acoustical comes from acoustic + -al, from Ancient Greek akoustikós ('pertaining to hearing'), from akoúein ('to hear'). Morphologically, the word is transparent: -al is attached to the older base named in the history, so the modern form packages an inherited or borrowed stem inside a later English derivative. In modern use, the adjective sense is "relating to sound or the science of sound", which keeps the etymology connected to the word's present dictionary value rather than treating it as a merely historical label. The wider word family includes related English forms such as acoustic, acoustics and derivatives such as acoustically, showing how the same base continues to support neighboring English vocabulary. The principal historical forms in the chain include Ancient Greek akoustikós, Ancient Greek akoúein, which anchor the explanation in named linguistic material.