As you read this post, read it aloud. Create a speech signal. Take in the sentences; listen to the words. Now, distinguish the individual sounds. As a literate adult, you are a practiced and experienced language processor. But how did you get that way? Where did it begin? Before you understood the concept of language, your brain was taking in the vast input in your environment and beginning to develop a sound system unique to you and your language(s). Before their first words are uttered, infants make impressive strides in developing sound systems.
The speech signal is not made up of perfectly spliced words and sounds. It is a fluid act of articulation. Infants absorb this information and use it to perceive speech around them. Bilingual infants have to listen the sounds of both of their languages and distinguish them from each other. For example, the placement of the tongue when making a /d/ sound in English is different than that of French. Bilingual infants need to be able to tell 1) that there is a difference between those two sounds and 2) correlate them with the correct language. Early in life, infants are equally successful at discriminating all sounds. However, by the end of the first year, infants tune their ears to their own language(s) and lose their ability to distinguish nonnative phones. In the case of /d/, at 6-8 months, all infants could distinguish between English-like and French-like /d/ sounds, but by 10-12 months, bilingual infants retained this ability and French monolingual infants were no longer able to tell the difference between the two types of /d/.
This process shows how efficient the mind is. Monolingual infants do not need to retain sounds used in other language to be successful communicators. However, it is important that bilingual infants are exposed to both languages from an early age. One study suggests that those infants who are more successful at discriminating sounds in their native language at 7 months have accelerated language learning abilities at 24 and 30 months. Yet, parents need not fret! The process of understanding sounds begins in utero. By counting the number of times infants only 33 hours old sucked on a pacifier, it was shown that they are already sensitive to familiar and unfamiliar sounds. While it is important to communicate often and well to your children once they are born, the ambient noise they are exposed to in the womb can help structure their phonetic systems.
It may seem as though bilingual infants are presented with a trickier task than their monolingual counterparts, but in reality, they are able to distinguish their two languages from the start. Though there is evidence that two languages influence each other, bilingual infants can recognize familiar sounds from unfamiliar sounds. Additionally, they can also distinguish between their two languages and a third, unfamiliar language. When creating sound systems, bilingual infants are on pace with monolingual infants, despite their need to make two distinct systems.
Speaking and interacting directly with infants can support their sound system development as well. Languages have different rules for how sounds can work together to create words. This is vital for a strong vocabulary. By looking at English and Japanese, two languages with different distributions of sounds, infants are able to use the cues provided by their mothers to better perceive the speech signal. Not only do infants listen to the sounds in their environment, they catalogue their occurrences. For example, by observing the distribution of the word “happy” with other sounds in the speech signal like “happy mommy,” “happy baby,” and “happy kitty”, the infants will begin to understand which sounds pair together to form words. They learn that “ha-“ and “-ppy” go together often and “-ppy” and “mo-“ rarely go together. While word learning and word saying come later in infant language acquisition, it’s important that they have the solid structure of sound system(s) to build upon.
Infants learn quickly and begin to explore their worlds even before they are born. Both monolingual and bilingual children are efficient at analyzing their environment and focusing on the languages of their families and communities. This means that ample exposure to sounds and infant-directed communication are crucial to development. While it may appear that bilingual infants have to work doubly hard, they are expert language learners and can connect the sounds they hear to the languages to which they belong. In turn, they are able to use the sound systems they build to master their languages as they grow.
 Maye, J., Werker, J.F., & Gerken, L. (2002). Infant sensitivity to distributional information can affect phonetic discrimination. Cognition, 82, B101-B111.
 Moon, C., Lagercrantz, H., & Kuhl, P.K. (2013). Language experienced in utero affects vowel perception after birth: a two-country study. Acta paediatrica (Oslo, Norway : 1992), 102(2), 156-160.
 Kuhl, P.K., Conboy, B.T., Padden, D., Nelson, T., & Pruitt, J. (2005). Early Speech Perception and Later Language Development: Implications for the “Critical Period”. Language Learning and Development, 3&4, 237-264.
 Sundara, M., Polka, L., & Molnar. (2008). Development of coronal stop perception: Bilingual infants keep pace with their monolingual peers. Cognition, 108, 232-242.
 Werker, J.F., & Byers-Heinlein, K. (2008). Bilingualism in infancy: first steps in perception and comprehension. Trends in Cognitive Sciences, 12, 144-151.
 Werker, J.F., Pons, F., Dietrich, C., Kajikawa, S., Fais, L., & Amano, S. (2007). Infant-directed speech supports phonetic category learning in English and Japanese. Cognition, 103, 147-162.