[eng] Spontaneous speech sound variation has been widely examined in studies of different kinds that put forward reduction processes to explain different acoustic realizations for a single phoneme, although there is still need to check the perceptual effects of such differences. This study focuses on voiceless labiodental fricative in Spanish, and it is based on previous research (Batllori, Blecua y Rost 2009, 2010, Blecua y Rost 2011), where acoustic realizations of /f/ in spontaneous speech were analysed. The aim of those studies was checking the existence of acoustic variation concerning /f/ and establishing the factors on which it depends. Items from 6 male subjects were analysed, obtained from a sample of spontaneous speech. Four allophones could be identified, and they were related to different steps in a continuum of articulatory reduction. The aim of this second part of the study is testing the perceptual effects of this variation in speakers. Two kinds of perception tests have been carried out to check whether the listeners perceived any difference or not and which phoneme they identified. The results of these tests showed that relaxed realizations of /f/ are mainly associated to /b/ which proves that such variation has effects at a perceptual level and can be related to phonetic change factors.