Babble to Speech Infants go through predictable stages when learning to speak. Young infants are attentive to voices and make speech-like sounds themselves. They begin with babbling. Then, they start saying single words, and, finally, graduate to saying two- and three-word phrases. Four- and five-year-old children have a large vocabulary and can include several ideas in one sentence. Acoustics: Infant vs. Adult Different languages contain different types of acoustic information that is often not perceptible to nonnative adults. In contrast, infants are sensitive to acoustical differences in all languages but slowly lose their ability to hear these differences. In this way, they tune in to their native language. Critical Period Language acquisition occurs during what is called a critical period. If children are not exposed to a language by the time they are three years old, they will have great difficulty in learning the language. The fact that the process of language development is similar regardless of the specific language environment leads some psychologists to say that speech is special. Categorical Perception Another piece of evidence that is used to support the speech-is-special idea is that of categorical perception. It is the perception of different stimuli as being categorically different. Consonant-vowel pairs such as ba, da, and ga differ in the acoustical properties of the consonant segment. When the difference between two such consonant-vowel pairs reaches a certain point, called the category boundary, listeners suddenly hear a new consonant-vowel pair. However, the acoustical parameters can vary quite a bit without being noticed by nonnative speakers.
This image is a spectrogram of the phonemes ba, da, and ga. (A phoneme is the basic unit of sound that contrasts between different sound utterances.) A spectrogram represents time on the x-axis, frequency in Hertz (Hz) on the y-axis, and intensity in orange (low) � red (high). The bottom portion of the image (white background) represents the sound waves of the phonemes. Although the acoustical changes are continual, the listener’s perception changes only at a certain point. When this phenomenon was first discovered, it was thought to be unique to speech. However, categorical perception has since been demonstrated in several aspects of music and a few other nonspeech stimuli.