Background: Previous research has found that high-frequency energy of speech signals decreased while wearing face masks. However, no study has examined the specific spectral characteristics of fricative consonants and vowels and the perception of clarity of speech in mask wearing. Aims: To investigate acoustic-phonetic characteristics of fricative consonants and vowels and auditory perceptual rating of clarity of speech produced with and without wearing a face mask. Methods & Procedures: A total of 16 healthcare workers read the Rainbow Passage using modal phonation in three conditions: without a face mask, with a standard surgical mask and with a KN95 mask (China GB2626-2006, a medical respirator with higher barrier level than the standard surgical mask). Speech samples were acoustically analysed for root mean square (RMS) amplitude (A[subscript RMS]) and spectral moments of four fricatives /f/, /s/, /[voiceless palato-alveolar fricative]/ and /z/; and amplitude of the first three formants (A1, A2 and A3) measured from the reading passage and extracted vowels. Auditory perception of speech clarity was performed. Data were compared across mask and non-mask conditions using linear mixed models. Outcomes & Results: The A[subscript RMS] of all included fricatives was significantly lower in surgical mask and KN95 mask compared with non-mask condition. Centre of gravity of /f/ decreased in both surgical and KN95 mask while other spectral moments did not show systematic significant linear trends across mask conditions. None of the formant amplitude measures was statistically different across conditions. Speech clarity was significantly poorer in both surgical and KN95 mask conditions. Conclusions & Implications: Speech produced while wearing either a surgical mask or KN95 mask was associated with decreased fricative amplitude and poorer speech clarity.