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experienced large amounts of informational masking even when the masking tones were presented to the ear opposite the one hearing the signal. Adults experienced much less informational masking, and when the signal and masking tones were presented to opposite ears, the masking tones had no effect on tone signal detection. The children, then, were apparently unable to ignore irrelevant sounds, even when they were heard in the “wrong” ear.
Children apparently also do not ignore echoes in rever- berant rooms (Litovsky, 1997). Although children can local- ize simple single sounds as well as do adults, when sounds have leading and lagging elements typical of reverberant rooms, children do not fuse those sounds into one image, as do adults. This may suggest that children are attending to the initial sound and its echo as independent sound sources. They have not yet learned to ignore the irrelevant echo.
Studies of children’s neurobiology suggest that the behavioral patterns seen in these studies are reflected in the neurological patterns in the brain. Patterns of neural excita- tion change as children age. The patterns become less diffuse and more specific as children age into adolescence and beyond. Recent results suggest that complex neurological changes, including integration of information from the sens- es, continue until the brain is fully mature, some time in the third decade of life (Giedd, 2004). Until neurological centers mature, children are more adversely affected by background noise and require more favorable SNR to fully recognize speech.
During the first years of life, infants and young children are also learning to selectively attend to the important sound patterns of their native language. In the first months of life, infants are equally sensitive to patterns and phonemes from all languages equally well. Within a few months, however, toddlers become better at detecting familiar phonetic pat- terns, and “lose” the ability to discriminate unfamiliar phonemes from other languages (Werker et al., 1981, Kuhl, 1991). Infants seem to be going through a period of devel- oping selective attention for speech sounds used by their immediate caregivers. Nittrouer (1996) has shown that throughout childhood children continue to learn and devel- op adult-like patterns of listening and attention that make them more efficient at understanding speech in their native language. At first, children pay attention to parts of the speech signal that are less reliable than others, but gradually they learn to use the acoustic information in adult-like ways. Children also don’t seem to be able to fill in missing infor- mation, using their knowledge of the language. When words are completely audible, children are able to identify the words nearly as well as adults; when the audibility of the words is reduced somewhat, however, adults can identify the partially audible words quite well, while children cannot (Stelmachowicz et al., 2000). The difference between chil- dren and adults for understanding partially heard words is quite remarkable. Children apparently do not use the partial information they hear to fill in the missing information. The younger the child, the larger the difference between their recognition and adult performance.
All of these data strongly suggest that all children require
more favorable acoustics than do adults for the kind of audi- tory learning that goes on in classrooms every day. Children are not just being unpredictably inattentive or easily dis- tractible; they are children, whose auditory systems are immature and still under development. (As one of my favorite father/teachers is fond of saying: “Children are so juvenile!”) Children start out life apparently wired to take in every sound and every language, and their experience shapes them into more efficient, focused listeners. Typically devel- oping children need acoustic conditions that allow them full, stable access to auditory signals in schools so that no matter where they sit in a classroom, they can hear every word.
Are some children more vulnerable to poor acoustics than others?
A second important finding is that children with spe- cial needs constitute a large proportion of the school popu- lations and these children need even more favorable acoustics. In particular, children who are learning English (who speak other languages at home) seem to be particu- larly vulnerable to background noise and reverberation. Children who are learning new speech sounds and speech contrasts (for example, “b” versus “v” for children who speak Spanish) require a more favorable signal-to-noise ratio than those who are functioning more comfortably in their first language (Nelson et al., 2005). All world travel- ers who have struggled to converse in noisy restaurants in unfamiliar languages can recall that background noise affects non-native listeners more than the locals. This need for favorable acoustics has been well quantified in the liter- ature for adults (e.g., Mayo, Florentine, and Buus, 1997). We know less about the needs of children (e.g., Crandell and Smaldino, 1996; Nelson et al., 2005) but it appears that children learning in a second language environment experi- ence a kind of double jeopardy in noisy classrooms that is related to their incomplete development as well as their unfamiliarity with a new language. In major cities around the country, children who speak other languages may con- stitute as many as 30–40% of a public school population.
Surprising numbers of school-aged children have some degree of hearing loss. Public health studies of thousands of children suggest that slight, sensorineural (inner ear) hearing loss is found in approximately 15% of the school population. Hearing loss, also, magnifies the detrimental effects of back- ground noise and reverberation. In addition to having decreased sensitivity for soft sounds, all people with sen- sorineural hearing loss have poorer auditory frequency reso- lution, making masking noises more effective. Adults with sensorineural hearing loss report that they tolerate less back- ground noise than expected simply on the basis of their reduced sensitivity to sound (Plomp, 1978). Children with hearing loss, then, can also be expected to experience the double effects of incomplete development and hearing loss. Children with other special needs also experience detrimen- tal effects of noise. These include children with hyperactivi- ty/attention deficit disorder, as well as other auditory learn- ing disorders. Evidence suggests (e.g., Warrier et al., 2004) that these children, too, are especially distracted by class-
30 Acoustics Today, October 2005