[en] We carry out theoretical investigations about distortions which may be present in directly computed energy or power spectral densities of signals made up of sub-signal components that have shorter time lengths compared to the time length of the ''mother'' signal itself. The analysis is divided into two distinct sections. In the first section, we consider signals whose composite sub-signals are simply connected together but do not overlap onto each other. This case is relevant to spectral analyses of a number of signals encountered often in scientific circles. These signals include amplitude-modulated complex signals whose modulation indices are equal to or greater than unity. In the second section, we consider signals whose composite sub-signals may overlap onto each other. It is shown that spectral densities computed in accordance with either section may harbour some distortions which, if not properly taken care of, can make genuine interpretations of the corresponding spectral densities partly erroneous. Some key features of these distortions are formulated and briefly discussed. (author)