How can you tell your "Aaahs" from your "Oooohs"?
Hopefully we've learned from the online spectrogram that different speech sounds are made up of different formants, or resonant frequencies. In fact, we can create vowel sounds by making sounds with just two resonant formant peaks. The following demo allows you to make such "artificial vowels" by choosing combinations of formants when you click the coordinate system on the left.
Admittedly, these relatively simple vowels do sound very artificial, but they are easy to recognize and tell apart, and they demonstrate that your ears must recognize patterns of formant peaks in order to process speech. This happens quickly and subconsciously, so that when you listen to someone speak, you are probably just thinking about the meaning of their words without any awareness of the processing of the frequency spectra that goes on in your brain.