Phonetic studies such as Quilis (1981) have found that Spanish voiced stops may surface as spirants with various degrees of constriction. These allophones are
not limited to regular fricative articulations, but range from articulations that involve a near complete oral closure to articulations
involving a degree of aperture quite close to vocalization Quilis, Antonio (1981), Fonética acústica de la lengua española [Acoustic phonetics of the Spanish language] (in Spanish), Gredos, ISBN978-84-249-0131-8