The high and low frequency components of speech are produced and perceived in different ways. The lower frequencies (roughly below 4KHz) are created by the voc

The high and low frequency components of speech are produced and perceived in different ways. The lower frequencies (roughly below 4KHz) are created by the vocal chords opening and closing at the fundamental frequency, and harmonics of this fundamental frequency (e.g. 100Hz + 2/3/400Hz etc harmonics), with this frequency spectrum then being modulated by the resonances of the vocal tract which change during pronunciation. What we perceive as speech is primarily the changes to these resonances (aka formants) due to articulation/pronunciation. The higher frequencies present in speech mostly comes from "white noise" created by the turbulence of forcing air out through closed teeth/etc (e.g. "S" sound), and our perception of these "fricative" speech sounds is based on onset/offset of energy in these higher 4-8KHz frequencies. Frequencies above 8KHz are not very perceptually relevant, and may be filtered out (e.g. not present in analog telephone speech).