the low dimensional system. This cut-off is consistent with that due to the finite thickness of the 2DES. At fractional quantum Hall effect minima (1/3, 2/5, 2/3) we observe that the cut-off to the interaction depends on the filling factor. At ν=1/3, the energy gap determined from these experiments is consistent with the theoretical prediction of the magnetoroton energy gap.