What aspect of formant frequencies is critical for speech perception?

The correct answer highlights that the relative spacing of formant frequencies is crucial for speech perception. Formants are resonant frequencies of the vocal tract and play a vital role in defining the characteristics of different speech sounds, particularly vowels. The specific pattern of formant frequencies—how closely spaced they are and their relationship to one another—provides important acoustic cues that the human auditory system uses to distinguish between different vowels and consonants.

For instance, the spacing between the first and second formants (F1 and F2) can help listeners identify whether a vowel is more open or closed or if it has a front or back quality. Changes in this spacing result in variations perceived as different phonemic categories. Thus, relative spacing is critical for mapping formant structures to the sound categories of speech that listeners recognize.

Amplitudes, durations, and strengths might influence how clearly a sound is perceived but do not directly define the specific vowel qualities that are recognized by their formant patterns. Therefore, while amplitude and strength may impact loudness and clarity, and duration can affect timing, they do not play the primary role that relative spacing does in discerning the sounds of spoken language.