Experimental studies on the complex susceptibility of soft magnetic composite materials (Ni and Ni-Zn spinels and YIG) in their region of spin resonance (SR) show that the SR frequency is a decreasing function of the magnetic concentration (demagnetising effects) which moreover tends to the natural spin resonance (NSR) frequency of the bulk material much before the unit concentration is reached. We take benefit of this remarkable property to show how powders could advantageously replace sintered materials to measure with no ambiguity (screening effects of domain wall oscillations) the NSR frequencies. Since composite materials are achieved by moulding processes under pressures, particular care of the technological stresses has been taken .