Question: Which acoustic elements are essential for the perception of speech?
How can we be so sure? Studies using sinewave replicas of natural utterances promote this conclusion.
What is the evidence? This page summarizes the findings of research done by Robert Remez and Philip Rubin, and their colleagues, and provides examples of sinewave synthesis for you to hear, along with information about this technique.
Most familiar synthetic speech aims to copy natural acoustic elements meticulously. That is why synthetic speech sounds voicelike, despite the mechanical quality of its articulation. In contrast, sinewave replication discards all of the acoustic attributes of natural speech, except one: the changing pattern of vocal resonances. By fitting 3 or 4 sinusoids to the pattern of resonance changes, sinusoidal signals preserve the dynamic properties of utterances without replicating the short-term acoustic products of vocalization.
If speech perception depended upon the particular sounds produced by talkers (the pop of the "p", the hiss of the "s", the hum of the "m", the click of the "k", or the buzz of the "z"), then sinusoidal signals lacking these attributes should not evoke impressions of consonants, vowels, words, etc. In fact, listeners who were asked to identify sinewave signals, reported "bad electronic music," "radio interference," etc., and no speechlike qualities. However, when asked to transcribe a "strangely-synthesized sentence," listeners readily reported the words of the natural utterances on which the sinewave signals were modeled. Below is an interactive introduction to the phenomenon of sinewave speech. Please judge for yourself.