Disciplinas de Estudio [DdE]> Any class or type of sound which
once existed, but has since been replaced, superseded, or has
otherwise ceased to be heard except as a museum artifact.
Disappearing sounds are generally those associated with social
activity, although some natural species and their sounds have
also become extinct. When the energy forms used in society
change, many direct and indirect acoustic changes result. As
travel by horse gave way to mechanized transport, the sounds of
shoed horsehooves, reins, sleighbells, and wagonwheels
disappeared, but so did the sound of the blacksmith and
wheelwright. In some cases an interesting morphology occurs if
the characteristic sound undergoes a metamorphosis, rather than
simple extinction. The development of the hunting horn and post
horn, for instance, through the steam whistle and compressed air
horn to the modern, electrically driven devices is a typical
such progression. Sometimes, changing materials form the basis
of acoustic change : earthenware to glass