Even unheard sounds can be perceived and not only the ear hears sound, so does the body. The thing that constitutes sound, changes in air pressure, happens obviously also when we cannot hear it, and even if we listen, we may not hear it. From a physical point of view there is little difference between the frequencies that fall outside the audible range and those that fall within it, both are functions of changes in air pressure. Sounds that have not yet been heard, those that are imagined, and those that have long been silenced are part of our listening activity, perceptually as real as the physical.