The two principal sources of sound in speech, voicing and frication, occur simultaneously in voiced fricatives as well as at the vowel-fricative boundary in phonologically voiceless fricatives. Instead of simply overlapping, the two sources interact. This paper is an acoustic study of one such interaction effect: the amplitude modulation of the frication component when voicing is present. Corpora of sustained and fluent-speech English fricatives were recorded and analyzed using a signal-processing technique designed to extract estimates of modulation depth. Results reveal a pattern, consistent across speaking style, speaker, and place of articulation, for modulation at f0 to rise at low voicing strengths and subsequently saturate. Voicing strength needed to produce saturation varied 60–66dB across subjects and experimental conditions. Modulation depths at saturation varied little across speakers but significantly for place of articulation (with [z] showing particularly strong modulation) clustering at approximately 0.4–0.5 (a 40%–50% fluctuation above and below unmodulated amplitude); spectral analysis of modulating signals revealed weak but detectable modulation at the second and third harmonics (i.e., 2f0 and 3f0)