There is disclosed a method and apparatus for changing the frequency content of an input spectrum and a method and apparatus for reducing the perceptibility of a component of an input signal. The first aspect involves adjusting frequency components of the input spectrum in response to a time varying adjustment frequency spectrum to produce an output frequency spectrum including adjusted frequency components of the input spectrum. The time varying input spectrum may be produced by selectively addressing a number of individual sub-spectra at different times. In addition, the input spectrum may be divided into a plurality of sub-spectra and each sub-spectrum may be operated on separately by a different adjustment frequency spectrum at different times. In addition, a perceptual model may be used to enhance the adjustment of the input spectrum or sub-spectra. The second aspect involves adjusting frequency components of a first analysis windowed frame of input time samples in response to an adjustment frequency spectrum, to produce an output frame of output time samples and synthesis windowing the output frame to produce a synthesis windowed frame of output time samples representing a time varying output signal having reduced perceptibility of the component. Different windows are used to produce different input spectra for use as the input spectrum on which the adjustment is made and for use in a perceptual model to provide maximum energy to the perceptual model to enable it to have its full effect in producing the adjustment frequency spectrum.