Acoustic characteristics of speech exhibit variability across individuals, while preserving shared phonetic information to listeners. In this paper, the general time-frequency pattern of individual speaker characteristics is discussed based on our previous research. The main target here is set at speaker-specific acoustic effects of the vocal tract in both higher and lower frequency ranges. To address the under-explored phenomena, two experiments were conducted. Firstly, simulations based on the transmission line model are used to explore how resonances in higher frequencies vary with different hypopharyngeal-cavity shapes. Secondly, speech signals emitted from the mouth and nostrils are recorded separately to observe potential factors for spectral irregularity in lower frequencies. From our findings, a time-frequency model of individual speaker characteristics is proposed that provides insights into how individuality is manifested in speech spectral patterns.