The name of an acoustically based phonological and phonetic feature. Compact sounds have a concentration of energy in the middle of the frequency range of the spectrum. An example is the vowel ɑ which has a relatively high first formant which is close to the frequency of the second formant. The opposite of compact is diffuse. A diffuse vowel, such as i, has no centrally located concentration of energy – the first and second formants are widely separated.

A diffuse vowel (left) and a compact vowel (right)