Humans are sensitive to sounds in the frequency range of 20-20,000Hz.  Lower frequencies are perceived as low-pitched such as a foghorn, where has higher frequencies are perceived high in pitch such as birds chirping.  However, speech is isolated to a select range of frequencies known as the speech frequencies.  They lie between roughly 250-8000 Hz.   The pitch of one’s voice is largely determined by the size and shape of the anatomical structures within the vocal tract.  Therefore, it is more common for women and children to have higher pitched voices.  Moreover, different speech sounds (phonemes or letters) typically fall into a certain frequency range.  Vowels tend to be loud and low-pitched whereas consonants tend to be quiet and middle to high-pitched.  Please note the graph below indicating where different English speech sounds are located on the frequency scale.  We are able to understand complex speech by attaching meaning to specific combinations of these speech sounds (words) when we are children.  Such combinations are sent from the inner ear, through the auditory nerve, to the speech and language centres of the brain for processing.