To understand speech emanating from a target source, the listener in a ‘cocktail-party effect’ (CPE) situation must perceptually separate a dynamically changing target from a dynamically changing background. Although these dynamic changes do not occur synchronously in the target and background, investigators have resorted to presenting concurrent speech signals synchronously, except for selected keyword epochs, in order to measure speech intelligibility under speech interference (Brungart, 2001) [2]. The present study follows the rationale of that research to investigate perceptual segregation of two concurrent streams of nonspeech signals with speech-like properties: periodic harmonic sounds with the two streams differing in fundamental frequency f₀ (in all experiments), FM-like trajectory of the center frequency of a formant-like resonance (in Experiment 1), and the rhythmic AM pattern of syllabic-rate envelope fluctuations. Results show that segregation of streams with dynamic formant trajectories is easier than that of steady-state formants and that both formant trajectory pattern and rhythmic pattern discrimination is easier with larger f₀ separation between the two streams. Since elderly individuals are known to have CPE deficits, the fact that in both experiments our elderly subjects have also demonstrated consistently poorer performance than the young suggest that FM- and AM-based segregation of streams may underlie speech understanding dynamics associated with the syllable, a unit serving as the organizational interface among the various tiers of linguistic representation.