Radio halos and relics are giant (Mpc-scale) diffuse synchrotron steep spectrum sources, observed at the center (halos) and the outskirts (relics) of a fraction of merging galaxy clusters. Their origin is believed to be related turbulent motions and shock waves dissipated in the intracluster gas during cluster mergers. In this talk I will present results from multifrequency studies of two interesting merging clusters, Abell 754 and Abell 697. Important properties of their hosted diffuse radio sources are revealed thanks to high sensitivity low frequency observations (GMRT <330 MHz). In A754 we confirm the existence of a peripheral relic, which interestingly coincides with a shock front detected in the X-ray gas. Such coincidence has been observed only in a couple of other clusters, and support the connection between merger shocks and relics. In A697 we find that the radio halo has a very steep spectrum, being one of the very few ultra-steep spectrum radio halos (USSRH) known so far. This result support the turbulent re-acceleration model for the origin of radio halos. I will finally show preliminary results from 150 MHz GMRT follow up observations of A697 and other two clusters, aimed at investigating the low frequency properties of USSRHs.